Grade 9 Final Exam Model Paper 2

Mixing gives out a lot of heat. If water is poured into concentrated acid, the first drops boil at once and spit the acid out, which can burn the skin and eyes; adding acid slowly to plenty of water spreads the heat safely.

The acceleration of a body falling freely under the Earth’s gravity; its average value near the surface is 9.8 m/s².

$d = \dfrac{v \times t}{2} = \dfrac{340 \times 0.8}{2} = 136\ \text{m}$

So that each gets the full mains voltage and can be switched on and off on its own; if one stops working, the others keep working.

Noise from lightning and electrical sparks mostly changes the height (amplitude) of a wave. In FM the sound is carried in the frequency, and the receiver ignores the amplitude, so the noise is not heard.

Keep the account private (or leave out the location and the school’s name), because strangers could use them to find her: what is posted publicly stays as part of her digital footprint.

When a burning splint (matchstick) is brought to the mouth of a jar of hydrogen, the gas burns with a squeaky “pop” sound.

Sodium is a very reactive metal: it reacts quickly with the oxygen and the water vapor in air (and violently with water), so it is kept under kerosene, with which it doesn’t react. Gold is very unreactive and doesn’t react with air or water.

Legumes have bacteria in their root nodules that fix nitrogen from the air, so they add nitrogen to the soil for the next crop.

• i.Least count = (60 − 50) mL ÷ 5 = 2 mL.
• ii.Volume = 50 mL + 3 × 2 mL = 56 mL.

Any two:

• a.Every species has one scientific name, used by scientists all over the world, whatever its local names are.
• b.No two species have the same scientific name, so there is no confusion.
• c.The genus name shows which other species it is closely related to.
• d.The names follow fixed rules (genus with a capital letter, species with a small letter, both in italics), so they are easy to recognize.

Her bags are likely to be spoiled by green and black molds and bacteria, and give few or no mushrooms. Straw from the field carries the spores of other fungi and bacteria, and in the warm, damp bag they grow faster than the mushroom spawn. She should boil the straw for about 15–20 minutes (or soak it in hot water) to kill them and soften the straw, then cool it before adding the spawn.

Archaeopteryx had features of two groups:

• a.like a bird, it had feathers and wings;
• b.like a reptile, it had teeth in its jaws, claws on its wings and a long bony tail.

So it links reptiles and birds, and shows that birds evolved from reptiles.

It is a reflex action.
Receptors in the skin of the finger → sensory nerve → spinal cord (connecting neuron) → motor nerve → muscles of the arm (effector), which pull the hand back.

To every action there is an equal and opposite reaction, and the two act on different bodies.
Example (any one): a gun recoils when it fires a bullet; a swimmer pushes the water back and moves forward; a rocket pushes gases down and rises; when you jump off a boat, the boat moves back.

No. The loaded truck has much more mass, so it has more inertia of motion. By $F = ma$, the same braking force gives it a smaller retardation ($a = F/m$), so it takes longer, and goes farther, before it stops. To stop it in the same time, a larger force is needed.

• i.$f = \dfrac{n}{t} = \dfrac{5}{2} = 2.5\ \text{Hz}$
• ii.$v = f \lambda = 2.5 \times 0.4 = 1\ \text{m/s}$

Sirius. The color of a star shows the temperature of its surface: blue stars are the hottest, then white and yellow ones, and red stars are the coolest. Sirius is bluish white, so its surface is much hotter than that of reddish Betelgeuse, a red supergiant.

• a.Transmitter: Muna’s phone. Its microphone takes in her voice (the input), and the phone turns it into a signal fit for sending.
• b.Channel: the radio waves from her phone to the nearest tower, and the cables, optical fibers or microwave links between the towers, which carry the signal to Kathmandu.
• c.Receiver: her brother’s phone, which picks up the signal and turns it back into sound in its earpiece (the output).

Any two:

• a.Not reply or hit back, which usually makes it worse.
• b.Keep a record: save the messages and take screenshots, with dates.
• c.Block the person and report the posts to the app.
• d.Tell a parent or teacher she trusts; if it is serious, report it to the Nepal Police Cyber Bureau.

Symbols: Al, O; valencies 3 and 2. Criss-crossing gives Al₂O₃.
Molecular mass = 2 × 27 + 3 × 16 = 54 + 48 = 102 amu.

No. There are 2 oxygen atoms on the left but only 1 on the right, so the equation breaks the law of conservation of mass: atoms are neither made nor destroyed in a reaction, so each element must have the same number of atoms on both sides. The balanced equation is 2Mg + O₂ → 2MgO (2 Mg and 2 O on each side).

