SEE Model Paper 3

$$\text{unit of } G = \frac{\text{N} \times \text{m}^2}{\text{kg} \times \text{kg}} = \frac{\text{kg m s}^{-2} \times \text{m}^2}{\text{kg}^2} = \text{m}^3\,\text{kg}^{-1}\,\text{s}^{-2}$$
(In derived units, N m²/kg².)

Because they live on land but need water to reproduce: their male gametes swim to the egg through a film of water.

Platelets (thrombocytes). They stick to the torn edge and to each other to make a plug, and help a clot to form.

When pressure is applied at any point of a liquid in a closed container, it is transmitted equally in all directions, throughout the liquid, and acts at right angles to the walls of the container.

Heat lost by the hot water = heat gained by the cold water. Let the final temperature be T.
$$2 \times 4200 \times (70 - T) = 3 \times 4200 \times (T - 20) \quad\Rightarrow\quad 140 - 2T = 3T - 60 \quad\Rightarrow\quad T = 40\ °\text{C}$$

The changing magnetic field would induce swirling currents (eddy currents) in a solid core, which would heat it and waste energy. The insulating layers break up these currents, so less energy is lost.

Fluorine’s nucleus has 9 protons and lithium’s only 3. The bigger positive charge pulls the same two shells in more tightly, so the fluorine atom is smaller.

Gases dissolve less in warm water. On the hot day, ammonia gas came out of the solution and built up pressure under the lid, which pushed the gas and the liquid out as soon as he opened it. The bottle should first be cooled in cold water or ice, so that the gas dissolves back in.

Because their carbon atoms are joined only by single covalent bonds, so each carbon already holds as many hydrogen atoms as it can and can take no more.

i. Bina: the left side, v, is in m s⁻¹, and so is u; but at² is m s⁻² × s² = m. A speed can’t be added to a distance, so her equation is wrong.
Kiran: v² and u² are in m² s⁻², and as is m s⁻² × m = m² s⁻². Every term has the same units, so his equation could be right.
ii. No. Numbers have no units, so a check by units can’t find a missing number. The correct equation is v² = u² + 2as: Kiran has left out the 2, and the units still match. Units can show that an equation is wrong, but not that it is right.

• i.The photo is cut into small packets of bits, and each packet carries the destination address and a number. Routers pass each packet on towards the address, and her sister's phone puts the packets back together in order by their numbers.
• ii.Each router chooses a good next step at that moment, so when one cable is broken, the packets simply take other routes round the break. They may arrive out of order, but the phone puts them back in order by their numbers and asks again for any that went missing.

• i.Any two: he clicked a link in an unknown message; he typed his password on a page he reached from that link; he gave the OTP to a caller (no real wallet company ever asks for an OTP or PIN); he believed the caller without checking.
• ii.Any two: never click links in messages that say an account is blocked or a prize is won, and open the real app yourself instead; never share an OTP or PIN with anyone; turn on two-step verification and use a strong, different password for each account; call the company’s own number to check; report the message and change the password at once.

Any two:

• a.The body is covered with hair (or fur).
• b.They have mammary glands, which make milk to feed the young.
• c.They are viviparous (give birth to young) and fertilization is internal.
• d.They are homeothermic (warm-blooded), breathe with lungs and have a four-chambered heart.
• e.They have external ears (pinnae) and teeth of different kinds.

Any two:

• i.19 × 4 = 76 beats a minute, within the normal range of 60 to 100.
• ii.31 × 4 = 124 beats a minute. No, she doesn’t have tachycardia: tachycardia is a heart rate above 100 a minute at rest. The heart rate rises with exercise in everyone and falls back when they rest.

• i.$$F = \frac{Gm_1m_2}{d^2} = \frac{6.67 \times 10^{-11} \times 10{,}000 \times 10{,}000}{5^2} = \frac{6.67 \times 10^{-3}}{25} = 2.67 \times 10^{-4}\ \text{N}$$
• ii.The force is very small, about a quarter of a thousandth of a newton, while each truck weighs about 98,000 N. The friction between the tyres and the road is far larger than this pull, so the trucks don’t move.

