Science and Environment

Science · Physics and chemistry · Topic 1 of 10

Light: Reflection, Refraction, Lenses and Defects of Vision

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Think of it like…

A wari dindi stepping from the road onto the riverbank sand at Pandharpur

Picture a dindi of warkaris walking arm in arm, ten abreast, on the tar road towards the Chandrabhaga at Pandharpur. The road is hard, so they walk fast. Near the ghat the road gives way to soft sand, and the row meets the sand at a slant. The warkari at the left end steps into the sand first and slows down, while those still on the road keep their pace. For a few steps the left end is dragging and the right end is racing, so the whole row swings round and ends up walking more squarely into the sand. When they later climb back onto the road, the opposite happens and the row swings the other way. If they had met the sand head-on, everyone would slow together and nobody would turn.

Light does exactly this. Its speed is about 3 × 10⁸ m/s in air, less in water and still less in glass. When a beam enters a denser medium at a slant, the side that enters first slows first and the beam bends towards the normal; coming out it bends away from the normal. A beam hitting the boundary straight on passes without bending. A lens is just a patch of sand shaped cleverly: thicker in the middle (convex) so the central part of the row is held back longest and the row closes in to a point, or thinner in the middle (concave) so the row spreads out. Your eye is such a lens, and spectacles are a second lens added when the eye’s own lens cannot bring the row to a point on the retina.

In everyday life	In the exam topic
Tar road where the row walks fast	Optically rarer medium (air), light travels faster
Soft sand where the row slows	Optically denser medium (water, glass), light travels slower
Row swinging when one end enters the sand first	Refraction: bending towards the normal on entering a denser medium
Row meeting the sand head-on and not turning	Light along the normal (angle of incidence 0°) passes undeviated
A sand patch thick in the middle that pulls the row to a point	Convex (converging) lens with a real focus
Adding a second sand patch so the row lands on the right spot	Spectacle lens correcting myopia (concave) or hypermetropia (convex)

The concept

Reflection obeys two laws: the angle of incidence equals the angle of reflection, and the incident ray, reflected ray and normal lie in one plane. A plane mirror forms a virtual, erect, laterally inverted image as far behind the mirror as the object is in front. A concave mirror converges light; it gives a magnified virtual image when the object is inside the focus (shaving and dentist’s mirrors) and is used as the reflector in torches and headlights with the bulb at the focus. A convex mirror always gives a small, erect, virtual image but covers a wide field of view, which is why it is the rear-view mirror of every ST bus and the mirror at blind turns on ghat roads. The mirror formula (NCERT sign convention) is 1/v + 1/u = 1/f, with f = R/2.

Refraction happens because light changes speed between media. The refractive index n = c / v (speed in vacuum ÷ speed in the medium); for glass n ≈ 1.5, so light moves at 3 × 10⁸ ÷ 1.5 = 2 × 10⁸ m/s in it. Snell’s law: sin i / sin r = constant. When light goes from a denser to a rarer medium at more than the critical angle, none of it escapes: this is total internal reflection, behind optical fibres, the sparkle of diamond (n ≈ 2.42, critical angle about 24°) and the desert or highway mirage. Atmospheric refraction makes stars twinkle (planets, being nearer discs, generally do not) and lets us see the Sun about two minutes before actual sunrise and after actual sunset. A prism splits white light into VIBGYOR (dispersion) because each colour has a slightly different speed in glass; violet deviates most, red least. A rainbow combines dispersion, refraction and internal reflection in raindrops and is always seen with the Sun behind you. The sky is blue because tiny air molecules scatter short (blue) wavelengths much more than long (red) ones; red light is scattered least, so it travels farthest, which is why danger signals and the setting Sun are red.

Lenses. A convex lens converges light and a concave lens diverges it. The lens formula is 1/v − 1/u = 1/f, and power P = 1/f with f in metres, measured in dioptres (D). Convex lenses have positive power, concave negative. For thin lenses in contact powers simply add: P = P₁ + P₂. This makes optometry numericals quick: an optician’s prescription of −2.0 D means a concave lens of focal length 50 cm.

The eye and its defects. The eye lens focuses images on the retina; the ciliary muscles change its focal length (accommodation). A normal adult eye sees clearly from the near point of 25 cm to infinity. Myopia (short-sightedness): distant objects are blurred because the image forms in front of the retina (eyeball too long or lens too curved); corrected with a concave lens. Hypermetropia (long-sightedness): near objects are blurred, image behind the retina (eyeball too short); corrected with a convex lens. Presbyopia: age weakens the ciliary muscles, so near vision fails; often both defects coexist and a bifocal lens is used. Astigmatism needs a cylindrical lens. Cataract is clouding of the eye lens, treated by surgery. Rods respond to dim light and cones to colour; night blindness is linked to vitamin A deficiency.

Key points & formulas

Refractive index n = c / v; light in glass (n = 1.5) travels at 2 × 10⁸ m/s
Power P = 1/f (f in metres); unit dioptre (D); convex +, concave −; P = P₁ + P₂ for lenses in contact
Mirror formula 1/v + 1/u = 1/f; lens formula 1/v − 1/u = 1/f; f = R/2 for mirrors
Convex mirror: rear-view mirror (wide field, erect diminished image). Concave mirror: headlights, shaving, dentist
Total internal reflection: optical fibre, diamond sparkle, mirage (denser → rarer, angle > critical angle)
Dispersion: violet deviates most, red least. Scattering: blue sky; red sunset and red danger signals
Atmospheric refraction: twinkling stars, Sun visible about 2 minutes before sunrise and after sunset
Near point of normal eye 25 cm, far point infinity; accommodation by ciliary muscles
Myopia → concave lens; hypermetropia → convex lens; presbyopia → bifocal; astigmatism → cylindrical
Cataract: opaque eye lens, corrected by surgery; rods = dim light, cones = colour

Worked example

A farmer from Satara can read a newspaper clearly only when he holds it at 1 m. His neighbour cannot see objects beyond 2 m clearly. Name each defect and find the power of the lens each needs (normal near point 25 cm).

Show solution

Farmer: near objects blurred, so this is hypermetropia. The lens must take an object at u = −25 cm and form a virtual image at his near point, v = −100 cm.
Lens formula: 1/f = 1/v − 1/u = 1/(−100) − 1/(−25) = −0.01 + 0.04 = 0.03 per cm, so f ≈ +33.3 cm.
P = 100 / f (cm) = 100 × 0.03 = +3 D, a convex lens.
Neighbour: distant objects blurred, so this is myopia. The lens must make objects at infinity appear to come from his far point, so f = −2 m.
P = 1 / (−2) = −0.5 D, a concave lens.

Answer: Farmer: hypermetropia, convex lens of +3 D. Neighbour: myopia, concave lens of −0.5 D.

Exam shortcut

Sign tells shape: “plus is convex, minus is concave.” And the defect name tells you what still works: myopia = my near vision is fine (needs concave); hypermetropia = horizon is fine, near is blurred (needs convex).

Common traps

Myopia is corrected by a concave lens, not convex. Candidates often reverse this because “short-sighted” sounds like it needs magnification.
Light bends towards the normal when entering a denser medium and away from it when leaving; total internal reflection happens only from denser to rarer.
Red light is scattered the least (that is why it is used for danger signals); blue is scattered more. Do not mix up “scattered most” with “visible farthest”.
Focal length must be in metres for power in dioptres: f = 25 cm gives P = 1/0.25 = +4 D, not 1/25.
A rainbow is seen in the direction opposite the Sun, so morning rainbows appear in the west.

Practice · tap an option to check

Q1. A student can read the blackboard notes in his notebook easily but finds the blackboard itself blurred from the last bench. Which defect does he have, and how is it corrected?

Q2. Two thin lenses of power +4 D and −2.5 D are placed in contact. The combination behaves as:

Q3. Consider the following statements:
1. A rainbow is seen in the same direction as the Sun.
2. Stars twinkle because of atmospheric refraction, while planets generally do not twinkle.
Which of the statements given above is/are correct?

Q4. Match List I with List II:
List I: a. Rear-view mirror of an ST bus b. Dentist’s mirror c. Correcting astigmatism d. Correcting presbyopia with both defects
List II: 1. Concave mirror 2. Convex mirror 3. Bifocal lens 4. Cylindrical lens

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Science · Physics and chemistry · Topic 2 of 10

Electricity and Magnetism: Ohm’s Law, Series/Parallel, Household Wiring, Electromagnetic Induction

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Think of it like…

Canal water from a dam reaching the fields of a village

Think of water released from Jayakwadi dam into the canal network of a Marathwada village. The height of the water in the dam decides how hard it pushes; the higher it stands, the more water rushes through. The amount of water flowing past a point every second is what the farmer actually gets. A narrow, weed-choked channel resists the flow, so for the same push less water reaches the field. Double the push, and the flow doubles; make the channel twice as narrow and the flow halves.

Now look at how the fields are fed. If the canal ran through field 1, then field 2, then field 3 in a single line, a blockage in field 1 would starve the others, and each field would get only what the previous one left. Instead each farmer gets his own branch straight from the main canal, so every field feels the same pressure and one farmer can close his gate without affecting the rest. The water meter at the outlet counts the total volume used over the season, and the irrigation department bills by that volume, not by how fast it flowed on one day. Electricity in a house is wired exactly like those branches, and the electricity meter counts “units” the same way.

