SEE Model Paper 6

A digital-to-analog converter (DAC).

Because its body is covered with hair, not feathers, and it gives birth to its young and feeds them milk from its mammary glands. Its wings are thin skin stretched between very long fingers.

Transfer RNA (tRNA).

A wide strap spreads the load’s weight over a larger area of the forehead, so the pressure $\left(P = \dfrac{F}{A}\right)$ on it is smaller and it hurts less.

$T = t + 273 = 37 + 273 = 310\ \text{K}$

A south pole. With the right-hand grip rule, the fingers curl clockwise as seen from that end, so the thumb points away from her, to the far end: the far end is the north pole. (Clockwise current at an end means S; anticlockwise means N.)

The galaxies are not flying apart through space: space itself is expanding, like the rubber between the dots, and carries them apart. (From any galaxy, all the others seem to move away; there is no center.)

Carbon dioxide is dissolved in the drink under high pressure. When the cap is opened, the pressure drops, the gas can no longer stay dissolved, and it escapes as bubbles.

No. Methylated spirit is ethanol with methanol added precisely so that nobody drinks it. Methanol is a poison: the body turns it into formaldehyde and formic acid, and as little as 10 mL can cause permanent blindness, and about 30 mL can kill.

• i.5 V is a high signal, 1, and 0 V a low signal, 0, so the signal is 10110010 (eight bits, one byte).
• ii.The voltage would rise and fall smoothly through every value between −V and +V, a wave with no flat steps. It would be an analog signal; her switched d.c. signal, with only two values that it jumps between (a square wave), is digital.

• i.X is a monocot (parallel venation, fibrous roots). Y is a dicot (reticulate venation, taproot).
• ii.Any one for each: X’s seed has one cotyledon, or the parts of its flower are in threes; Y’s seed has two cotyledons (it splits into two halves), or the parts of its flower are in fours or fives.

Any two:

• a.Its flowers stay closed and pollinate themselves, so the plants stay pure; they can also be cross-pollinated by hand.
• b.Its life cycle is short, so many generations can be studied quickly.
• c.It has many clear pairs of contrasting characters (tall or dwarf, round or wrinkled seed).
• d.Each plant gives many seeds, so there are many offspring to count.
• e.It is easy to grow.

• i.When one of the parents’ gametes was made by meiosis, the two chromosomes of pair 21 failed to separate, so that gamete got an extra chromosome 21 (24 instead of 23). At fertilization, 24 + 23 gave a zygote with 47.
• ii.The gametes would have 46 chromosomes, like body cells, so the zygote would have 46 + 46 = 92, and the number would double every generation (46, 92, 184 …).

Angiography is a test that shows the blood vessels: a dye that X-rays can’t pass through is put into the blood through a thin tube guided into the coronary arteries, and a live X-ray video is taken, so any narrowing or blockage shows up.
Treatment, any one: angioplasty and stenting (a balloon on a catheter squashes the plaque and a metal-mesh stent holds the artery open), or coronary artery bypass surgery (a healthy vessel is sewn on to carry blood round the blockage).

• i.It is a fungus that grows inside a caterpillar in the soil, feeds on it and kills it. In winter it is seen as the dead caterpillar (the “worm”); in spring and summer the fungus’s brown stalk grows up out of the caterpillar’s head, above the ground, like a blade of grass.
• ii.So much is collected that it is now listed as vulnerable. If the stalks are all picked before they release their spores, few new caterpillars are infected, and it could disappear (along with the income it gives mountain families).

• i.$$t = \frac{12 - 3}{23 - 3} \times 100 = \frac{9}{20} \times 100 = 45\ °\text{C}$$
• ii.The upper fixed point is the steam above water boiling at one atmosphere of pressure. The air pressure in Kathmandu is lower, so water there boils at only about 95 °C. His “100 °C” mark is really at about 95 °C, so every reading above 0 °C is too high (the water was really about 43 °C).

Near (the book): the ciliary muscles contract, the suspensory ligaments slacken, and the eye lens bulges thicker: its focal length becomes shorter, so it bends the spreading rays from the book strongly enough to focus them on the retina.

Far (the mountain): the ciliary muscles relax, and the lens is pulled thin: its focal length becomes longer, just right to focus the nearly parallel rays on the retina. This adjustment is called accommodation.

• i.He put the focal length in centimeters; it must be in meters. f = 20 cm = 0.2 m, so $P = \dfrac{1}{0.2\ \text{m}} = +5\ \text{D}$.
• ii.Rojan’s lens. Its power is greater (+5 D against +2 D) and its focal length shorter (20 cm against 50 cm); a more strongly curved, thicker lens bends light more.

• i.No light will come out into the air: all of it is reflected back into the water, obeying the law of reflection. This is total internal reflection; 60° is greater than the critical angle of water, 49°.
• ii.Total internal reflection happens only when light goes from a denser to a rarer medium. From air into water the light goes from rarer to denser, so it bends towards the normal; its angle of refraction is always less than the angle of incidence and can never reach 90°, so some light always enters the water.

i. It is a (positive) catalyst: it speeds up the decomposition of potassium chlorate without being changed itself. Her results show both: B gave oxygen much faster, and the same 1 g of black manganese dioxide was got back at the end.
$$\text{2KClO}_3 \xrightarrow{\text{MnO}_2,\ \Delta} \text{2KCl} + \text{3O}_2$$
ii. No. The oxygen comes only from the potassium chlorate, and both tubes had the same 5 g of it. A catalyst changes only the rate of a reaction, not how much product it gives (it isn’t used up), so B gives the same amount of oxygen, only sooner.

