SEE Model Paper 9

Seven: the meter, kilogram, second, kelvin, candela, ampere and mole.

Hollow, air-filled (pneumatic) bones make the body light, which helps birds to fly.

Choosing parent plants or animals with the characters we want (more milk, more grain, resistance to disease) and breeding them, so that their offspring have those characters.

Trees take in carbon dioxide to make their food and store its carbon in their wood. With fewer trees, less carbon dioxide is taken from the air, and burning or rotting the cut trees releases the carbon they stored, so the amount of this greenhouse gas in the air rises.

The summit is about 8.8 km farther from the Earth’s center, and g = GM/(R + h)² gets smaller as the distance from the center grows. (It is only about 0.03 m/s² less, because 8.8 km is tiny next to the Earth’s radius.)

Area = 0.20 m × 0.30 m = 0.06 m².
$$P = \frac{F}{A} = \frac{600\ \text{N}}{0.06\ \text{m}^2} = 10{,}000\ \text{Pa}$$

8.6 × 9.46 × 10¹² km = 81.4 × 10¹² km ≈ 8.1 × 10¹³ km.

Pure oxygen has about five times the concentration of oxygen in air, so oxygen molecules collide with the hot wood far more often, and the burning becomes much faster.

The black specks are carbon: 2Mg + CO₂ → 2MgO + C. Burning magnesium is hot enough to pull the oxygen out of carbon dioxide, which shows that carbon dioxide contains carbon (and that it can’t put out every fire).

$$s = \frac{Q}{m\,\Delta T}$$
The unit of heat (energy) is the joule, J = kg m² s⁻²; the unit of mass is kg and of temperature difference K. So
$$\text{unit of } s = \frac{\text{kg m}^2\,\text{s}^{-2}}{\text{kg} \times \text{K}} = \text{m}^2\,\text{s}^{-2}\,\text{K}^{-1}$$
(This is the J/(kg °C) of the tables: a change of 1 °C is the same size as a change of 1 K.)

• i.$\dfrac{10^6}{10^8\ \text{per s}} = 0.01\ \text{s}$: a hundredth of a second.
• ii.$\dfrac{10^{16}}{10^8\ \text{per s}} = 10^8\ \text{s}$, and $\dfrac{10^8\ \text{s}}{3.16 \times 10^7\ \text{s}} \approx 3.2$ years. Each extra word multiplies the time by 10,000, which is why a long password of random words is so much safer than a short one.

• i.An earthworm breathes through its skin: oxygen dissolves in the thin film of moisture on it and passes into the blood vessels below. A dry skin can’t take in oxygen, so the worm suffocates.
• ii.Its tunnels let air and rainwater into the soil, and it digests dead leaves in the soil it swallows and leaves fine, rich castings, so it plows and enriches the soil.

• i.Each round doubles the number: 1 → 2 → 4 → 8 … After n rounds there are 2ⁿ cells, and 2¹⁰ = 1,024, so 10 rounds.
• ii.Each cell gives four haploid cells by meiosis: 250 × 4 = 1,000 sperms, each with 23 chromosomes (half of 46).

Any two:

• a.It carries digested food (glucose, amino acids, fats) to all the cells, and wastes such as urea to the organs that remove them.
• b.It carries enzymes and hormones made by glands to the parts of the body that need them.
• c.Its protein fibrinogen helps the blood to clot.
• d.It keeps the right amount of water in the blood, and its chemical make-up and pH steady.
• e.It spreads heat around the body, keeping the temperature even.

• i.Pasang: bradycardia (a resting heart rate below 60 a minute). Rina: tachycardia (a resting heart rate above 100 a minute).
• ii.Training has made his heart strong, so it pumps more blood with each beat. It can send the body all the blood it needs with fewer beats, so a slow resting rate is normal in fit athletes.