Gravity still pulls the astronauts and the station: it is what keeps the station in orbit. But the station is falling freely round the Earth all the time, and the astronauts fall with it at the same acceleration. So they don’t press on the floor or pull on anything: their apparent weight is zero, and they float. This is weightlessness.

• i.$Q = ms(T_2 - T_1) = 100 \times 4200 \times (55 - 15) = 16{,}800{,}000\ \text{J}$ (16.8 MJ)
• ii.$$\Delta T = \frac{Q}{ms} = \frac{16{,}800{,}000}{100 \times 2100} = 80\ °\text{C}$$

Twice the rise in water's temperature: the same heat warms a liquid with half the specific heat capacity twice as much. (That is why water, with its high specific heat capacity, is used: it stores a lot of heat with a small rise in temperature.)

• i.The light travels in the endoscope's optical fibers. It meets the edge of each fiber's core at more than the critical angle, so it is totally internally reflected again and again and stays in the core, whether the fiber is straight or bent.
• ii.No. Total internal reflection happens only when light goes from a denser into a rarer medium. The cladding has a lower refractive index than the core, which makes the core the denser medium. With cladding of the same glass, nothing would totally reflect the light at the core's edge: it would leak out through the sides, and little would reach the far end.

• i.For the same power, a higher voltage needs a smaller current (power = voltage × current). The heat wasted in the wires grows with the square of the current, so a small current wastes much less energy on the long journey.
• ii.By step-down transformers: at substations and on poles near the houses, they lower the voltage in stages to 230 V.

• i.1s², 2s² 2p⁶, 3s² 3p³. Its third (outermost) shell holds 5 electrons, so it is in group VA (15) and period 3. (X is phosphorus.)
• ii.The element with atomic number 7 (nitrogen, 1s², 2s² 2p³). It is in the same group as X but one period higher, so its atom is smaller, and its nucleus pulls an extra electron more strongly.

i. $$\text{Fe} + \text{CuSO}_4 \rightarrow \text{FeSO}_4 + \text{Cu}$$
The reddish-brown layer is copper, and the pale green solution is iron(II) sulphate. It is a single displacement reaction.
ii. No. Silver is below copper in the reactivity series (K > Na > Ca > Mg > Al > Zn > Fe > Pb > (H) > Cu > Hg > Ag > Au), so it is less reactive and can’t displace copper from copper sulphate. Iron is above copper, so it can.

• i.The sample from near the kilns (pH 4.4) is acid rain. Even clean rain dissolves a little carbon dioxide from the air and is slightly acidic, with a pH of about 5.6; only rain with a pH below 5.6 is called acid rain.
• ii.Sulphur dioxide (SO₂), from burning coal:

$$2\text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3 \qquad \text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4$$
(Oxides of nitrogen from the hot kilns add nitric acid as well.)

• i.Straight up on the comb stands for the direction of the sun, so they should fly 90° to the left of the sun. The longer the waggle run, the farther the food, roughly one second of waggling for each kilometer, so the food is about 2 km away.
• ii.A forager sucks nectar (mostly water, with sucrose) into her honey stomach, and the bees add an enzyme (invertase) that splits the sucrose into glucose and fructose. House bees spread it in the cells and fan it with their wings to evaporate the water; when the water is down to under about 18%, they seal the cell with wax.
• iii.Honey has so little water and so much sugar that it pulls water out of any germ that lands in it. It is also slightly acidic, and an enzyme the bees add slowly makes a little hydrogen peroxide, which kills germs.
• iv.As the bees suck nectar, they carry pollen from flower to flower and pollinate the mustard, in the neighbors' fields too (they forage a kilometer or more from the hive), so more pods and seeds form.

• i.GG × gg: the parents give only G and only g gametes, so every F₁ plant is Gg, with green pods.
• ii.F₁ × F₁ is Gg × Gg; each parent gives G and g gametes.

• iii.The ratio is 3 green : 1 yellow, so about $\tfrac{1}{4} \times 600 =$ 150 plants with yellow pods. The other 450 are green, and 1 in 3 of them is pure (GG : Gg = 1 : 2): about 150 pure GG plants.
• iv.The law of purity of gametes (segregation). The alleles G and g sit together in the F₁ hybrid without mixing; when it makes gametes they separate, and each gamete gets only one of them. Two g gametes can meet at fertilization, giving gg plants with yellow pods.

