SEE Model Paper 10

Cutting off the unwanted parts at the start and the end of a clip, so that only the part you want to keep is left.

Protista: they are eukaryotic, made of one cell with a nucleus.

The body turns beta-carotene into vitamin A, so golden rice helps to prevent the blindness caused by a lack of vitamin A in people who live mostly on rice.

A blood vessel is called an artery because it carries blood away from the heart, whatever the blood is like. The pulmonary artery carries blood from the right ventricle to the lungs.

He needs a concave lens that makes distant objects seem to be at his far point, so its focal length is f = −80 cm = −0.8 m.
$$P = \frac{1}{f} = \frac{1}{-0.8\ \text{m}} = -1.25\ \text{D}$$

When the face of the card lies along the field lines (the card is parallel to the lines), no lines pass through it, so the flux through it is zero.

The galaxies are moving away from us, the farther ones faster, so the universe is expanding. Run backwards, everything was once packed together, which supports the Big Bang.

X: 2, 8, 3, so its valency is 3. Y: 2, 6, so its valency is 2. Crossing the valencies: X₂Y₃ (this is aluminium oxide, Al₂O₃).

Alumina melts only above 2,000 °C. Cryolite lowers its melting point to about 950 °C, which saves a great deal of energy (and the molten mixture also conducts electricity better).

• i.Bandwidth is the range of frequencies that a channel (or a signal) takes up; the wider the bandwidth, the more data it can carry each second.
• ii.The statement is true only for raw, uncompressed digital sound and video. Before the bits are sent, the encoder compresses the data into as few bits as possible (for example, sending only the parts of a picture that change from one frame to the next). Each compressed channel takes up only a small part of the bandwidth one analog channel needed, so several channels fit into the space of one.

Any two:

• The body is covered with feathers.
• The forelimbs are wings; the hind limbs are for walking, perching or swimming.
• The bones are hollow and filled with air (pneumatic bones).
• They are homeothermic (warm-blooded), with a four-chambered heart.
• They breathe with lungs, helped by air sacs.
• Fertilization is internal, and they lay eggs (oviparous).

• i.Square: 4 × 1 cm = 4 cm. Hexagon: 6 × 0.62 cm = 3.72 cm. The hexagon needs $\dfrac{4 - 3.72}{4} \times 100\% = 7\%$ less wall for the same space.
• ii.Wax is very costly for the bees: each kilogram of it costs them 6–8 kg of honey. Less wall means less wax, so more honey is left as food. Round cells can’t fit together without gaps, which would waste space (and wax around the gaps); of the shapes that fit together with no gaps, the hexagon needs the least wall.

i. All the F₁ plants are Gg (green). Each makes gametes G and g in equal numbers:

ii. GG : Gg : gg = 1 : 2 : 1, so green : yellow = 3 : 1. Yellow (gg) = ¼ × 1,200 = 300 plants; pure green (GG) = ¼ × 1,200 = 300 plants (the other 600 are hybrid green, Gg).

• i.$\dfrac{705}{224} \approx 3.15$, so the ratio is about 3 : 1. That is the ratio from crossing two hybrids, Pp × Pp: so each F₁ plant, though purple, carried a hidden gene for white (p), and made P and p gametes in equal numbers.
• ii.Which pollen grain fertilizes which egg is a matter of chance, like tossing coins, so the real numbers wander a little from 3 : 1. The more plants are counted, the closer the ratio comes to 3 : 1; with only a few plants, chance could hide the pattern completely.

• i.Spring balance: W = mg = 3 kg × 1.62 m/s² = 4.86 N. Beam balance: it still balances with 3 kg of standard masses.
• ii.A spring balance measures weight, the pull of gravity on the bag, which is smaller on the Moon because its g is smaller. A beam balance compares the bag with standard masses; the Moon pulls both sides less by the same factor, so they still balance. It measures mass, which is the same everywhere.

i. $g = \dfrac{GM}{R^2}$. On P the mass is 4M and the radius 3R:
$$g_P = \frac{G(4M)}{(3R)^2} = \frac{4}{9} \times \frac{GM}{R^2} = \frac{4}{9} \times 9.8 \approx 4.36\ \text{m/s}^2$$
ii. No. g depends on the radius as well as the mass, and the radius counts as its square. P’s mass makes g 4 times bigger, but standing 3 times farther from its center makes it 9 times smaller, so P pulls things less than half as hard as the Earth.