• i.A convex lens gives a real, inverted image of the same size only when the object is at 2F, and the image then forms at 2F on the other side. So 2f = 60 cm, and f = 30 cm.
• ii.f = 30 cm = 0.30 m, so $P = \dfrac{1}{f} = \dfrac{1}{0.30\ \text{m}} \approx +3.3\ \text{D}$.

i. Vₚ = 33 kV = 33,000 V, Vₛ = 11 kV = 11,000 V, Nₚ = 4,500.
$$N_s = \frac{N_p \times V_s}{V_p} = \frac{4500 \times 11000}{33000} = 1{,}500\ \text{turns}$$
ii. The secondary: a transformer gives out almost the same power it takes in (power = voltage × current), so with one third of the voltage the secondary carries about 3 times the current of the primary.

• i.In each cycle the current rises, falls to zero, reverses and falls to zero again: it is zero twice a cycle. 2 × 50 = 100 times a second.
• ii.The filament is white-hot and can’t cool down in a hundredth of a second, so its glow hardly changes while the current is near zero. (In any case, the eye can’t see flicker faster than about 50 to 60 times a second.)

Ramesh wrote oxygen as single atoms (O), but oxygen gas is made of molecules of two atoms (O₂). Sita changed the formula of the product: MgO₂ is not magnesium oxide. To balance an equation, only the coefficients in front may be changed, never the formulae.
Correct equation: $\text{2Mg} + \text{O}_2 \rightarrow \text{2MgO}$ (2 Mg and 2 O on each side).

• i.Potassium hydroxide, an alkali, absorbed the carbon dioxide, so the pressure of the gas inside the tube fell. The air pressing on the water outside then pushed water up into the tube.
• ii.$\text{2KOH} + \text{CO}_2 \rightarrow \text{K}_2\text{CO}_3 + \text{H}_2\text{O}$ (K 2 = 2, O 4 = 4, H 2 = 2, C 1 = 1).

• i.$\text{2C}_2\text{H}_2 + \text{5O}_2 \rightarrow \text{4CO}_2 + \text{2H}_2\text{O}$ (C 4 = 4, H 4 = 4, O 10 = 8 + 2).
• ii.The triple bond opens and takes in two molecules of hydrogen (hydrogenation):

$$\text{C}_2\text{H}_2 + \text{2H}_2 \xrightarrow{\text{Ni}} \text{C}_2\text{H}_6$$

• i.60 ÷ 3 = 20 liters. Any one use: refined into biodiesel; making soap; lighting lamps; making glycerine; rubbed on aching muscles and joints; put on cattle to kill lice.
• ii.The seeds are poisonous: they contain toxins (curcin and phorbol esters) that cause severe vomiting and diarrhea, even after only a few seeds.

• i.Egg → larva → pupa → adult. A change of body form through these four stages is complete metamorphosis.
• ii.A worker develops in 21 days (egg 3, larva 6, pupa 12) and a drone in 24 days (egg 3, larva 7, pupa 14), so the drones come out 3 days after the workers.
• iii.On about day 9 (3 days as an egg and 6 as a larva), when the larva is fully grown. Sealed in, it turns into a pupa, which doesn’t eat or move while its legs, wings and other adult organs form, and after 12 days the adult worker chews through the wax cap.
• iv.0.5 kg × 6 to 8 = 3 to 4 kg of honey. The bees don’t have to make new wax, so they spend that honey and their time on filling the comb with more honey, which he can harvest.

i. A copied chromosome is two identical sister chromatids held together at the centromere. Each chromatid is one very long DNA molecule wound on histone proteins, and a gene is one stretch of that DNA.

• ii.Any two: they carry the genes that decide the characters of the organism; they pass the genetic information to the daughter cells at every cell division, and to the next generation through the gametes; the sex chromosomes decide the sex; through crossing over and mutation they produce variation.
• iii.A mutation is a change in the DNA, the genetic code, of a chromosome (for example a missing or wrong nucleotide). If it is in an important gene, the protein that gene codes for may not work, and if it happens in a gamete, it can be passed on to the offspring.