In everyday life	In the exam topic
Height of water in the dam (push)	Potential difference V, in volts
Water flowing past a point per second	Current I, in amperes
Narrow, weed-choked channel	Resistance R, in ohms; V = IR
Fields fed one after another on a single channel	Series connection: same current, resistances add
Each field on its own branch from the main canal	Parallel connection: same voltage, appliances work independently
Season’s water volume on the outlet meter	Energy in kWh (“units”) on the electricity meter

The concept

Ohm’s law: at constant temperature the current through a conductor is proportional to the potential difference across it, V = IR. Resistance depends on the material and shape: R = ρL / A, so a longer wire has more resistance and a thicker wire less. The resistivity ρ is a property of the material: silver and copper are the best common conductors (copper is used for wiring), while nichrome, an alloy with high resistivity and a high melting point that does not oxidise easily, is used in heaters, irons and toasters. Bulb filaments are made of tungsten (melting point about 3380 °C) and the bulb is filled with an inert gas such as argon or nitrogen.

Series and parallel. In series, the same current flows through every resistor and R = R₁ + R₂ + R₃; one break stops everything (old decorative light strings). In parallel, every branch gets the same voltage and 1/R = 1/R₁ + 1/R₂ + 1/R₃; the total is always less than the smallest branch. Household appliances are connected in parallel so each gets the full supply voltage and can be switched on or off independently. Heat produced (Joule’s law) H = I²Rt, and power P = VI = I²R = V²/R. The commercial unit of energy is the kilowatt-hour: 1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J = 1 “unit” on the bill.

Household wiring. Indian mains supply is alternating current at 50 Hz; NCERT gives the potential difference between live and neutral as 220 V. NCERT’s colour convention is live = red, neutral = black, earth = green (newer wiring often uses brown, blue and green-yellow). The fuse (a wire of low melting point) or MCB is connected in series with the live wire; it melts or trips when overloading or a short circuit pushes the current too high. The earth wire is connected to the metal body of appliances like irons, fridges and geysers so that if the live wire touches the body, current flows to earth instead of through a person.

Magnetism and induction. Oersted (1820) showed that a current produces a magnetic field; field lines run from north to south outside a magnet, and a current-carrying solenoid with a soft-iron core is an electromagnet. A current-carrying conductor in a magnetic field feels a force (direction by Fleming’s left-hand rule): this is the principle of the electric motor. Faraday (1831) discovered electromagnetic induction: a changing magnetic field through a coil induces a current (direction by Fleming’s right-hand rule). This is the principle of the generator (dynamo) and the transformer, which works only on AC. Power is transmitted at very high voltage because, for the same power, a higher voltage means a smaller current and much smaller I²R loss in the lines.

Key points & formulas

Ohm’s law V = IR; R = ρL/A; SI unit of resistivity ohm-metre (Ω m)
Series: R = R₁ + R₂ + …; parallel: 1/R = 1/R₁ + 1/R₂ + … (total less than the smallest)
P = VI = I²R = V²/R; heat H = I²Rt (Joule’s law)
1 kWh = 3.6 × 10⁶ J = 1 unit; units = (watts × hours) ÷ 1000
Indian mains: AC, 50 Hz; appliances in parallel; fuse/MCB in series with the live wire
NCERT wire colours: live red, neutral black, earth green
Nichrome: heating elements; tungsten: bulb filament; copper: wiring
Oersted (1820): current produces magnetic field; Faraday (1831): electromagnetic induction
Motor: Fleming’s left-hand rule; generator: Fleming’s right-hand rule; transformer works only on AC
High-voltage transmission reduces current and hence I²R losses

Worked example

A house in Nagpur runs 3 fans of 75 W for 8 hours a day, 5 LED bulbs of 10 W for 6 hours a day and a 1500 W geyser for 1 hour a day. Find the energy used in a 30-day month and the bill at an assumed flat rate of ₹8 per unit.

Show solution

Fans: 3 × 75 W × 8 h = 1800 Wh per day.
LED bulbs: 5 × 10 W × 6 h = 300 Wh per day.
Geyser: 1500 W × 1 h = 1500 Wh per day.
Total = 1800 + 300 + 1500 = 3600 Wh = 3.6 kWh = 3.6 units per day.
Month: 3.6 × 30 = 108 units. Bill: 108 × ₹8 = ₹864.

Answer: 108 units, costing ₹864 at ₹8 per unit. Note that the geyser alone uses nearly as much (1500 Wh) as all three fans together (1800 Wh) in one-eighth of the time: high wattage matters as much as long hours.

Exam shortcut

For two resistors in parallel use “product over sum”: R = R₁R₂ / (R₁ + R₂). For n equal resistors R in parallel the total is R/n. For units, always do watts × hours ÷ 1000.

Common traps

The fuse goes in the live wire, not the neutral; a fuse in the neutral would leave the appliance live even after it blows.
In parallel the voltage is the same across each branch; in series the current is the same. Candidates swap these.
An electric motor converts electrical to mechanical energy (left-hand rule); a generator converts mechanical to electrical energy by induction (right-hand rule).
A “unit” of electricity is 1 kWh, an energy unit, not a power unit. A 100 W bulb uses 1 unit only after running 10 hours.
A transformer cannot work on steady DC, because induction needs a changing magnetic field.

Practice · tap an option to check

Q1. Resistors of 3 Ω and 6 Ω are connected in parallel, and this combination is connected in series with a 4 Ω resistor across a 12 V battery. The current drawn from the battery is:

Q2. An electric iron of 1000 W is used for 30 minutes every day. How many units of electricity does it consume in 30 days?

Q3. Consider the following statements about household wiring:
1. Appliances are connected in parallel so that each receives the same voltage and can be operated independently.
2. The fuse is connected in the neutral wire.
Which of the statements given above is/are correct?

Q4. Which of the following devices works on the principle of electromagnetic induction?

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Science · Physics and chemistry · Topic 3 of 10

Atomic Structure, Isotopes and the Periodic Table

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Think of it like…

Allotting flats in a Mumbai cooperative housing society

A new cooperative housing society in Thane is allotting flats. The first secretary numbered them by the rent each family paid. It mostly worked, but a few families ended up next to the wrong neighbours, and he had to leave some flats empty, saying “a family that fits here will come later.” His successor numbered flats by a fixed registration number that never changes, and every oddity disappeared.

In the building, each floor fills from left to right, and once a floor is full a new floor begins. Flats stacked one above another in the same column have the same layout, the same number of windows facing outside, so the families in them behave alike: all the corner-flat families keep plants on the balcony, all the top-row families complain about water. Two brothers may share the same surname and registration number but one carries more luggage than the other; they weigh differently but behave identically at society meetings.

That is the periodic table. Mendeleev (1869) arranged elements by atomic mass (rent) and left gaps for undiscovered elements; Moseley (1913) showed the true basis is atomic number (registration number). Periods are floors, groups are columns, the outer “windows” are valence electrons, and the brothers with different luggage are isotopes: same protons, different neutrons.

In everyday life	In the exam topic
Flats numbered by rent, with a few misfits and empty flats	Mendeleev’s table by atomic mass, with gaps for undiscovered elements
Fixed registration number that never changes	Atomic number Z (number of protons): basis of the modern periodic law
Floors filling left to right	Periods (7): a new shell starts each period
Flats stacked in the same column with the same layout	Groups (18): same number of valence electrons, similar properties
Brothers with the same registration number but different luggage	Isotopes: same Z, different mass number (different neutrons)

The concept

Discovering the atom’s parts. J.J. Thomson discovered the electron (1897) with cathode rays and proposed the “plum pudding” model. Rutherford’s gold-foil experiment showed that most alpha particles pass straight through but a few bounce back, so the atom is mostly empty space with a tiny, dense, positive nucleus (1911). Bohr (1913) put electrons in fixed energy levels (shells K, L, M, N) that do not radiate energy, explaining atomic stability. Chadwick discovered the neutron (1932). A shell n can hold at most 2n² electrons (K = 2, L = 8, M = 18), and the outermost shell holds at most 8; so sodium (Z = 11) is 2, 8, 1 and chlorine (Z = 17) is 2, 8, 7.

Atomic number and mass number. Atomic number Z = number of protons (= electrons in a neutral atom). Mass number A = protons + neutrons, so neutrons = A − Z. Isotopes have the same Z but different A (protium, deuterium and tritium of hydrogen; ¹²C and ¹⁴C); they have the same chemical properties because chemistry depends on electrons. Isobars have the same A but different Z (⁴⁰Ar and ⁴⁰Ca). Isotones have the same number of neutrons. Uses of isotopes are frequently asked: U-235 as nuclear fuel, C-14 for dating old organic remains, Co-60 for cancer radiotherapy, I-131 for thyroid disorders, and heavy water (D₂O) as a moderator in Indian pressurised heavy water reactors. The atomic mass printed for chlorine (35.5) is a weighted average of its isotopes ³⁵Cl and ³⁷Cl.

Periodic table history. Döbereiner’s triads (middle element’s mass ≈ average of the other two) and Newlands’ law of octaves (every eighth element similar, worked only up to calcium) came first. Mendeleev (1869) arranged the 63 then-known elements by atomic mass and boldly left gaps, predicting eka-aluminium (gallium), eka-silicon (germanium) and eka-boron (scandium). His table could not place hydrogen or isotopes properly and had a few mass inversions. Moseley (1913) showed that properties are periodic functions of atomic number, which is the modern periodic law. The modern long form has 18 groups and 7 periods and 118 elements, with the lanthanides and actinides placed separately.