• i.When steam touches the cooler skin, it condenses into water, and gives out the large extra heat it took in when it boiled (about 2,260 J for every gram) before it even begins to cool. That extra heat goes into the skin, so the burn is deeper.
• ii.Glass is a poor conductor of heat. The inner surface heats up and expands at once, while the outer surface stays cold and doesn’t expand. In thick glass the difference between the inside and the outside is bigger, so the strain is bigger and the glass cracks.

The bulb is beyond 2F₁ (2f = 24 cm). One ray parallel to the axis is bent through F₂; one ray through the optical center O goes straight on. Where they meet is the image:

The image is real, inverted and diminished (smaller than the bulb), and it forms on the other side of the lens between F₂ and 2F₂ (in fact about 17 cm from the lens).

• i.Light traveling in the water and glass meets the inner wall of the tube, with air behind it, at an angle bigger than the critical angle. It can’t pass into the rarer air, so it is totally internally reflected to our eye, and the wall looks like a mirror.
• ii.With water inside, there is water on both sides of the thin glass wall and no surface with air behind it. Light goes from water into glass and back into water, a denser medium each time or almost the same, so there is no total internal reflection: the light passes through and the shine is gone.

i. Subshells fill in order of energy, and 4s fills before 3d:
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶
ii. Its last electron enters a d subshell (3d), so it is in the d-block; its outermost shell is the fourth, so it is in period 4.

• Alcohols: the hydroxyl group, –OH; for example ethanol, C₂H₅OH.
• Carboxylic acids: the carboxyl group, –COOH; for example ethanoic acid (acetic acid, in vinegar), CH₃COOH.

• i.Phosphates are plant nutrients, like a fertilizer, so the algae in the pond grew and spread very fast.
• ii.The thick algae die and rot, and the bacteria that decompose them use up the oxygen dissolved in the water (the algae also block the light). Fish need that oxygen to breathe, so they die.

• i.Independent variable: the height from which the ball is dropped. Dependent variable: the depth of the dent. Controlled variables (any one): the same ball, the same kind and dampness of sand, the sand smoothed flat each time, the ball let go without pushing.
• ii.She changed two more things at once (the ball and the sand), as well as the height. If the dents are different, she can’t tell whether the height, the bigger ball or the dry sand caused it, so it isn’t a fair test. Only the independent variable may change.
• iii.g is in m s⁻² and h in m. For $\sqrt{2g/h}$: $\sqrt{\text{m s}^{-2} / \text{m}} = \sqrt{\text{s}^{-2}} = \text{s}^{-1}$, which is not a unit of speed, so it is surely wrong. For $\sqrt{2gh}$: $\sqrt{\text{m s}^{-2} \times \text{m}} = \sqrt{\text{m}^2\,\text{s}^{-2}} = \text{m s}^{-1}$, the unit of speed, so it could be correct (and it is, from v² = u² + 2gh with u = 0).
• iv.Yes, $\sqrt{gh}$ is also in m s⁻¹. But a unit check can’t tell her it is wrong, because numbers have no units: a missing 2 (or ½) slips through. Matching units show only that a formula could be correct; if they don’t match, it is surely wrong.

• i.It lies between the left auricle and the left ventricle. It lets oxygenated blood flow from the left auricle into the left ventricle, and stops it from flowing back when the ventricle contracts.
• ii.They are the sounds of valves shutting: “lub” is the tricuspid and bicuspid valves closing as the ventricles begin to contract; “dub” is the aortic and pulmonary (semilunar) valves closing as the ventricles relax.
• iii.Because the valve leaks, some blood is pushed back into the left auricle instead of all of it going out through the aorta. (That leaking blood makes the swishing murmur.)
• iv.Less oxygenated blood goes out through the aorta to his body, so his muscles get less oxygen when they need more, as on the stairs, and he tires quickly. The blood also backs up from the left auricle into the pulmonary veins and the lungs, which makes him breathless.