• i.Taxol, a medicine for cancer. They should have taken only as much as the trees could grow back: some bark from each tree, never stripped from the whole trunk.
• ii.A tiny piece of a panch aunle plant is grown on a jelly of nutrients in a clean glass jar, where it multiplies into hundreds of identical small plants. These can be raised and planted out in the pastures without digging up any more plants from the wild.
• iii.The Endangered Wild Fauna and Flora International Trade Control Act, 2073 (Nepal's law for the world agreement CITES). It sets rules on selling and trading rare wild plants and animals (and on breeding, growing and research).
• iv.Any two: collect only as much as can grow back (for a herb, leave some plants to make seed); raise saplings of the rare plants in a nursery and plant them out; protect the places where they grow (control grazing, forest fires and cutting); stop theft and smuggling; hold awareness programs in the community. Plants left growing give a harvest year after year, but a plant dug up and sold all at once is gone for good.

i. The part (the object) is between the principal focus F₁ and the optical center O. A ray from the top of the object parallel to the principal axis passes, after the lens, through F₂; a ray through O goes straight on. The two rays spread apart on the other side and never meet. Traced backwards (dashed), they seem to meet on the same side as the object, farther from the lens than it, where the image forms.

• ii.Any two: it is virtual (it can’t be caught on a screen), upright (erect), magnified, and on the same side of the lens as the object.
• iii.$f = 5\ \text{cm} = 0.05\ \text{m}$, so

$$P = \frac{1}{f} = \frac{1}{0.05\ \text{m}} = +20\ \text{D}$$

• i.Galaxies are moving away from us, and the farther away a galaxy is, the faster it is moving away: $v = Hd$, where v is the galaxy’s speed of recession, d its distance and H Hubble’s constant.
• ii.If the galaxies are moving apart now, they were closer together in the past; going back far enough, everything was together in one extremely hot, dense state. That is what the Big Bang theory says: the universe began from such a state about 13.8 billion years ago and has been expanding ever since.
• iii.The cosmic microwave background: the afterglow of the Big Bang, light released about 380,000 years after the beginning and stretched by the expansion into weak microwaves.
• iv.No. It is space itself that is expanding, everywhere at once, so from any galaxy all the others seem to move away, like dots on a balloon being blown up. No dot, and no galaxy, is the center.

• i.Grinding (crushing) → concentration (removing the gangue) → oxidation (roasting or calcination) → reduction → refining.
• ii.Calcination: heating the ore strongly without air (calamine is a carbonate ore).

$$\text{ZnCO}_3 \xrightarrow{\Delta} \text{ZnO} + \text{CO}_2$$
iii. Zinc oxide is heated with carbon (coke):
$$\text{ZnO} + \text{C} \xrightarrow{\Delta} \text{Zn} + \text{CO}$$
iv. Distillation. Zinc has a low boiling point (907 °C): the impure zinc is heated until it boils, and the vapor is cooled back into pure zinc, leaving the impurities behind.

• i.Saponification: fat or oil + sodium hydroxide → soap + glycerol. Soap dissolves less in salty water, so the salt makes the soap separate and float to the top as a solid, leaving the glycerol in the water (salting out).
• ii.A soap molecule has a long hydrocarbon tail that dissolves in oil (hydrophobic) and an ionic head that loves water (hydrophilic). The tails dig into the grease and the heads stay in the water, so the soap wraps tiny droplets of grease into balls called micelles, which are rinsed away.
• iii.Mostly right. Soap is biodegradable: microbes break it down, so it doesn't pollute for long. Washing powder contains phosphates, which are a fertilizer for water plants: wastewater full of them makes rivers choke with algae, which use up the oxygen fish need. (Still, no soapy water should run straight into a river; it should go to a soak pit or a treatment plant.)
• iv.Wood ash is alkaline: it contains potassium carbonate (potash), which turns grease into a kind of soap, and its grains scrub. Lemon juice contains citric acid, which dissolves the dull green-black layer of copper compounds on the brass.