Trends. Across a period (left to right), nuclear charge rises, so atomic radius decreases, metallic character decreases and electronegativity increases. Down a group, new shells are added, so atomic radius increases and metallic character increases. Group 1 = alkali metals (one valence electron, highly reactive), group 17 = halogens (seven valence electrons), group 18 = noble gases (complete octet, very unreactive). Fluorine is the most electronegative element. Mercury is the metal that is liquid at room temperature and bromine the non-metal; oxygen is the most abundant element in the Earth’s crust and aluminium the most abundant metal.

Key points & formulas

Thomson: electron (1897); Rutherford: nucleus (1911); Bohr: energy levels (1913); Chadwick: neutron (1932)
Neutrons = A − Z; maximum electrons in a shell = 2n²
Isotopes: same Z, different A; isobars: same A, different Z; isotones: same neutrons
Isotope uses: U-235 fuel, C-14 dating, Co-60 cancer therapy, I-131 thyroid, D₂O moderator
Mendeleev (1869): by atomic mass, left gaps (gallium, germanium, scandium predicted)
Moseley (1913): modern periodic law based on atomic number
Modern table: 18 groups, 7 periods, 118 elements
Across a period: radius ↓, metallic character ↓, electronegativity ↑; down a group: radius ↑
Most electronegative: fluorine; liquid metal: mercury; liquid non-metal: bromine
Most abundant in Earth’s crust: oxygen (element), aluminium (metal)

Worked example

Natural chlorine is about 75% ³⁵Cl and 25% ³⁷Cl (Z = 17). (a) How many neutrons are in each isotope? (b) What average atomic mass should appear in the periodic table? (c) Will the two isotopes react differently with sodium?

Show solution

Neutrons = A − Z. For ³⁵Cl: 35 − 17 = 18. For ³⁷Cl: 37 − 17 = 20.
Weighted average = 35 × 0.75 + 37 × 0.25 = 26.25 + 9.25 = 35.5.
Both isotopes have 17 protons and 17 electrons with configuration 2, 8, 7, so both gain one electron to form Cl⁻.
Chemical behaviour depends on electrons, not neutrons, so both form NaCl in the same way; only physical properties that depend on mass (density, rate of diffusion) differ slightly.

Answer: (a) 18 and 20 neutrons; (b) 35.5 u; (c) No, isotopes have identical chemical properties.

Exam shortcut

For “which is not a trend across a period” questions remember one rule: across a period the nucleus pulls harder, so anything that needs a strong pull (electronegativity, ionisation energy, non-metallic character) goes up, and anything that needs a loose hold (size, metallic character) goes down. Down a group the opposite.

Common traps

Isotopes have the same chemical properties but slightly different physical properties; isobars are different elements.
Mendeleev used atomic mass; the modern law uses atomic number (Moseley), not Mendeleev.
Atomic radius decreases across a period even though more electrons are added, because the extra protons pull all the electrons in the same shell closer.
Rutherford discovered the nucleus, not the neutron; the neutron came much later from Chadwick (1932).

Practice · tap an option to check

Q1. Uranium-235 has atomic number 92. How many neutrons does its nucleus contain?

Q2. Isotopes of an element have:

Q3. Arrange the following in chronological order:
1. Rutherford’s nuclear model of the atom
2. Thomson’s discovery of the electron
3. Chadwick’s discovery of the neutron
4. Bohr’s model of fixed energy levels

Q4. Consider the following statements about the modern periodic table:
1. Elements are arranged in order of increasing atomic mass.
2. Atomic size generally decreases from left to right across a period.
Which of the statements given above is/are correct?

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Science · Physics and chemistry · Topic 4 of 10

Acids, Bases, Salts and pH (everyday chemicals)

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Think of it like…

The soil health card at the Krishi Vigyan Kendra

Two farmers collect their soil health cards on the same day. Sakharam grows paddy on red laterite soil near Ratnagiri, where heavy rain has washed away the calcium; his card reads pH 5, “acidic.” The officer tells him to spread lime (calcium oxide or slaked lime), a base, to bring the soil towards neutral. Tukaram grows sugarcane on black soil near Baramati, where years of flood irrigation have left sodium salts behind; his card reads pH 9, “alkaline (sodic).” He is told to apply gypsum and flush the salts out.

Sakharam asks: “Is my soil only a little more acidic than his? 5 and 9 are close.” The officer laughs. “Each step on this card is ten times. From pH 9 to pH 5 is four steps, so your soil has 10 × 10 × 10 × 10 = 10,000 times more hydrogen ions per litre.” Adding just the right amount of lime makes the acid and base cancel, leaving neutral salt and water in the soil. That is the whole chapter: acids give H⁺ ions, bases give OH⁻ ions, they cancel each other in neutralisation, and pH is a logarithmic scorecard of how many H⁺ ions remain.

In everyday life	In the exam topic
Rain-leached laterite soil reading pH 5	Acid: gives H⁺ ions; pH below 7
Sodic black soil reading pH 9	Base (alkali): gives OH⁻ ions; pH above 7
Spreading lime on acidic soil	Neutralisation: acid + base → salt + water
Each step on the soil card is a tenfold change	pH is logarithmic: pH = −log[H⁺]
The ideal reading near 7 that most crops like	Neutral solution, pH 7 at 25 °C

The concept

Acids taste sour, turn blue litmus red and produce H⁺ (as H₃O⁺) ions in water; bases taste bitter, feel soapy, turn red litmus blue and produce OH⁻ ions; bases soluble in water are called alkalis. Strong acids (HCl, H₂SO₄, HNO₃) and strong bases (NaOH, KOH) ionise almost completely; acetic acid, citric acid and ammonium hydroxide are weak. Acids react with metals to give hydrogen gas and with carbonates and bicarbonates to give carbon dioxide (that is why lemon juice fizzes with baking soda). Aqua regia, a 3 : 1 mixture of concentrated HCl and HNO₃, dissolves gold and platinum.

pH = −log[H⁺]. At 25 °C pure water has pH 7; below 7 is acidic and above 7 basic. Because the scale is logarithmic, a fall of one unit means ten times more H⁺: pH 3 is 1000 times more acidic than pH 6. Everyday values asked in exams: gastric juice (HCl in the stomach) about 1.5–2, lemon juice about 2, rain becomes “acid rain” below 5.6, milk about 6.5, human blood about 7.4, and tooth enamel starts to corrode when the mouth’s pH falls below 5.5 (hence toothpaste is basic). Antacids such as milk of magnesia (Mg(OH)₂) neutralise excess stomach acid. An ant or nettle sting injects methanoic (formic) acid; rubbing baking soda neutralises it.

Natural sources: vinegar – acetic acid; lemon and orange – citric acid; tamarind – tartaric acid; tomato – oxalic acid; sour milk and curd – lactic acid; ant and nettle sting – methanoic acid. Indicators: litmus (from lichens) is red in acid, blue in base; phenolphthalein is colourless in acid and pink in base; methyl orange is red in acid and yellow in base; turmeric turns reddish-brown in base (a turmeric stain on clothes turns red with soap).

Important salts. Common salt (NaCl) electrolysed as brine in the chlor-alkali process gives NaOH, Cl₂ and H₂. Bleaching powder (CaOCl₂) is used to disinfect drinking water. Baking soda is sodium hydrogen carbonate (NaHCO₃), used in baking powder and antacids. Washing soda is Na₂CO₃·10H₂O, used to remove permanent hardness of water. Plaster of Paris is CaSO₄·½H₂O; mixed with water it sets into gypsum (CaSO₄·2H₂O), which is why doctors use it for fracture casts. The water molecules held in a crystal are called water of crystallisation (blue copper sulphate, CuSO₄·5H₂O, turns white on heating as it loses them).

Key points & formulas

pH = −log[H⁺]; 7 neutral at 25 °C; one pH unit = tenfold change in H⁺
Approximate pH: gastric juice 1.5–2, lemon 2, milk 6.5, blood 7.4; acid rain below 5.6; enamel corrodes below 5.5
Tamarind – tartaric; tomato – oxalic; curd – lactic; vinegar – acetic; ant sting – methanoic (formic)
Phenolphthalein: colourless in acid, pink in base; methyl orange: red in acid, yellow in base
Baking soda NaHCO₃; washing soda Na₂CO₃·10H₂O; bleaching powder CaOCl₂
Plaster of Paris CaSO₄·½H₂O; gypsum CaSO₄·2H₂O
Chlor-alkali process (brine electrolysis) → NaOH + Cl₂ + H₂
Antacid: milk of magnesia Mg(OH)₂; acidic soil corrected with lime, sodic soil with gypsum
Aqua regia = conc. HCl : conc. HNO₃ = 3 : 1, dissolves gold

Worked example

A Konkan paddy field shows pH 5 and a Baramati sugarcane field shows pH 9. (a) How many times greater is the hydrogen-ion concentration in the paddy soil? (b) What should each farmer add?

Show solution

pH is −log[H⁺], so each unit is a factor of 10.
Difference = 9 − 5 = 4 units, so the ratio = 10⁴ = 10,000.
The paddy soil is acidic: add a base such as lime (CaO or Ca(OH)₂) to neutralise the excess H⁺.
The sugarcane soil is sodic (alkaline): apply gypsum (CaSO₄·2H₂O), whose calcium replaces the sodium, then leach the salts out with good drainage.

Answer: (a) 10,000 times; (b) lime for the acidic paddy soil, gypsum for the sodic sugarcane soil.

