SEE Model Paper 4

The candela (cd).

A gene is a segment of DNA in a chromosome that carries the code for a particular character of an organism, and passes it from parents to offspring.

It lets seawater into its ballast tanks. Its weight increases and becomes more than the upthrust on it, so it goes down.

$$Q = ms(T_2 - T_1) = 3 \times 2010 \times (30 - 10) = 120{,}600\ \text{J}$$

Diamond has a very high refractive index (2.42), so its critical angle is very small, only about 24°. Light that enters it meets its faces inside at more than 24° almost every time, so it is totally internally reflected again and again and comes back out of the top towards the eye.

Each turn of the coil gives one complete cycle, so the frequency is 25 Hz. The current changes direction twice in each cycle: 2 × 25 = 50 times a second.

It swells into a red giant, puffs off its outer layers as a planetary nebula, and its core is left as a white dwarf, which slowly cools and fades.

The extra carbon dioxide and water turn the insoluble calcium carbonate (the milkiness) into calcium hydrogencarbonate, which dissolves:
$$\text{CaCO}_3 + \text{H}_2\text{O} + \text{CO}_2 \rightarrow \text{Ca(HCO}_3)_2$$

No. Aluminium is very reactive (high in the reactivity series), so its oxide is very stable and carbon can’t take the oxygen away from it. Aluminium oxide is reduced only by electrolysis.

• i.Independent variable: the load (weight) he hangs on the spring, which he chooses. Dependent variable: the length (or extension) of the spring, which he measures.
• ii.Any one controlled variable: the same spring; the same meter scale, read in the same way; the same way of hanging the loads. The load, the independent variable, goes on the x-axis (and the length on the y-axis).

Any two:

• a.They are soft-bodied animals.
• b.The body has a head, a visceral mass, a muscular foot, and a mantle, which in most makes a hard shell of calcium carbonate.
• c.The head carries tentacles and eyes.
• d.The muscular foot is used for gliding or swimming.
• e.They breathe through the body surface, gills or a lung-like pulmonary sac.

• i.The leech belongs to Annelida; the centipede to Arthropoda.
• ii.Any one: the centipede has jointed legs, the leech has no legs; the centipede has a hard exoskeleton (of chitin), the leech a soft, moist skin (through which it breathes).

• i.It depends on the food. All larvae are fed royal jelly for the first three days. A future queen keeps getting royal jelly for as long as she is a larva; a future worker is switched to bee bread (pollen mixed with honey).
• ii.They choose a young larva from a fertilized egg, not more than about three days old, enlarge its cell, and feed it only on royal jelly, so it grows into a queen.

Any two:

• i.A always pairs with T, and G with C, so the other strand is T A C C G T A G.
• ii.Each A pairs with a T, so thymine is also 30%. A and T together make 60%, leaving 40% for G and C. Each G pairs with a C, so they are equal: guanine is 40 ÷ 2 = 20%.

• i.Inbreeding. Closely related animals carry many of the same genes, so mating them generation after generation makes it more likely that a kid gets two copies of a harmful gene: inherited defects become more common.
• ii.He should crossbreed them: mate his does with a healthy, unrelated buck, for example of a good breed such as Boer or Jamunapari. The kids get different genes from the two parents and are often stronger than either (hybrid vigor).

• i.Low: Bishnu (below 90/60). Normal: Kamala. A little raised: Laxmi (between 120/80 and 139/89). High: Dorje (both numbers above 140/90), if it is the same when measured again.
• ii.No. High blood pressure often has no symptoms for years (it is called the “silent killer”), but all the while it damages the arteries, heart, kidneys, eyes and brain, and can lead to a heart attack or a stroke. He should see a doctor and have it checked regularly.

• i.As a glacier melts back, its meltwater collects in a lake behind the moraine, a ridge of loose rock and buried ice that the glacier pushed up long ago. This natural dam is weak: an avalanche falling into the lake can send a wave over it, or the buried ice can melt, and the dam suddenly gives way (a glacial lake outburst flood, GLOF).
• ii.Any one: cutting channels to lower the water, as at Tsho Rolpa in Dolakha (3 m, in 2000) and Imja Tsho (3.4 m, in 2016); setting up early-warning sirens in the valleys below.

i. A ray from the top of the candle parallel to the principal axis spreads out after the lens as if it came from the principal focus F₁ on the candle’s side; a ray through the optical center O goes straight on. The two rays spread apart and never meet, but traced backwards (dashed) they seem to meet between F₁ and O, where the image forms.

ii. Any two: it is virtual (it can’t be caught on a screen), upright (erect), diminished (smaller than the candle), and between F₁ and O on the same side as the candle (10 cm from the lens).

