SEE Model Paper 7

$$\text{unit of power} = \frac{\text{unit of work}}{\text{unit of time}} = \frac{\text{kg m}^2\,\text{s}^{-2}}{\text{s}} = \text{kg m}^2\,\text{s}^{-3}$$
So 1 W = 1 kg m² s⁻³.

Fungi (eukaryotic, with a cell wall but no chlorophyll; they feed on dead and decaying matter).

Mitosis. Before each division the DNA of every chromosome is copied, and each daughter cell gets one copy, so the new cells have the same chromosomes as the old skin cells. Mitosis is the division for growth and repair.

The forces are equal, but the Earth’s mass is enormous, so by a = F/m the same force gives it an acceleration far too small to notice; the mango, with a tiny mass, gets an acceleration of 9.8 m/s². (The Earth does move, but by much less than the width of an atom.)

The pressure of a liquid increases with depth (P = hρg), so the water pushes hardest on the bottom of the wall. A thicker base can withstand that greater pressure.

$$\mu = \frac{\text{real depth}}{\text{apparent depth}} \quad\Rightarrow\quad \text{real depth} = 1.33 \times 0.9\ \text{m} = 1.2\ \text{m}$$
The pond is about 1.2 m deep, deeper than it looks.

The phone’s battery and electronic circuits need direct current, but the socket gives alternating current. The charger contains a rectifier, which changes a.c. into d.c. (and it also lowers the voltage).

Limestone is a flux. In the furnace it breaks down into lime (CaO), which combines with the sand to form a glassy waste, slag (CaO + SiO₂ → CaSiO₃). The molten slag floats on the molten iron and is drained off separately, so the sand is removed.

Propane-1,2,3-triol, C₃H₅(OH)₃: a trihydric alcohol with one –OH group on each carbon of propane.

• i.The variety and the watering are controlled variables. Only the independent variable, the amount of compost, should change; if anything else differed too, he could not tell whether a difference in yield came from the compost or from the other change.
• ii.One plant may do well or badly for reasons that have nothing to do with the compost: a weak seed potato, a disease, a pest, a wetter corner of the field. With many plants in each plot he can take the average yield, so a single odd plant can’t mislead him, and the result is more reliable.

• i.8 GB = 8 × 1024 MB = 8,192 MB. Time = 8,192 MB ÷ 100 MB per minute ≈ 81.9 minutes, so about 81 minutes (nearly 82).
• ii.A video is about 30 complete pictures every second, each made of millions of colored dots stored as bytes, plus the sound. A song is the sound alone, so a minute of video holds far more information.

• i.He should use them only with the permission of the people who made them (or use ones whose makers allow it), and cite the source. Songs, photos and videos online are the intellectual property of the people who made them.
• ii.To protect his privacy and security. Anything posted online can be seen and copied by strangers and stays for years, and someone could use his number or address to cheat, trouble or harm him. A student’s public details should be few: his name is enough.

Any two:

• a.The body is covered by a moist skin without scales.
• b.The young (tadpoles) live in water and breathe with gills; the adults breathe with lungs and their moist skin.
• c.The heart has three chambers: two atria and one ventricle.
• d.They are poikilothermic (cold-blooded).
• e.They lay their eggs in water; in frogs and toads fertilization is external.
• f.They have two pairs of limbs, and the body is divided into head and trunk (some have a tail).

• i.Bees make honey from the nectar of flowers. Mustard flowers in winter, and litchi and chiuri flower later, so by following the flowers his bees always have nectar and pollen to collect, and he gets more honey all through the year.
• ii.Any one: it is Nepal’s own bee, at home in the hills; it lives in hollows, so it can be kept in traditional log and wall hives; it is gentle; it is good at defending itself against mites and hornets.

Any two:

• a.It is the best way for couples to have a child when they can’t conceive (blocked tubes, low sperm count, unexplained infertility).
• b.It can use the couple’s own eggs and sperm, or a donor’s.
• c.It is more successful than the other assisted reproductive techniques.
• d.Embryos can be tested for some chromosomal and genetic disorders before they are placed in the uterus, so parents who carry an inherited disease can have a healthy child.

• i.$\dfrac{450\ \text{mL}}{5000\ \text{mL}} \times 100\% = 9\%$: less than a tenth of his blood.
• ii.His body replaces the lost plasma within a day or two, but it takes weeks for the red bone marrow to make enough new red blood cells. Waiting three months lets his red cells (and hemoglobin) come back to normal, so that giving blood again does not make him anemic.