Exam shortcut

To compare two pH values, subtract and raise 10 to that power: difference of 2 → 100 times, 3 → 1000 times. Never compare pH values by simple division.

Common traps

pH 3 is not “twice as acidic” as pH 6; it is 10³ = 1000 times more acidic, because the scale is logarithmic.
Tomato contains oxalic acid and tamarind tartaric acid; examiners swap these with citric.
Plaster of Paris (½ H₂O) and gypsum (2 H₂O) differ only in water of crystallisation; PoP turns into gypsum on setting.
Baking soda (NaHCO₃) and washing soda (Na₂CO₃·10H₂O) are different salts with different uses.
Phenolphthalein stays colourless in acid; it does not turn red.

Practice · tap an option to check

Q1. Which of the following pairs of natural source and the acid it contains is correctly matched?

Q2. Solution P has pH 3 and solution Q has pH 6. Which of the following is correct?

Q3. Consider the following statements:
1. Phenolphthalein turns pink in a basic solution.
2. Acidic soils are treated with quicklime or slaked lime.
Which of the statements given above is/are correct?

Q4. The chemical formula of Plaster of Paris is:

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Science · Biology and health · Topic 5 of 10

Cell, DNA and Heredity (Mendel, sex determination, genetic disorders)

Mark as done

Think of it like…

A cooperative sugar factory and its master register

Walk through a cooperative sugar factory near Kolhapur during the crushing season. At the gate, security checks who and what comes in: that is the cell membrane. The head office keeps the master register with every instruction for running the factory; nobody takes the register out, they only send photocopies of the needed page to the shop floor. That register is DNA in the nucleus, and the photocopy is messenger RNA. On the floor, workers read the photocopy and build things: those are the ribosomes making proteins. The boiler house burns bagasse to make steam and power for everything: the mitochondria burning food to make ATP. The packing and dispatch section is the Golgi apparatus, and the cleaning crew that breaks down waste is the lysosomes.

Now think of a family that runs such a factory. When two families unite in marriage, the new household gets one copy of each register page from each side. If one side’s page says “tall” and the other “dwarf”, the stronger (dominant) instruction is followed, but the weaker page is still kept in the cupboard and can be passed on. This is Mendel’s idea: traits are carried in pairs of factors (genes), one from each parent, and they separate again when the next generation is formed.

In everyday life	In the exam topic
Factory gate with security	Cell membrane: selectively permeable
Master register kept in the head office	DNA in the nucleus (chromosomes)
Photocopy sent to the shop floor; workers build from it	mRNA read by ribosomes to make proteins
Boiler house making steam and power	Mitochondria making ATP (powerhouse, has its own DNA)
One copy of each register page from each family	One allele from each parent; dominant allele expressed, recessive carried

The concept

The cell. Robert Hooke saw cells in cork (1665); Leeuwenhoek first saw living cells. Schleiden and Schwann (1838–39) proposed the cell theory, and Virchow added that all cells arise from pre-existing cells (1855). Prokaryotic cells (bacteria) have no membrane-bound nucleus or organelles; eukaryotic cells do. Plant cells have a cellulose cell wall, chloroplasts and a large central vacuole; animal cells have none of these but have centrioles. Mitochondria (the powerhouse) and chloroplasts have their own DNA and ribosomes. Ribosomes (without a membrane) make proteins, the endoplasmic reticulum forms the transport network, the Golgi apparatus packages and dispatches, and lysosomes (the “suicide bags”) contain digestive enzymes.

DNA and chromosomes. Watson and Crick (1953), using Rosalind Franklin’s X-ray data, proposed the double helix: two sugar-phosphate strands with bases paired A–T (two hydrogen bonds) and G–C (three). RNA has uracil in place of thymine. A gene is a segment of DNA that codes for a protein. Humans have 46 chromosomes (23 pairs): 22 pairs of autosomes and one pair of sex chromosomes (XX in females, XY in males). Mitosis produces two identical cells for growth and repair; meiosis halves the chromosome number to make gametes (sperm and egg have 23 each), so the zygote restores 46.

Mendel studied seven contrasting traits in the garden pea (Pisum sativum) and presented his work in 1865; it was rediscovered in 1900 by de Vries, Correns and Tschermak. His laws: dominance (in a hybrid only the dominant trait shows), segregation (the two factors separate into different gametes) and independent assortment (different traits are inherited independently). A monohybrid cross Tt × Tt gives phenotype ratio 3 tall : 1 dwarf and genotype ratio 1 TT : 2 Tt : 1 tt. A dihybrid cross gives 9 : 3 : 3 : 1.

Sex determination and disorders. The mother always gives an X; the father’s sperm carries either X or Y, so the father’s sperm decides the sex of the child, with a 50 : 50 chance each time. Chromosomal disorders: Down syndrome (trisomy 21), Turner syndrome (45, XO, female), Klinefelter syndrome (47, XXY, male). Haemophilia and red-green colour blindness are X-linked recessive, so they are far more common in males. Sickle-cell anaemia and thalassaemia are autosomal recessive; sickle-cell comes from a single base change that replaces glutamic acid by valine in the β-globin chain. Sickle-cell disease is notably common in tribal belts of Maharashtra (such as Gadchiroli, Chandrapur and Nandurbar); the National Sickle Cell Anaemia Elimination Mission was launched on 1 July 2023 with the aim of eliminating it as a public health problem by 2047.

Key points & formulas

Hooke 1665 (cork cells); cell theory Schleiden and Schwann 1838–39; Virchow 1855 (cells from cells)
Prokaryote: no nuclear membrane (bacteria); plant cell: cell wall, chloroplast, large vacuole
Mitochondria: ATP, own DNA; ribosome: protein synthesis; lysosome: suicide bag; Golgi: packaging
DNA double helix: Watson and Crick, 1953; A–T, G–C; RNA has uracil instead of thymine
Humans: 46 chromosomes = 22 pairs autosomes + XX or XY; gametes have 23
Mendel: garden pea, 7 traits; laws of dominance, segregation, independent assortment
Monohybrid F₂: 3 : 1 phenotype, 1 : 2 : 1 genotype; dihybrid F₂: 9 : 3 : 3 : 1
Father’s sperm (X or Y) decides the child’s sex
Down = trisomy 21; Turner = XO; Klinefelter = XXY; haemophilia and colour blindness X-linked recessive
Sickle-cell anaemia: autosomal recessive; Elimination Mission launched 1 July 2023, target 2047

Worked example

Mendel crossed pure round-yellow (RRYY) pea plants with pure wrinkled-green (rryy) plants. The F₁ plants were self-pollinated and gave 1600 F₂ seeds. How many seeds of each type are expected?

Show solution

F₁ is RrYy, all round and yellow, since round and yellow are dominant.
Selfing RrYy gives the dihybrid ratio 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green (16 parts).
One part = 1600 ÷ 16 = 100.
Round-yellow = 9 × 100 = 900; round-green = 300; wrinkled-yellow = 300; wrinkled-green = 100.
Two new combinations (round-green and wrinkled-yellow) appear, which proves that the two traits assort independently.

Answer: 900 round-yellow, 300 round-green, 300 wrinkled-yellow, 100 wrinkled-green.

Exam shortcut

Ratios to memorise as numbers: monohybrid 3 : 1 (out of 4), dihybrid 9 : 3 : 3 : 1 (out of 16). Divide the total by 4 or 16 first, then multiply.

Common traps

The father, not the mother, determines the sex of the child; blaming the mother for daughters has no scientific basis.
Turner syndrome (XO) affects females; Klinefelter syndrome (XXY) affects males.
Mitochondria and chloroplasts have their own DNA; ribosomes have no membrane and are found in prokaryotes too.
In a Tt × Tt cross, 3/4 show the dominant trait but only 1/4 are pure dominant (TT); the 3 : 1 is phenotype, 1 : 2 : 1 is genotype.
Sickle-cell anaemia is autosomal recessive, not X-linked like haemophilia.

Practice · tap an option to check

Q1. In humans, the sex of the child is determined by:

Q2. A heterozygous tall pea plant (Tt) is crossed with another heterozygous tall plant (Tt). Out of 400 offspring, how many are expected to be dwarf?

Q3. Match List I (disorder) with List II (cause):
a. Down syndrome b. Turner syndrome c. Klinefelter syndrome d. Sickle-cell anaemia
1. XXY sex chromosomes 2. Trisomy of chromosome 21 3. Point mutation in the β-globin gene 4. XO sex chromosomes

Q4. Which of the following cell organelles is correctly matched with its function?

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Science · Biology and health · Topic 6 of 10

Human Body Systems: Digestion, Circulation, Respiration, Nervous and Endocrine

Mark as done

Think of it like…

Mumbai’s dabbawalas, local trains and control room

Every morning, home kitchens in Mumbai cook food and break it into portions; that is digestion, turning big food into pieces small enough to carry. The dabbawalas pick up the tiffins and carry them along fixed routes on the local trains to every office; that is blood carrying oxygen and nutrients through the arteries. In the afternoon they bring back the empty boxes; that is blood carrying carbon dioxide and waste back through the veins. Churchgate and CST are the busy hubs where everything is sorted and pushed out again, like the heart, and the separate loop to the fresh-air yard where boxes are aired is the lung circuit.

When a train breaks down, the control room phones the station master and the response is instant and goes to one place only: that is the nervous system. When the railways change the monsoon timetable, they send a printed circular to every station; it is slow to arrive but reaches everyone and lasts for weeks: that is a hormone from an endocrine gland, poured into the blood and acting on many organs over a long time.