• i.The layers of warm and cool air keep moving, so the light from a star is bent now a little this way, now a little that way, and because a star is so far away that it is just a point of light, its brightness and position flicker. The steady one is most likely a planet (such as Venus). A planet is much closer and looks like a tiny disc; each point of the disc flickers on its own, but the flickers average out, so it shines steadily.
• ii.Light from the Sun still below the horizon passes from rarer into denser air and bends towards the normal, down round the curve of the Earth (atmospheric refraction). So it reaches our eyes, and we see the Sun about 2 minutes before it actually rises.

• i.The reaction makes water. If the tube sloped upwards, drops of cool water would run back onto the hot glass at the bottom and crack it.
• ii.Ammonia is a base, so it reacts with sulphuric acid, an acid (making ammonium sulphate), and would be lost. Quicklime (calcium oxide) is a base too, so it takes up only the water and leaves the ammonia alone.

• i.Six carbons with only single bonds: hexane. Five carbons with a double bond: pentene (pent-1-ene).
• ii.For an alkyne, CₙH₂ₙ₋₂ with n = 4 gives C₄H₆: CH≡C–CH₂–CH₃, butyne (but-1-yne).

i. It gives off acetylene (ethyne):
$$\text{CaC}_2 + 2\text{H}_2\text{O} \rightarrow \text{C}_2\text{H}_2 + \text{Ca(OH)}_2$$
Industrial calcium carbide carries traces of poisonous arsenic and phosphorus compounds; it can also cause skin irritation, burns and eye irritation. (Nepal’s food rules forbid selling fruit ripened with it.)
ii. The skin is evenly bright yellow while the stalk end is still green; there may be a grayish powder or a garlic-like smell; the inside is hard and sour. A safe way: wrap the mangoes in a jute sack, or pack them in hay, husk or asuro leaves, or keep them in a closed bag with a ripe fruit, so that their own ethylene ripens them.

i. Source (Sita’s voice) → input transducer (the microphone) → encoder → modulator → channel (radio waves and optical fiber) → demodulator → decoder → output transducer (the loudspeaker) → output (her voice, as sound, to her brother’s ear).

• ii.The encoder decides which 0s and 1s to send: it turns the signal into bits, compresses them and can add check bits or a code. The modulator decides how to carry them: it puts the bits onto a carrier wave (a radio wave or light) to travel through the channel.
• iii.Broadband. Each message is modulated onto a carrier of its own frequency, so many messages share the same fiber at once; broadband is used over long distances. (Baseband sends the bits as they are, one signal at a time, over short distances.)

• i.Diabetes (diabetes mellitus) is a condition in which the blood has more glucose than normal, because the pancreas makes too little insulin or the cells stop responding to it.
• ii.Any two: great thirst and hunger; frequent urination; blurred vision; losing weight and feeling tired; frequent dizziness; muscle cramps or numbness in the hands and feet; wounds that heal slowly.
• iii.Purines, from food and from the body’s own DNA and RNA, are broken down, mainly in the liver, into uric acid, which the kidneys pass out in the urine. Too much of it forms crystals in the joints and causes gout (and stones in the kidneys).
• iv.Any two: exercise regularly; drink enough water, a little at a time, many times a day; avoid alcohol (especially beer) and smoking; eat less red meat, organ meat and seafood, and fewer purine-rich foods; avoid sugary drinks.

i. The stone starts from rest (u = 0), so $h = \tfrac{1}{2}at^2$ and $a = \dfrac{2h}{t^2}$.
Trial 1: $a_1 = \dfrac{2 \times 5.0}{1.00^2} = 10.0\ \text{m/s}^2$
Trial 2: $a_2 = \dfrac{2 \times 10.0}{1.44^2} = \dfrac{20}{2.0736} = 9.65\ \text{m/s}^2$
Trial 3: $a_3 = \dfrac{2 \times 15.0}{1.75^2} = \dfrac{30}{3.0625} = 9.80\ \text{m/s}^2$
ii. Average: $\dfrac{10.0 + 9.65 + 9.80}{3} = 9.82\ \text{m/s}^2$, about 9.8 m/s².
Speed in trial 3: $v = u + at = 0 + 9.80 \times 1.75 = 17.15\ \text{m/s}$, about 17 m/s. (Or $v = \sqrt{2ah} = \sqrt{2 \times 9.80 \times 15} = \sqrt{294} = 17.1\ \text{m/s}$.)