• i.At Base Camp the air has only about half as much oxygen in each breath as at sea level. More red cells, with more hemoglobin, let his blood carry more oxygen to his cells: this is part of acclimatization.
• ii.Many Sherpas carry gene variants (such as EPAS1) that let their bodies use the oxygen they have more efficiently, so they need no extra red cells. Too many red cells (polycythemia) make the blood thick, so it flows slowly and the heart must work harder.

• i.$$F = \frac{G m_1 m_2}{d^2} = \frac{6.67 \times 10^{-11} \times 1.99 \times 10^{30} \times 5.97 \times 10^{24}}{(1.5 \times 10^{11})^2} = \frac{79.24 \times 10^{43}}{2.25 \times 10^{22}} = 3.52 \times 10^{22}\ \text{N}$$
• ii.$$a = \frac{F}{m} = \frac{3.52 \times 10^{22}\ \text{N}}{5.97 \times 10^{24}\ \text{kg}} = 5.9 \times 10^{-3}\ \text{m/s}^2$$

This small acceleration, always towards the Sun, bends the Earth’s path into its orbit.

• i.The eye keeps an image for about a tenth of a second. When the disc spins fast, each spot of the retina receives all seven colors one after another, faster than the eye can tell them apart, so the colors add up and the disc looks white, as the seven colors of sunlight together make white light.
• ii.Paints are not pure spectrum colors, and each painted sector reflects only part of the light falling on it, so the mixture is dimmer than white light: grayish-white.

• i.A run-of-river station uses the river’s flow as it comes. In winter the rivers carry much less water, so less water turns the turbines and less electricity is made. Kulekhani stores monsoon water in a reservoir behind its dam and uses it to make electricity in the dry months.
• ii.Any one: hydropower burns no fuel, so it gives out (almost) no carbon dioxide and causes no climate change or air pollution; it uses Nepal’s own renewable rivers, while diesel must be imported and paid for and will run out.

• i.1s², 2s² 2p³ (shells 2, 5). Five electrons in the outermost shell, the second shell: group VA (15), period 2. (X is nitrogen.)
• ii.It needs three more electrons to complete its octet, so its valency is 3. With hydrogen (valency 1) it forms XH₃ (ammonia, NH₃).

• i.Ethane, C₂H₆, minus one hydrogen gives the ethyl radical, C₂H₅–. Joined to –OH it gives ethanol, C₂H₅OH (ethyl alcohol).
• ii.The general formula is CₙH₂ₙ₊₁OH. For n = 3, 2n + 1 = 7, so the alcohol is C₃H₇OH (propanol).

• i.Plants can use only part of the urea spread. Much of the rest washes down into the ground and into wells as nitrate, which harms babies who drink the water; and too much urea, year after year, makes the soil acidic and hard.
• ii.Any two: use organic manure or compost together with less urea; use fertilizer only as advised by an agricultural expert, after a soil test; spread only as much as the crop needs, at the right time.

• i.A hybrid is not pure: it carries different alleles for many characters. When its seed is sown (like Mendel’s F₂), the alleles separate and recombine, so the offspring split up into different types. The qualities of a hybrid are not passed on unchanged to the next generation, which is why hybrid seed must be bought fresh each year.
• ii.The male chauri are sterile (only the females are fertile), so chauri can’t be bred with each other; each new chauri must come from crossing a yak with a cow (or a bull).
• iii.Any one: the calves combine the desired qualities of both parents, such as the Jersey’s high milk yield and the local cow’s strength in the local conditions; the calves may be better than either parent (hybrid vigor); with artificial insemination she needs no expensive bull of her own.
• iv.The local breeds carry genes, such as hardiness in the hills and resistance to local diseases, that crossbreds may lose. If they die out, those genes are lost for ever, biodiversity falls, and herds of genetically alike animals could all be struck by one disease or be unable to cope with future changes.

• i.No. One cold winter is weather, the state of the air at one place and time. Climate is the average weather of a place over a long time, about 30 years. Records show that Nepal’s average temperature has been rising (by about 0.06 °C a year since the 1970s), so the climate is changing even if one winter is cold.
• ii.The climate has become warmer, so the lower orchards are now too warm for apples, and growers move them higher up, where it is cooler. Many plants and animals in Nepal are being found higher up the mountains for the same reason.
• iii.As the climate warms, the mosquitoes that carry dengue can now live and breed in the warmer hill towns, such as Kathmandu, where it used to be too cold for them.
• iv.Any one, with how it helps: cook on an electric cooker or induction stove powered by Nepal’s hydropower instead of LPG or firewood (less carbon dioxide released); plant trees and protect the forest (trees take in carbon dioxide); walk, cycle or use public transport for short trips (less fuel burnt); switch off lights and use LED bulbs (less energy used).