In everyday life	In the exam topic
Home kitchens breaking food into tiffin portions	Digestion: enzymes break food into absorbable molecules
Dabbawalas delivering full tiffins	Arteries and RBCs carrying oxygen and nutrients to tissues
Collecting empty boxes for the return trip	Veins carrying CO₂ and wastes back
Sorting hub that pushes boxes out again	Heart pumping blood (double circulation)
Control room phone call: instant, one target	Nervous system: fast electrical signals
Printed circular to every station: slow, wide, lasting	Hormones in the blood: slow, widespread, long-lasting

The concept

Digestion. In the mouth, salivary amylase (ptyalin) starts breaking starch into maltose. In the stomach, HCl kills germs and provides the acidic medium in which pepsin digests proteins; mucus protects the stomach lining. The liver (the largest gland) makes bile, stored in the gall bladder; bile contains no enzyme but emulsifies fats into tiny droplets and makes the medium alkaline. Pancreatic juice brings trypsin (proteins), lipase (fats) and amylase (starch). Digestion is completed in the small intestine, whose finger-like villi absorb the products into the blood. The large intestine mainly absorbs water.

Circulation. The human heart has four chambers. The right side receives deoxygenated blood from the body (through the vena cava) and sends it to the lungs; the left side receives oxygenated blood from the lungs and pumps it to the body through the aorta. Blood passes through the heart twice per round: double circulation. The exceptions asked in every exam: the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood. The heartbeat starts at the SA node (natural pacemaker). Normal blood pressure is about 120/80 mm Hg. RBCs carry haemoglobin and live about 120 days (old ones are destroyed in the spleen); WBCs fight infection; platelets help clotting. Landsteiner discovered the ABO groups: O is the universal donor and AB the universal recipient (for red cells).

Respiration. Air reaches the alveoli of the lungs, where oxygen diffuses into blood and CO₂ comes out; the diaphragm and rib muscles drive breathing. Cellular respiration takes place in the mitochondria: glucose + oxygen → CO₂ + water + energy (ATP). When oxygen is short, as in sprinting, muscles respire anaerobically and form lactic acid, which causes cramps. Yeast respires anaerobically to give ethanol and CO₂ (fermentation).

Control and coordination. The neuron is the unit of the nervous system. The cerebrum handles thinking, memory and voluntary actions; the cerebellum maintains balance and posture; the medulla oblongata controls involuntary actions such as heartbeat, breathing and blood pressure; the hypothalamus links the nervous and endocrine systems. A reflex (pulling your hand from a hot tawa) is routed through the spinal cord without waiting for the brain. Endocrine glands release hormones into the blood: the pituitary (the “master gland”) secretes growth hormone (excess: gigantism, deficiency: dwarfism); the thyroid makes thyroxine, which needs iodine (deficiency: goitre); the pancreas’s β-cells release insulin, which lowers blood sugar (deficiency: diabetes), and α-cells release glucagon, which raises it; the adrenal gland releases adrenaline for “fight or flight”.

Key points & formulas

Starch digestion begins in the mouth (salivary amylase); protein digestion begins in the stomach (pepsin)
Bile (from liver, stored in gall bladder) emulsifies fats; it contains no enzymes
Villi of the small intestine absorb digested food; large intestine absorbs water
Pulmonary artery: deoxygenated blood; pulmonary vein: oxygenated blood
SA node = pacemaker; normal BP about 120/80 mm Hg; RBC lifespan about 120 days
O = universal donor, AB = universal recipient (ABO system, Landsteiner)
Anaerobic respiration in muscles → lactic acid; in yeast → ethanol + CO₂
Cerebrum: thinking; cerebellum: balance; medulla: heartbeat and breathing
Pituitary = master gland; thyroxine needs iodine; insulin (β-cells) lowers blood sugar
Adrenaline: emergency (fight or flight) hormone from the adrenal gland

Worked example

Trace the path of a red blood cell that has just picked up oxygen in the lungs until it delivers oxygen to a leg muscle and returns to the lungs. Name the vessels and chambers in order.

Show solution

Lungs → pulmonary vein (a vein, yet carrying oxygenated blood) → left atrium.
Left atrium → left ventricle, which has the thickest wall because it must pump blood to the whole body.
Left ventricle → aorta → arteries → capillaries in the leg muscle, where oxygen is released and CO₂ picked up.
Capillaries → veins → vena cava → right atrium → right ventricle.
Right ventricle → pulmonary artery (an artery, yet carrying deoxygenated blood) → lungs. The cell passed through the heart twice: double circulation.

Answer: Lungs → pulmonary vein → left atrium → left ventricle → aorta → leg capillaries → vena cava → right atrium → right ventricle → pulmonary artery → lungs.

Exam shortcut

Arteries go Away from the heart, veins return to it; the oxygen content does not decide the name. That single rule resolves the pulmonary artery/vein trap.

Common traps

Bile has no enzyme; it only emulsifies fat. Lipase comes from the pancreas.
Pulmonary artery carries deoxygenated blood and pulmonary vein carries oxygenated blood.
Balance and posture are the cerebellum’s job; the cerebrum handles thinking and voluntary actions.
Insulin lowers blood sugar; glucagon raises it.
Pepsin works in the acidic stomach; trypsin works in the alkaline small intestine.

Practice · tap an option to check

Q1. Which blood vessel carries deoxygenated blood from the heart to the lungs?

Q2. Consider the following statements:
1. Bile contains the enzyme lipase, which digests fats.
2. Pepsin acts in the alkaline medium of the small intestine.
Which of the statements given above is/are correct?

Q3. A person who has had a head injury walks unsteadily and cannot keep his balance, though his memory and speech are normal. Which part of the brain is most likely affected?

Q4. Match List I (gland) with List II (hormone):
a. Pancreas (β-cells) b. Thyroid c. Adrenal d. Pituitary
1. Thyroxine 2. Growth hormone 3. Insulin 4. Adrenaline

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Science · Biology and health · Topic 7 of 10

Nutrition, Vitamins, Diseases and Immunity (vaccines, pathogens)

Mark as done

Think of it like…

Defending a Sahyadri hill fort

Think of a hill fort like Raigad or Sinhagad. Its stone walls, steep cliffs and gates keep most raiders out without any fighting; that is the skin, mucus, tears and stomach acid. If a raider slips in, the general guards on duty attack anyone who does not belong, without asking who he is: that is innate immunity, the white blood cells that swallow germs and cause inflammation and fever.

The fort also has an elite unit that learns the exact face of each enemy. The first time a new enemy attacks, the unit takes days to study him and respond, and the fort suffers. But afterwards the commander keeps portraits of that enemy, and the next time the unit strikes within hours. That is acquired immunity: B-cells make antibodies that fit one particular germ, and memory cells remember it. A wise commander does not wait for a real attack. He stages a mock drill with a straw dummy dressed as the enemy, so the unit learns the face without any risk; that is a vaccine. And sometimes an allied fort sends ready-trained soldiers for a short time (the soldiers return home after the danger passes): that is passive immunity, like antibodies a baby gets through its mother’s milk or an anti-venom injection. A well-fed garrison fights better, which is why nutrition is part of the same story.

In everyday life	In the exam topic
Stone walls, cliffs and gates	First line of defence: skin, mucus, tears, stomach acid
Guards attacking anyone who does not belong	Innate immunity: phagocytic WBCs, fever, inflammation
Elite unit that learns each enemy’s face and keeps portraits	Acquired immunity: B-cells (antibodies), T-cells, memory cells
Mock drill with a straw dummy enemy	Vaccine: weakened, killed or partial germ trains active immunity
Allied soldiers lent for a short while	Passive immunity: ready-made antibodies (colostrum, anti-venom), short-lived
A well-fed garrison	Balanced diet: proteins, vitamins and minerals keep immunity strong

The concept

Nutrition. A balanced diet supplies carbohydrates and fats (energy), proteins (body building), vitamins and minerals (protection), water and roughage. Vitamins A, D, E and K are fat-soluble (stored in the body); B-complex and C are water-soluble. Deficiency diseases: A – night blindness and xerophthalmia; B1 (thiamine) – beriberi; B3 (niacin) – pellagra; B12 (cobalamin, contains cobalt) – pernicious anaemia; C (ascorbic acid) – scurvy (bleeding gums); D – rickets in children, osteomalacia in adults (made in skin by sunlight); K – failure of blood clotting (made by gut bacteria). Minerals: iron – anaemia; iodine – goitre; calcium – weak bones and teeth. Protein-energy malnutrition shows as kwashiorkor (protein deficiency, swollen belly) or marasmus (deficiency of both protein and calories, wasting).

Pathogens and diseases. Bacterial: tuberculosis (Mycobacterium tuberculosis, BCG vaccine), cholera (Vibrio cholerae), typhoid (Salmonella typhi, Widal test), tetanus, diphtheria, whooping cough, leprosy, plague. Viral: polio, measles, chickenpox, hepatitis, rabies, influenza, COVID-19, dengue, chikungunya and AIDS (HIV, which destroys helper T-cells; screened by ELISA). Protozoan: malaria (Plasmodium, carried by the female Anopheles mosquito), amoebic dysentery, kala-azar (Leishmania, sandfly). Fungal: ringworm, athlete’s foot. Worms: filariasis (Wuchereria, spread by Culex), roundworm. Vectors are a favourite: Aedes (day-biting) spreads dengue, chikungunya and Zika; female Anopheles spreads malaria; Culex spreads filariasis and Japanese encephalitis.