• i.$54\ \text{km/h} = \dfrac{54 \times 1000\ \text{m}}{3600\ \text{s}} = 15\ \text{m/s}$
• ii.At the top, v = 0; going up, g = −9.8 m/s². From $v^2 = u^2 + 2gh$:

$$0 = 15^2 + 2 \times (-9.8) \times h \;\Rightarrow\; h = \frac{225}{19.6} \approx 11.5\ \text{m}$$

• iii.Time to the top, from $v = u + gt$: $0 = 15 - 9.8\,t \Rightarrow t = \dfrac{15}{9.8} \approx 1.53\ \text{s}$. It takes as long to fall back, so it is in the air for about 3.06 s.
• iv.$h = \dfrac{u^2}{2g} = \dfrac{225}{2 \times 3.7} = \dfrac{225}{7.4} \approx 30.4\ \text{m}$, more than two and a half times as high, because Mars pulls it back less strongly.

• i.Heat lost by the hot body = heat gained by the cold body (the principle of calorimetry), provided no heat is lost to the surroundings.
• ii.$Q = ms(T_1 - T) = 3 \times 4200 \times (85 - 40) = 567{,}000\ \text{J} = 567\ \text{kJ}$
• iii.The cold water gains this heat:

$$m = \frac{Q}{s(T - T_2)} = \frac{567000}{4200 \times (40 - 10)} = \frac{567000}{126000} = 4.5\ \text{kg}$$
So she must add 4.5 liters.
iv. No, it would end a little below 40 °C. The cold steel bucket also takes in some of the heat given out by the hot water, so less heat is left for the cold water. (Steel conducts heat well and takes in more heat than the thin plastic.) She would need a little less cold water.

• i.$\mu = \dfrac{\sin i}{\sin r} = \dfrac{0.766}{0.451} \approx 1.7$
• ii.$v = \dfrac{c}{\mu} = \dfrac{3 \times 10^8\ \text{m/s}}{1.7} = 1.76 \times 10^8\ \text{m/s} = 1.76 \times 10^5\ \text{km/s}$
• iii.$\sin c = \dfrac{1}{\mu} = \dfrac{1}{1.7} = 0.588$, and sin 36° = 0.588, so c ≈ 36°.
• iv.No. The light is going from a denser medium (glass) to a rarer one (air), and its angle of incidence, 40°, is greater than the critical angle, 36°. So it is totally internally reflected back into the glass.

• i.A (lithium): 1s², 2s¹. B (fluorine): 1s², 2s² 2p⁵.
• ii.A: group IA (1), period 2. B: group VIIA (17), period 2. Both have two shells; A has one valence electron and B seven.
• iii.$\text{2Li} + \text{F}_2 \rightarrow \text{2LiF}$ (Li 2 = 2, F 2 = 2). Lithium gives one electron and fluorine takes one, so the formula is LiF.
• iv.B (fluorine). Both are in group VIIA and need one electron, but fluorine has only two shells while chlorine has three, so its atom is smaller and its nucleus pulls in an electron more strongly. The reactivity of non-metals decreases down a group.

i. Roasting: a sulphide ore is heated strongly in a supply of air.
$$\text{2Cu}_2\text{S} + \text{3O}_2 \rightarrow \text{2Cu}_2\text{O} + \text{2SO}_2$$
(Cu 4 = 4, S 2 = 2, O 6 = 2 + 4.)

• ii.$\text{Cu}_2\text{O} + \text{C} \rightarrow \text{2Cu} + \text{CO}$ (Cu 2 = 2, O 1 = 1, C 1 = 1). Carbon is the reducing agent: it takes the oxygen.
• iii.Mass of Cu₂S = 2 × 63.5 + 32 = 159, of which copper is 127.

$$\%\ \text{Cu} = \frac{127}{159} \times 100 = 79.9\,\% \qquad \text{copper in 500 kg} = 500 \times \frac{127}{159} \approx 399\ \text{kg}$$
iv. Electro-refining. Distillation works only for metals with low boiling points, such as mercury and zinc; copper boils at far too high a temperature. In electro-refining, the impure copper is the anode and pure copper builds up on the cathode.