• i.The principle of calorimetry (principle of mixtures): when a hot body and a cold body are in contact, heat lost by the hot body = heat gained by the cold body, if no heat escapes.
• ii.Let the final temperature be T.

$$m_1 s_1 (450 - T) = m_2 s_2 (T - 20)$$
$$0.5 \times 460 \times (450 - T) = 2.3 \times 4200 \times (T - 20)$$
$$230(450 - T) = 9660(T - 20)$$
$$103{,}500 - 230T = 9660T - 193{,}200$$
$$9890T = 296{,}700 \quad\Rightarrow\quad T = 30\ °\text{C}$$
The sickle and the water both end at 30 °C. (Check: the sickle gives out 230 × 420 = 96,600 J, and the water takes in 9660 × 10 = 96,600 J.)
iii. The water has a much larger mass (2.3 kg against 0.5 kg) and a much higher specific heat capacity (4,200 against 460 J/kg °C), so it needs about 42 times as much heat as the sickle for each degree its temperature changes. The same heat therefore changes the water’s temperature far less.

i. A cataract: as people grow old, the proteins of the eye lens can clump together, so the lens becomes cloudy; light can’t pass cleanly to the retina, and in the end the person can become blind. It is treated by surgery: through a tiny cut, the cloudy lens is broken up and removed, and a small clear artificial lens, an intraocular lens (IOL), is put in its place. (Nepal’s Tilganga Institute makes such lenses cheaply.)

• ii.Color blindness is the inability to tell certain colors apart (most often red and green) because some cone cells of the retina don’t work properly. It is mostly inherited, and the genes for the red and green cones are on the X chromosome: a boy has only one X, so one faulty gene makes him color-blind, while a girl would need it on both her X chromosomes.
• iii.Corneal transplantation is replacing a damaged cornea with a healthy one by surgery. The healthy corneas come from people who pledged to donate their eyes after death; in Nepal the Nepal Eye Bank at the Tilganga Institute of Ophthalmology collects, stores and distributes them.

• i.It is a meteor: a grain of space rock burning up as it rushes through our air, not a star. A meteorite is a piece of space rock that survives the fall and lands on the ground.
• ii.Andromeda is 2.5 million light-years away, so its light has taken 2.5 million years to reach us. We see the light that left it then, so we see it as it was then: looking far away means looking back in time.
• iii.The Moon is moving sideways along its orbit at about 1 km/s. The Earth’s pull bends its path into a curve, so it is falling towards the Earth all the time but keeps missing it, going round instead of falling in.
• iv.The stars of the Milky Way are held together by the gravity of the galaxy. The expansion carries apart only distant groups of galaxies, between which gravity is too weak to hold them together: it is the space between them that stretches.

i. $$\text{Mg(OH)}_2 + 2\text{HCl} \rightarrow \text{MgCl}_2 + 2\text{H}_2\text{O}$$
An acid–base (neutralization) reaction: a base and an acid form a salt and water.
ii. $$2\text{NaHCO}_3 \xrightarrow{\Delta} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2$$
A decomposition (thermal decomposition) reaction: one reactant breaks into three products on heating.

• iii.Saliva contains an enzyme, amylase, which turns the starch of the rice into sugar. An enzyme is a biological catalyst, so this shows the effect of a catalyst on the rate of reaction.
• iv.Inside the closed cooker the pressure is higher, so the water boils at about 120 °C instead of below 100 °C. At this higher temperature the reactions that soften the food go much faster.

• i.Ammonia is a refrigerant: liquid ammonia takes in a large amount of heat from its surroundings when it evaporates, so it keeps the store cold.
• ii.Ammonia is extremely soluble in water (about 700 liters of the gas dissolve in one liter of water at 20 °C), so the water in the cloth dissolves much of the ammonia before it reaches his nose and lungs.
• iii.Thick white fumes appear at the leak: tiny specks of solid ammonium chloride, formed where the ammonia meets the hydrogen chloride from the rod.

$$\text{NH}_3\text{(g)} + \text{HCl(g)} \rightarrow \text{NH}_4\text{Cl(s)}$$
iv. Ammonia is poisonous if it leaks, which would be dangerous in a home; home fridges use other gases (such as isobutane or R-134a) instead.