Immunity. Innate immunity is present from birth and non-specific. Acquired immunity is specific and has memory: B-lymphocytes produce antibodies (humoral immunity) and T-lymphocytes kill infected cells (cell-mediated). Active immunity is made by your own body after infection or vaccination and lasts long; passive immunity uses ready-made antibodies (from the mother across the placenta and in colostrum, or anti-venom and anti-tetanus serum) and is quick but short-lived. Antibiotics such as penicillin (Alexander Fleming, 1928) kill bacteria but are useless against viruses, so taking them for a common cold only breeds resistance.

Vaccines and milestones. Edward Jenner developed the smallpox vaccine (1796) and Louis Pasteur the rabies vaccine. Jonas Salk made the injectable polio vaccine (IPV) and Albert Sabin the oral vaccine (OPV). The WHO declared smallpox eradicated in 1980. India’s last wild polio case was in January 2011, and India (with the WHO South-East Asia Region) was certified polio-free on 27 March 2014. India runs the Universal Immunisation Programme (1985) and Mission Indradhanush (launched December 2014) to reach unvaccinated children.

Key points & formulas

Fat-soluble: A, D, E, K; water-soluble: B-complex, C
A – night blindness; B1 – beriberi; B3 – pellagra; B12 – pernicious anaemia; C – scurvy; D – rickets; K – clotting failure
Iron – anaemia; iodine – goitre; kwashiorkor = protein deficiency; marasmus = protein + calorie deficiency
Bacterial: TB, cholera, typhoid, tetanus, leprosy; viral: polio, measles, dengue, rabies, AIDS; protozoan: malaria, kala-azar
Aedes: dengue, chikungunya (day-biting); female Anopheles: malaria; Culex: filariasis, Japanese encephalitis
B-cells: antibodies; T-cells: cell-mediated immunity; HIV attacks helper T-cells
Active immunity: infection or vaccine (long-lasting); passive: ready-made antibodies (short-lived)
Antibiotics act on bacteria, not viruses; penicillin – Alexander Fleming, 1928
Jenner – smallpox vaccine 1796; smallpox declared eradicated 1980; India certified polio-free 27 March 2014
Universal Immunisation Programme 1985; Mission Indradhanush December 2014

Worked example

Consider the following statements:
1. A newborn fed colostrum acquires active immunity against several diseases.
2. A person given anti-rabies vaccine after a dog bite develops active immunity.
3. Antibiotics shorten the duration of viral fever.
Which of the statements given above is/are correct?

Show solution

Statement 1: colostrum (first milk) carries ready-made antibodies (mainly IgA) from the mother. The baby’s body did not make them, so this is passive immunity. Wrong.
Statement 2: a rabies vaccine contains inactivated virus, so the person’s own immune system produces antibodies and memory cells. That is active immunity. Correct.
Statement 3: antibiotics act on bacterial structures such as the cell wall; viruses have none, so antibiotics do not cure viral fever. Wrong.
Only statement 2 survives.

Answer: 2 only.

Exam shortcut

Ask “who made the antibodies?” If your own body made them (after infection or a vaccine), it is active. If they were handed over ready-made (mother, serum, anti-venom), it is passive.

Common traps

Vitamin B1 deficiency causes beriberi; pellagra is B3 (niacin). Examiners swap them.
Colostrum gives passive, not active, immunity.
Dengue is spread by the day-biting Aedes mosquito, not Anopheles (malaria) or Culex (filariasis).
Antibiotics do not work against viruses (common cold, flu, dengue, COVID-19).
Kwashiorkor is mainly protein deficiency; marasmus is deficiency of both protein and calories.

Practice · tap an option to check

Q1. Which of the following pairs of vitamin and deficiency disease is NOT correctly matched?

Q2. Dengue fever, which rises in Maharashtra after the monsoon, is transmitted by:

Q3. Consider the following statements:
1. Antibiotics are effective against viral infections such as the common cold.
2. Antibodies passed from mother to baby through colostrum provide passive immunity.
Which of the statements given above is/are correct?

Q4. Arrange the following events in chronological order:
1. India certified polio-free
2. Edward Jenner’s smallpox vaccine
3. WHO declares smallpox eradicated
4. Alexander Fleming discovers penicillin

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Science · Environment · Topic 8 of 10

Ecology: Food Chains, Energy Flow (10% law), Biogeochemical Cycles

Mark as done

Think of it like…

A cooperative sugar factory’s crushing season

Follow sugarcane into a cooperative factory at crushing time. Tonnes of cane go in, but only roughly a tenth of that weight comes out as sugar. The rest is bagasse burnt in the boiler, molasses, and water. Most of what entered was used up just running the process, and the heat from the boiler leaves through the chimney and never comes back. If a sweet-maker then turns the sugar into pedhe, he too loses part of it on the way. Each step keeps only a small share of what it received.

Yet not everything is lost. The press mud and boiler ash are carted back to the members’ fields as manure, and their nutrients grow next season’s cane. So there are two different flows in the same factory: energy (the fuel’s heat) goes one way and escapes, while matter (nutrients in the press mud) goes round and round.

An ecosystem is the same. Green plants (producers) capture sunlight. A grasshopper that eats grass gets only about 10% of the energy the grass stored, because the grass used most of it for its own living and much was never eaten; the frog that eats the grasshopper gets about 10% of that, and so on. Energy flows in one direction and is lost as heat at every step, which is why food chains rarely go beyond four or five links. Nutrients like carbon, nitrogen and phosphorus, however, are returned to the soil and air by decomposers and used again: the biogeochemical cycles.

In everyday life	In the exam topic
Cane entering the factory	Energy fixed by producers (green plants)
Only about a tenth comes out as sugar at each stage	Lindeman’s 10% law of energy transfer between trophic levels
Boiler heat escaping up the chimney	Energy lost as heat in respiration; energy flow is unidirectional
Press mud carted back to the fields as manure	Nutrients recycled by decomposers: biogeochemical cycles
Each extra processing step leaves less sugar	Fewer organisms and less energy at higher trophic levels; chains stay short

The concept

An ecosystem is a community of living organisms interacting with its non-living environment. Producers (green plants, phytoplankton) make food by photosynthesis; primary consumers (herbivores) eat them; secondary and tertiary consumers (carnivores) follow; decomposers (bacteria and fungi) break down dead matter and return nutrients to the soil. Each step is a trophic level. A food chain beginning with living plants is a grazing food chain; one beginning with dead organic matter is a detritus food chain. In nature chains interlink into a food web, which makes the ecosystem more stable.

Energy flow. Plants capture only a small part of the sunlight reaching them (NCERT: only 2–10% of the photosynthetically active radiation). At each trophic level most of the energy is used in respiration and lost as heat, and only about 10% passes to the next level. This is Lindeman’s 10% law (Raymond Lindeman, 1942). Energy flow is therefore unidirectional: from the Sun to producers to consumers, never back. That is why the pyramid of energy is always upright. Pyramids of numbers and biomass can be inverted: a single large tree supporting many insects gives an inverted pyramid of numbers, and in the sea a small biomass of fast-reproducing phytoplankton supports a larger biomass of zooplankton and fish, giving an inverted pyramid of biomass.

Biogeochemical cycles move matter round and round. In the carbon cycle, photosynthesis takes CO₂ out of the air, and respiration, decomposition, burning of fossil fuels and forest fires put it back; the oceans are a huge carbon sink. In the nitrogen cycle, atmospheric N₂ (about 78% of air) is fixed by Rhizobium in the root nodules of legumes (gram, tur, soybean), by free-living Azotobacter and by cyanobacteria such as Anabaena and Nostoc (useful in paddy fields), and by lightning. Nitrosomonas converts ammonia to nitrite and Nitrobacter converts nitrite to nitrate (nitrification); Pseudomonas returns nitrogen to the air (denitrification). The phosphorus cycle is a sedimentary cycle with no significant gaseous phase: phosphorus comes from weathering of rocks.

Other terms asked: a habitat is where an organism lives and its niche is its functional role; an ecotone is a transition zone between two ecosystems (such as a mangrove between land and sea) and often has more species (edge effect). Ecological succession is the gradual change of a community over time; on bare rock, lichens are the pioneers, ending in a stable climax community. Toxic substances such as DDT and mercury, which are not broken down, increase in concentration at each higher trophic level: biomagnification.

Key points & formulas

Trophic levels: producers → primary consumers → secondary → tertiary; decomposers recycle
10% law: Raymond Lindeman, 1942; only about 10% of energy passes to the next level
Energy flow is unidirectional; nutrients cycle
Pyramid of energy always upright; number (tree) and biomass (ocean) can be inverted
Plants capture only 2–10% of photosynthetically active radiation (NCERT)
N₂ fixation: Rhizobium (legume nodules), Azotobacter (free-living), Anabaena and Nostoc (cyanobacteria)
Nitrification: Nitrosomonas (NH₃ → NO₂⁻), Nitrobacter (NO₂⁻ → NO₃⁻); denitrification: Pseudomonas
Phosphorus cycle: sedimentary, no significant atmospheric phase
Pioneer species on bare rock: lichens; ecotone shows edge effect
Biomagnification: concentration of DDT or mercury rises up the food chain

Worked example

In a grassland near Solapur, grass fixes 10,000 J of energy. Using the 10% law, find the energy available to (a) grasshoppers, (b) frogs that eat grasshoppers, (c) snakes that eat frogs. If a hawk eating the snakes needs 5 J, how much energy must the grass fix?