• i.In-situ conservation, because the tigers are protected in their own natural habitat (the forests of the national parks), not bred away from it as in a zoo.
• ii.To know the real state of the species: how many tigers there are, where they live and whether their number is rising or falling, so that conservation can be planned and checked. Camera traps photograph each tiger, and each tiger’s stripes are different, so they can be counted without catching them.
• iii.The parks are nearly full: there may now be more tigers than the forests can feed, so young tigers looking for their own area, and old or injured ones that can’t hunt wild prey, wander out to where goats and cattle are easy to catch.
• iv.Any two: pay people quickly and fairly for the animals and lives lost; keep forest corridors so tigers can move between parks instead of through villages; build strong night sheds for livestock; share the parks’ income with the buffer-zone communities (30 to 50 %), so they gain from the tigers; warn people and avoid going alone into the forest at dawn and dusk.

• i.A floating body displaces liquid whose weight is equal to its own weight.
• ii.Volume under water = 25 cm × 8 cm × 3.5 cm = 700 cm³ = 0.0007 m³. Upthrust = weight of water displaced = Vρg = 0.0007 × 1,000 × 9.8 = 6.86 N.
• iii.The block floats, so its weight = upthrust = 6.86 N, and its mass = 6.86 ÷ 9.8 = 0.7 kg. Its whole volume = 25 × 8 × 5 = 1,000 cm³ = 0.001 m³, so its density = 0.7 kg ÷ 0.001 m³ = 700 kg/m³.
• iv.It must still displace 0.7 kg of liquid: volume of oil = 0.7 ÷ 920 ≈ 0.000761 m³ = 761 cm³. Depth = 761 cm³ ÷ (25 × 8) cm² ≈ 3.8 cm. That is less than its 5 cm height, so it still floats, but sinks a little deeper than in water because the oil is less dense.

• i.X: the current stays at 3 A in the same direction all the time. Any one source: a cell, a battery or a solar panel.
• ii.One cycle of Y takes 0.025 s, so $f = \dfrac{1}{T} = \dfrac{1}{0.025\ \text{s}} = 40\ \text{Hz}$. It changes direction twice in each cycle: 80 times a second.
• iii.It rises from zero to its peak in a quarter of a cycle: $\tfrac{1}{4} \times 0.025\ \text{s} = 0.00625\ \text{s}$ (about 6 ms).
• iv.Under X the needle turns and stays turned across the wire, because the steady current makes a steady magnetic field. Under Y the field reverses 80 times a second; the needle is too heavy to swing back and forth so fast, and the pushes in the two directions cancel, so it hardly moves from its north–south direction (at most it trembles).

• i.A combination reaction: two substances, nitrogen and hydrogen, combine to form a single new substance, ammonia.
• ii.Iron is a (positive) catalyst: it speeds up the reaction by giving it a path with a lower energy hill. It is not used up or permanently changed, and comes out at the end as it went in, so it is not a reactant and is written above the arrow.
• iii.Both reactants are gases. A high pressure squeezes the gas molecules closer together, so they collide more often, and the rate increases.
• iv.At a higher temperature the molecules move faster, so they collide more often, and, more importantly, a bigger share of the collisions have enough energy (the activation energy) to react.

i. A strong solution of sodium bicarbonate fills the cylinder; a small glass bottle of concentrated sulphuric acid hangs near the top under a plunger; striking the plunger breaks the bottle, and the carbon dioxide made forces the liquid out through the nozzle.

• ii.$$2\text{NaHCO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O} + 2\text{CO}_2$$
• iii.Its jet is mostly water. Water conducts electricity, so on an electrical fire it can give a shock; and oil is lighter than water, so it floats on the water and spreads the fire.