• iii.The fall from 5 m is the shortest, only about 1 s. Pressing the stopwatch takes a reaction time of about 0.1–0.2 s, and a small error in timing a short fall makes a big error in a = 2h/t² (t is squared). A taller drop, and several trials averaged, give a better result.
• iv.No. The acceleration due to gravity, $g = \dfrac{GM}{R^2}$, doesn’t depend on the mass of the falling body: the heavier stone is pulled harder, but it is also harder to speed up by exactly the same factor. With no air resistance it falls with the same acceleration and takes the same time, about 1.75 s.

• i.$Q_1 = ms(T_2 - T_1) = 1.5 \times 4200 \times (89 - 9) = 1.5 \times 4200 \times 80 = 504{,}000\ \text{J}$
• ii.$Q_2 = 0.5 \times 884 \times 80 = 35{,}360\ \text{J}$. (So the stove supplies at least 504,000 + 35,360 = 539,360 J in all.)
• iii.Namche Bazaar is high (about 3,440 m), where the air is thinner and its pressure is lower. A liquid boils when its vapor can push bubbles open against the pressure of the air above, so at a lower air pressure water boils at a lower temperature, about 89 °C.
• iv.Heat lost by the hot water = heat gained by the cold water. Let the final temperature be T; s is the same and cancels:

$$0.5 \times (89 - T) = 0.3 \times (T - 9)$$
$$44.5 - 0.5T = 0.3T - 2.7 \quad\Rightarrow\quad 0.8T = 47.2 \quad\Rightarrow\quad T = 59\ °\text{C}$$
(It lies between 9 °C and 89 °C, nearer 89 °C, because there is more hot water.)

• i.$\dfrac{N_p}{N_s} = \dfrac{V_p}{V_s} \Rightarrow N_s = \dfrac{N_p \times V_s}{V_p} = \dfrac{400 \times 2200}{220} = 4{,}000$ turns.
• ii.Q: $V_s = \dfrac{V_p \times N_s}{N_p} = \dfrac{220 \times 100}{1100} = 20\ \text{V}$.

R: $N_p = \dfrac{N_s \times V_p}{V_s} = \dfrac{60 \times 11000}{220} = 3{,}000$ turns.

• iii.P is step-up: its secondary has more turns than its primary (4,000 > 400), so its output voltage is higher. Q and R are step-down: their secondaries have fewer turns (100 < 1,100; 60 < 3,000), so their output voltages are lower.
• iv.No. A transformer doesn’t make energy. Power = voltage × current, and almost all the input power comes out, never more. With ten times the voltage, the output current is about ten times smaller: a transformer trades voltage for current.

• i.C has 7 electrons in its outermost shell, the third: group VIIA (17), period 3. (It is chlorine.)
• ii.A and B (sodium and potassium) are both in group IA with one valence electron, but B has one more shell, so its atom is bigger and its outer electron is farther from the nucleus and more loosely held. B loses it more easily, so it is more reactive.
• iii.Metals react by losing electrons and non-metals by gaining them. Down a group the atoms get bigger: an outer electron is lost more easily (so the reactivity of metals increases), but a new electron is pulled in less strongly (so the reactivity of non-metals decreases). D (fluorine) is small, with a strong pull, so it gains an electron more readily than C.
• iv.A has valency 1 (it gives one electron) and C has valency 1 (it takes one), so the formula is AC (NaCl, common salt). E has a full outer shell of eight electrons (an octet), so it has no need to lose, gain or share electrons: it is a noble gas.

• i.P: double displacement (NaCl + AgNO₃ → AgCl↓ + NaNO₃: two compounds swap partners, and silver chloride is the white precipitate). R: decomposition (CaCO₃ → CaO + CO₂: one reactant breaks into two products, by heat).
• ii.$\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2$. The gas is hydrogen. (It is a single displacement reaction.)
• iii.The acid used up (neutralized) the base: HCl + NaOH → NaCl + H₂O. Once no base was left, phenolphthalein, which is pink only in a base, became colorless. This is an acid–base (neutralization) reaction, and it gives out heat.
• iv.No. Copper is below hydrogen in the reactivity series (K > Na > Ca > Mg > Al > Zn > Fe > Pb > (H) > Cu > Hg > Ag > Au), so it is less reactive than hydrogen and can’t displace it from the acid. Zinc is above hydrogen, so it can.