Show solution

Grasshoppers (primary consumers): 10% of 10,000 = 1,000 J.
Frogs (secondary consumers): 10% of 1,000 = 100 J.
Snakes (tertiary consumers): 10% of 100 = 10 J.
The hawk is four transfers above the grass: grass → grasshopper → frog → snake → hawk.
Work backwards, multiplying by 10 at each step: snake = 5 × 10 = 50 J; frog = 500 J; grasshopper = 5,000 J; grass = 50,000 J (that is, 5 ÷ 0.1⁴).

Answer: (a) 1,000 J, (b) 100 J, (c) 10 J. For the hawk to get 5 J, the grass must fix 50,000 J.

Exam shortcut

Count the arrows, then remove one zero per arrow. 10,000 J at the producer with three arrows to the snake leaves 10 J. Going backwards, add one zero per arrow.

Common traps

The pyramid of energy can never be inverted; pyramids of number and biomass can.
The 10% law was given by Lindeman, not by Odum or Darwin.
Nitrosomonas makes nitrite and Nitrobacter makes nitrate; Rhizobium fixes nitrogen and Pseudomonas denitrifies.
The phosphorus cycle has no major gaseous phase; carbon and nitrogen cycles do.
Energy is not recycled; only matter is. Decomposers return nutrients, not energy, to producers.

Practice · tap an option to check

Q1. In the food chain grass → grasshopper → frog → snake, the grass fixes 20,000 J of energy. According to the 10% law, the energy available to the snake is:

Q2. Which ecological pyramid is always upright?

Q3. Match List I (organism) with List II (role in the nitrogen cycle):
a. Rhizobium b. Nitrosomonas c. Nitrobacter d. Pseudomonas
1. Denitrification 2. Nitrite to nitrate 3. Ammonia to nitrite 4. Symbiotic nitrogen fixation in legume nodules

Q4. Consider the following statements:
1. In an ecosystem energy flows in one direction, while nutrients are recycled.
2. The 10% law of energy transfer was proposed by Charles Darwin.
Which of the statements given above is/are correct?

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Science · Environment · Topic 9 of 10

Biodiversity and Conservation (hotspots, IUCN categories, Maharashtra’s national parks and tiger reserves)

Mark as done

Think of it like…

A tribal community seed bank in the Sahyadri hills

In the hill villages of the Sahyadri, some farming families keep dozens of traditional varieties of rice, nachni, varai and beans, each suited to a different slope, soil or rainfall. When hybrids arrived, most families switched, and many old varieties quietly disappeared. A few women started a community seed bank. They keep the varieties alive in two ways: by persuading farmers to keep growing them in their own fields, where they keep adapting to local conditions, and by storing seeds in clay pots in the bank as a back-up in case a variety fails everywhere.

They also keep a register that grades every variety: “still common”, “getting rare”, “only two families grow it”, “only in our pots now”, “gone”. They notice that their own valley has an unusually large number of varieties found nowhere else, and that most of the old fields have already been converted, so the valley is both very rich and very threatened.

That is biodiversity conservation. The varieties, the crops and the different field types are genetic, species and ecosystem diversity. Growing them in the field is in-situ conservation (national parks, tiger reserves, sacred groves); storing seeds is ex-situ (seed banks, zoos, botanical gardens). The register is the IUCN Red List, and the rich but threatened valley is a biodiversity hotspot.

In everyday life	In the exam topic
Many varieties of one crop, many crops, many field types	Genetic, species and ecosystem diversity
Farmers keep growing old varieties in their own fields	In-situ conservation: national parks, sanctuaries, tiger reserves, sacred groves
Seeds stored in clay pots in the bank	Ex-situ conservation: seed banks, zoos, botanical gardens
Register grading each variety from common to gone	IUCN Red List categories: LC, NT, VU, EN, CR, EW, EX
A valley very rich in unique varieties but mostly already converted	Biodiversity hotspot: high endemism and heavy habitat loss

The concept

Biodiversity exists at three levels: genetic (variation within a species, like many rice varieties), species, and ecosystem. India is one of the world’s megadiverse countries. A biodiversity hotspot (concept by Norman Myers, criteria now used by Conservation International) must have at least 1,500 endemic vascular plant species and must have lost at least 70% of its original habitat. There are 36 hotspots worldwide; India has parts of four: the Himalaya, Indo-Burma, the Western Ghats and Sri Lanka, and Sundaland (Nicobar Islands). Maharashtra’s Sahyadri ranges lie in the Western Ghats hotspot.

IUCN Red List categories, from safest to worst: Least Concern (LC), Near Threatened (NT), Vulnerable (VU), Endangered (EN), Critically Endangered (CR), Extinct in the Wild (EW) and Extinct (EX), plus Data Deficient (DD) and Not Evaluated (NE). Only VU, EN and CR together count as “threatened”. Maharashtra examples: the Great Indian Bustard is Critically Endangered (Great Indian Bustard Sanctuary in Solapur district); the Forest Owlet, rediscovered in 1997 and found in Melghat, is Endangered. Maharashtra’s state animal is the Indian giant squirrel (shekru), the state bird the yellow-footed green pigeon (harial), and in 2015 it became the first state to name a state butterfly, the Blue Mormon.

Conservation. In-situ conservation protects species in their natural habitat: national parks, wildlife sanctuaries, tiger reserves, biosphere reserves and community-protected sacred groves (devrai in the Sahyadri). Ex-situ conservation keeps them outside: zoos, botanical gardens, seed and gene banks, and cryopreservation. Laws and programmes: Wildlife (Protection) Act, 1972; Project Tiger, launched on 1 April 1973 with nine reserves; the National Tiger Conservation Authority (statutory, set up under a 2006 amendment of the Wildlife Act); the Biological Diversity Act, 2002, which set up the National Biodiversity Authority (Chennai), State Biodiversity Boards and local Biodiversity Management Committees that maintain People’s Biodiversity Registers. Internationally, the Convention on Biological Diversity came out of the 1992 Rio Earth Summit, and CITES regulates trade in endangered species.

Maharashtra’s tiger reserves (verified against the NTCA list, November 2026): six. Melghat (mainly Amravati district; among the first nine reserves of Project Tiger in 1973–74; includes Gugamal National Park), Tadoba-Andhari (Chandrapur; Tadoba, set up in 1955, is Maharashtra’s oldest national park), Pench (Nagpur; continuous with Pench in Madhya Pradesh), Sahyadri (Chandoli National Park plus Koyna Wildlife Sanctuary, in the Western Ghats across Satara, Sangli, Kolhapur and Ratnagiri districts), Nawegaon-Nagzira (Gondia and Bhandara) and Bor (Wardha, with a part in Nagpur; notified in July 2014, the newest, with the smallest core area). Five of the six lie in Vidarbha; Sahyadri is the only one in western Maharashtra. Other protected areas asked: Sanjay Gandhi National Park in Mumbai (with the Kanheri caves), Bhimashankar Wildlife Sanctuary (the shekru’s home) and Nandur Madhameshwar in Nashik, Maharashtra’s first Ramsar wetland, followed by Lonar crater lake in Buldhana and Thane Creek.

Key points & formulas

Three levels of biodiversity: genetic, species, ecosystem
Hotspot criteria: at least 1,500 endemic vascular plants and at least 70% habitat lost; 36 worldwide; India has parts of 4
India’s hotspots: Himalaya, Indo-Burma, Western Ghats and Sri Lanka, Sundaland (Nicobar)
IUCN threatened categories: Vulnerable, Endangered, Critically Endangered; Near Threatened is not “threatened”
Great Indian Bustard: Critically Endangered; Forest Owlet (Melghat): Endangered
In-situ: national parks, sanctuaries, tiger reserves, sacred groves; ex-situ: zoos, botanical gardens, seed banks
Wildlife (Protection) Act 1972; Project Tiger 1 April 1973; Biological Diversity Act 2002
Maharashtra has 6 tiger reserves: Melghat, Tadoba-Andhari, Pench, Sahyadri, Nawegaon-Nagzira, Bor
Melghat – Amravati; Tadoba-Andhari – Chandrapur; Pench – Nagpur; Nawegaon-Nagzira – Gondia/Bhandara; Bor – Wardha; Sahyadri – Satara/Sangli/Kolhapur/Ratnagiri
Tadoba (1955): oldest national park in Maharashtra; Bor (2014): newest tiger reserve; Nandur Madhameshwar: first Ramsar site of Maharashtra

Worked example

Consider the following statements about Maharashtra’s tiger reserves:
1. Maharashtra has six tiger reserves.
2. Melghat was among the first nine tiger reserves of Project Tiger.
3. The Sahyadri Tiger Reserve lies in Vidarbha.
Which of the statements given above is/are correct?

Show solution

Statement 1: NTCA lists Melghat, Tadoba-Andhari, Pench, Sahyadri, Nawegaon-Nagzira and Bor for Maharashtra, i.e. six. Correct.
Statement 2: Project Tiger began in 1973 with nine reserves, and Melghat (Amravati) was one of them. Correct.
Statement 3: Sahyadri is formed from Chandoli National Park and Koyna Wildlife Sanctuary in the Western Ghats of western Maharashtra, not Vidarbha. Wrong.
Five of the six reserves are in Vidarbha, which is why statement 3 is a tempting trap.