• i.Sunlight passes through the clear plastic and warms the soil and plants, which give out long-wave infrared. The plastic stops the warm air from blowing away and mixing with the cold air outside, and it holds back some of the infrared, so the heat stays inside (the artificial greenhouse effect).
• ii.Any one: plants of any season can be grown all year round; summer crops can be grown in winter or in cold places; flowers, leafy vegetables, fruits and seedlings can be produced; plants of cold places can be kept alive; farmers can earn more by selling vegetables out of season.

• i.The yeast in marcha turns sugar into ethanol and carbon dioxide only without air (C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂). With air, other microbes would turn the alcohol into vinegar.
• ii.Ethanol boils at 78 °C, below water’s 100 °C, so when the brew is heated, its vapor is richer in alcohol than the brew; condensing that vapor gives a liquid with more alcohol.

• i.No. Wood ash is alkaline: it contains potassium carbonate (potash), which turns grease into a kind of soap that washes away; the paste also scrubs.
• ii.Lemon juice is acidic: its citric acid dissolves the dull green-black layer of copper compounds on the brass, so the shiny metal shows.

• i.Independent: the height from which the car is let go (what he changes). Dependent: the distance the car rolls along the floor (what he measures).
• ii.In step 2 he changed the floor (carpet instead of cement), and in step 3 he pushed the car, so it did not start from rest. The kind of floor and the starting speed should both have been controlled (kept the same).
• iii.No. Three things changed at once (the height, the floor and the push), so he can’t tell which one caused each distance: the carpet slowed the car at 20 cm, and the push sped it up at 30 cm. It was not a fair test, so it can’t show whether height matters.
• iv.Use the same car, the same ramp and the same floor every time, and let the car go from rest each time, changing only the height (say 10, 20 and 30 cm). Repeat each height three times and take the average distance. The height, the independent variable, goes on the horizontal x-axis, and the distance up the vertical y-axis.

• i.A forager sucks up nectar, mostly water (often 60–80%) with sucrose, into her honey stomach. On the way home and in the hive, the bees add an enzyme (invertase) that splits the sucrose into the simpler sugars glucose and fructose. House bees spread the nectar thinly in the cells and fan it with their wings, so that the water evaporates. When the water has dropped below about 18%, they seal the cell with a wax cap.
• ii.The bees had not finished drying it: honey in unsealed cells still had too much water. Watery honey ferments, which made it frothy and sour.
• iii.Ripe 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 makes a little hydrogen peroxide, a germ killer.

i. From the top: plasma (a pale yellow liquid, about 55% of the blood); a thin buffy coat of white blood cells and platelets (less than 1%); and the red blood cells at the bottom (about 45%), the heaviest part.

• ii.The platelets stuck together and released chemicals that, step by step, turned the soluble plasma protein fibrinogen into long threads of fibrin. The fibrin threads made a net that trapped the red blood cells: a clot.
• iii.It is serum: plasma with its fibrinogen removed (used up in making the clot). So, unlike plasma, serum cannot clot.
• iv.No. A normal count is 1.5 to 4.5 lakh platelets per mm³; 40,000 is far too low. Bleeding from too few platelets is thrombocytopenia, which dengue causes. Hemophilia is an inherited disease in which a clotting protein is missing, while the platelets are normal.

• i.The balance read zero, and the stone stopped pulling on its thread: the thread went loose, and the stone stayed where it was in the frame, as if floating (weightlessness).
• ii.u = 0, h = 2.5 m. $v^2 = u^2 + 2gh = 0 + 2 \times 9.8 \times 2.5 = 49$, so $v = \sqrt{49} = 7\ \text{m/s}$.
• iii.$v = u + gt \Rightarrow t = \dfrac{v - u}{g} = \dfrac{7 - 0}{9.8} = 0.71\ \text{s}$
• iv.No. Gravity kept pulling all the time; it is what made everything fall. The balance, the frame and the stone all fell together with the same acceleration g, so nothing pulled on anything else. What became zero is the apparent weight, not the force of gravity.

• i.Pushed in: the needle moves to one side. Held still: it comes back to zero. Pulled out: it moves to the other side.
• ii.An e.m.f. is induced only while the magnetic flux linked with the coil changes. A moving magnet changes the number of field lines through the coil; a still one gives a steady flux, so no e.m.f. is induced and no current flows (electromagnetic induction).
• iii.When there is relative motion between a conductor and a magnet, an e.m.f. is induced in the conductor, and its size is directly proportional to the rate of change of the magnetic flux linked with it.
• iv.Any two: use a coil with more turns; use a stronger magnet; move the magnet faster.

• i.Magnesium conducts well: a metal. Sulphur doesn’t conduct: a non-metal. Silicon conducts, but much less than a metal and more than a non-metal: a metalloid.
• ii.1s², 2s² 2p⁶, 3s² 3p² (shells 2, 8, 4). Four electrons in the third shell: group IVA (14), period 3.
• iii.Because it is a semiconductor: as a metalloid it conducts electricity less than metals and more than non-metals, which is what the tiny switches (transistors) in chips need.
• iv.With four valence electrons it shares four: valency 4. Chlorine has valency 1, so the formula is SiCl₄.

A block of impure copper is the anode, joined to the positive terminal of the battery; a thin sheet of pure copper is the cathode, joined to the negative terminal. Both hang in copper sulphate solution (the electrolyte) in a voltameter. Copper leaves the anode as ions and is deposited on the cathode; the impurities stay in the solution or fall below the anode as anode mud.
$$\text{at the anode: } \text{Cu} \rightarrow \text{Cu}^{2+} + \text{2e}^- \qquad \text{at the cathode: } \text{Cu}^{2+} + \text{2e}^- \rightarrow \text{Cu}$$
Reason: electro-refined copper is very pure (about 99.99%), and electric wires need such pure copper, because even tiny impurities make copper conduct much worse.