Answer: 1 and 2 only.

Exam shortcut

For the six tiger reserves remember “My Tiger Plays Safely Near Bor” in order of creation: Melghat, Tadoba, Pench, Sahyadri, Nawegaon-Nagzira, Bor. Only S is in the west; all the rest are in Vidarbha.

Common traps

Maharashtra has six tiger reserves (as of November 2026). Umred-Karhandla and Tipeshwar are wildlife sanctuaries, not tiger reserves.
Near Threatened is not a “threatened” category; only VU, EN and CR are.
Tadoba-Andhari is in Chandrapur and Melghat in Amravati; examiners swap these two.
A zoo or seed bank is ex-situ; a sacred grove or national park is in-situ.
The Western Ghats hotspot is officially “Western Ghats and Sri Lanka”, and India has parts of four hotspots, not two or three.

Practice · tap an option to check

Q1. Which of the following tiger reserve–district pairs is correctly matched?

Q2. Which one of the following IUCN Red List categories is NOT counted among the “threatened” categories?

Q3. Consider the following statements:
1. India has parts of four of the world’s 36 biodiversity hotspots.
2. The Western Ghats form part of the “Western Ghats and Sri Lanka” hotspot.
Which of the statements given above is/are correct?

Q4. Which of the following is an example of ex-situ conservation?

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Science · Environment · Topic 10 of 10

Climate Change, Ozone Depletion and Pollution (greenhouse effect, Kyoto, Paris, Montreal Protocol)

Mark as done

Think of it like…

A polyhouse and its shade net on a Nashik flower farm

A flower grower near Nashik builds a polyhouse for roses. Sunlight passes easily through the plastic film and warms the soil and plants. The warm soil then gives off heat as invisible long-wave (infrared) radiation, which the film does not let out so easily, so the inside stays warmer than the outside, even at night. A thin film keeps the roses comfortable; if he keeps adding layers, the polyhouse overheats and the plants wilt. Above the polyhouse he stretches a shade net that cuts the harshest rays before they even reach the film. If a hailstorm tears the net, the roses get scorched even though the polyhouse temperature has not changed.

The Earth is wrapped in the same two layers, and they are two different problems. The polyhouse film is the blanket of greenhouse gases (water vapour, carbon dioxide, methane, nitrous oxide): without it, the Earth would be frozen, but burning coal, petrol and forests keeps adding layers, so the planet is overheating (global warming). The shade net is the ozone layer in the stratosphere, which absorbs harmful ultraviolet radiation; CFCs from old fridges and sprays tore holes in it. The world fixed the net with the Montreal Protocol; reducing the extra film is the job of the Kyoto Protocol and the Paris Agreement.

In everyday life	In the exam topic
Sunlight passing through the polyhouse film	Short-wave solar radiation passes through the atmosphere
Warm soil’s heat trapped by the film	Greenhouse gases absorb and re-emit long-wave (infrared) radiation
Adding extra layers of film	Rising CO₂, methane, N₂O from fossil fuels, farming, deforestation: global warming
Shade net cutting the harshest rays	Stratospheric ozone absorbing harmful UV radiation
Hailstorm tearing the shade net	CFCs and other chlorine/bromine compounds causing ozone depletion (ozone hole)
Repairing the net vs removing extra film	Montreal Protocol (ozone) vs Kyoto and Paris (climate)

The concept

The greenhouse effect is natural and necessary: greenhouse gases absorb the infrared radiation emitted by the Earth’s surface and re-radiate part of it downward, keeping the average surface temperature at about 15 °C instead of roughly −18 °C. The main greenhouse gases are water vapour (the most abundant), carbon dioxide (the largest contributor to human-caused warming), methane (from paddy fields, livestock, landfills; far more potent per molecule than CO₂ but shorter-lived), nitrous oxide (from fertilisers), and fluorinated gases (HFCs, PFCs, SF₆). The enhanced greenhouse effect from human emissions causes global warming, melting glaciers, sea-level rise and more extreme rain and droughts. The IPCC (set up in 1988 by WMO and UNEP) assesses the science.

Ozone depletion is a separate problem. The ozone layer lies in the stratosphere (roughly 15–35 km up) and absorbs harmful ultraviolet radiation (all UV-C and most UV-B), which causes skin cancer, cataract and crop damage. Chlorofluorocarbons (CFCs) used in refrigerators, ACs and aerosol sprays drift up, and UV light releases chlorine atoms that destroy ozone in a chain reaction; halons (fire extinguishers) supply bromine. The ozone hole appears over Antarctica every southern spring. The Vienna Convention (1985) set the framework and the Montreal Protocol (signed 16 September 1987, which is why 16 September is World Ozone Day) phases out ozone-depleting substances; together with the Vienna Convention it was among the first UN treaties to be ratified by every UN member state. The Kigali Amendment (2016) phases down HFCs, which do not harm ozone but are powerful greenhouse gases. Ozone at ground level, by contrast, is a pollutant formed in smog.

Climate treaties. The UNFCCC was adopted at the 1992 Rio Earth Summit. The Kyoto Protocol (adopted 1997, in force 2005) set legally binding emission cuts only for developed (Annex I) countries, following the principle of common but differentiated responsibilities, and created market mechanisms such as the Clean Development Mechanism and emissions trading (carbon credits). The Paris Agreement (COP21, December 2015, in force November 2016) applies to all countries: it aims to keep warming well below 2 °C above pre-industrial levels and to pursue 1.5 °C, with each country setting its own Nationally Determined Contribution (NDC) and a global stocktake every five years. India announced its target of net-zero emissions by 2070 at COP26 in Glasgow (2021).

Pollution. Air: PM2.5 (particles 2.5 µm or smaller, which reach deep into the lungs), PM10, SO₂, NO₂, CO, ozone; India’s National Air Quality Index (2014) uses eight pollutants and six categories from Good to Severe. SO₂ and NOₓ cause acid rain (pH below 5.6). Water: sewage raises the biochemical oxygen demand (BOD), so higher BOD means more polluted water; fertiliser runoff rich in nitrogen and phosphorus causes eutrophication and algal blooms that suffocate fish; mercury poisoning caused Minamata disease and non-degradable pesticides like DDT biomagnify. Noise is measured in decibels. The 1984 Bhopal gas tragedy (methyl isocyanate) led to the Environment (Protection) Act, 1986; the National Green Tribunal was set up in 2010.

Key points & formulas

Natural greenhouse effect keeps Earth near 15 °C (would be about −18 °C without it)
Major GHGs: water vapour, CO₂, CH₄, N₂O, HFCs, PFCs, SF₆; CO₂ is the largest human-caused contributor
Ozone layer: stratosphere; absorbs UV; destroyed by chlorine from CFCs and bromine from halons
Vienna Convention 1985; Montreal Protocol 1987 (World Ozone Day 16 September); Kigali Amendment 2016 (HFCs)
UNFCCC 1992 (Rio); Kyoto 1997 (in force 2005), binding cuts only for developed countries
Paris Agreement 2015 (COP21): well below 2 °C, pursue 1.5 °C; NDCs from all countries
India: net-zero target 2070, announced at COP26 Glasgow (2021)
Acid rain: pH below 5.6 (SO₂, NOₓ); higher BOD = more polluted water
Eutrophication: excess N and P → algal bloom → oxygen depletion
Bhopal 1984 (methyl isocyanate) → Environment (Protection) Act 1986; NGT 2010

Worked example

Consider the following statements:
1. CFCs contribute both to ozone depletion and to global warming.
2. HFCs, which replaced CFCs, are controlled under the Kyoto Protocol and not under the Montreal Protocol.
3. The ozone hole is the main cause of global warming.
Which of the statements given above is/are correct?

Show solution

Statement 1: CFCs release chlorine that destroys stratospheric ozone, and they also absorb infrared radiation, so they are greenhouse gases too. Correct.
Statement 2: HFCs were among the six gases in the Kyoto basket, but the 2016 Kigali Amendment brought their phase-down under the Montreal Protocol as well. So “not under the Montreal Protocol” is wrong.
Statement 3: The ozone hole lets in more UV radiation; it is not the main driver of warming, which comes from the build-up of greenhouse gases, chiefly CO₂. Wrong.
Only statement 1 survives.

Answer: 1 only.

Exam shortcut

Pair each treaty with one word: Vienna and Montreal = ozone; Kigali = HFCs; Kyoto = binding cuts for rich countries; Paris = NDCs for everyone, 1.5–2 °C.

Common traps

Ozone depletion and global warming are different problems; the ozone hole does not cause global warming.
The Montreal Protocol deals with ozone-depleting substances; the Kyoto Protocol deals with greenhouse gases.
Kyoto set binding targets only for developed countries; India had no binding target under it.
Ground-level ozone is a pollutant; stratospheric ozone is protective.
Higher BOD means more polluted water, not cleaner water.

Practice · tap an option to check

Q1. The Montreal Protocol is concerned with:

Q2. Consider the following statements:
1. The Kyoto Protocol set legally binding emission targets for all countries, including India.
2. Under the Paris Agreement, every party must prepare and communicate Nationally Determined Contributions.
Which of the statements given above is/are correct?

Q3. Arrange the following in chronological order:
1. Adoption of the Paris Agreement
2. Signing of the Montreal Protocol
3. Adoption of the Kyoto Protocol
4. Kigali Amendment to the Montreal Protocol

Q4. Which of the following statements about ozone is correct?

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