Yes, I agree.

• a.CFCs release chlorine high in the atmosphere, which destroys ozone; HFCs contain no chlorine, so they don’t harm the ozone layer.
• b.But HFCs are strong greenhouse gases that add to global warming, so they too are now being phased out (the Kigali Amendment to the Montreal Protocol) in favor of gases such as isobutane.

• Parenchyma: stores food and water (and, as chlorenchyma with chlorophyll, makes food by photosynthesis).
• Collenchyma: gives the plant flexible support and mechanical strength, in young stems and leaf stalks, so they bend without breaking.
• Sclerenchyma: gives the plant strength and hardness (its dead, thick-walled cells form the fibers and hard shells).
• Xylem: carries water and minerals from the roots up to the leaves (and gives support).

• i.Mutualism (symbiosis): both benefit. The alga makes food by photosynthesis for both, and the fungus gives it shelter, water and minerals.
• ii.Commensalism: the egret benefits (an easy meal of insects), and the buffalo is neither helped nor harmed.
• iii.Competition: both need the same food, which is limited, so each gets less.
• iv.Parasitism: the tapeworm (parasite) benefits by taking digested food from the intestine, and the human (host) is harmed.

• i.Any two: briquettes give less smoke, so the kitchen and the children’s air stay cleaner; they reduce the cutting of firewood and save the forests; they give more heat and are easy to store and carry; they use weeds and crop waste; they give income to the local group.
• ii.They are made from biomass (weeds, husk, crop waste), which grows again every season, so the supply does not run out.
• iii.No. Burning charcoal makes carbon monoxide, a poisonous gas with no color or smell; in a closed room without ventilation it can kill a person, often in their sleep. Briquettes must be used only where there is fresh air.

• i.By echolocation: a bat sends out ultrasound and listens to the echoes that come back from insects and obstacles, which tell it where they are.
• ii.For acoustic protection: soft, rough materials absorb sound instead of reflecting it, so there are no echoes to mix with the sound and spoil it, and the sound in the hall stays clear.
• iii.The ship sends a pulse of ultrasound down to the sea bed and records the time the echo takes to come back. Knowing the speed of sound in water, the depth is worked out as depth = speed × time ÷ 2, because the sound has gone down and up again.

Using $E = P \times n \times t$, with P in kW and t in hours:

• i.Bulbs: $\dfrac{9}{1000} \times 5 \times 6 = 0.27\ \text{kWh}$
• ii.Television: 0.1 × 1 × 5 = 0.5 kWh; rice cooker: 0.6 × 1 × 1.5 = 0.9 kWh; iron: 1 × 1 × 0.5 = 0.5 kWh.

Total in a day = 0.27 + 0.5 + 0.9 + 0.5 = 2.17 kWh

• iii.In a month = 2.17 × 30 = 65.1 units
• iv.Bill = 65.1 × Rs 10 = Rs 651

i. Nuclear fission is the splitting of a heavy, unstable nucleus into two smaller nuclei. When a neutron hits a uranium-235 nucleus, the nucleus splits into two middle-sized nuclei (such as barium and krypton), giving out two or three new neutrons and a great deal of energy. The new neutrons can split more nuclei, a chain reaction.
${}^{235}_{92}\text{U} + {}^{1}_{0}\text{n} \rightarrow {}^{141}_{56}\text{Ba} + {}^{92}_{36}\text{Kr} + 3\,{}^{1}_{0}\text{n} + \text{energy}$
ii. Nuclear fusion is the joining of light nuclei to make a heavier nucleus. In the Sun’s core, at a temperature of millions of degrees and a huge pressure, four hydrogen nuclei join to make one helium nucleus. The helium formed is slightly lighter than the four hydrogen nuclei, and the lost mass becomes energy (E = mc²), which the Sun gives out as light and heat.

• i.Any two:
• a.Shorts and sandals: he should wear protective clothing (gloves, long sleeves, boots, a mask and goggles).
• b.Spraying on a windy day, into the wind: the spray will blow into his face; he should spray on a calm day, with the wind behind him.
• c.Selling the next morning: he should wait the number of days given on the label before harvesting, so the poison breaks down and doesn’t reach people’s food.
• ii.Jholmol: chop strong-smelling local plants (such as neem, timur, asuro, titepati, garlic), mix them with cow urine and a little dung, keep it covered for about 15 days, then mix one part with three to five parts of water and spray it. (Neem leaves or ash also work.)
• iii.DDT does not break down for years. It pollutes soil and water, kills useful insects and fish, and builds up along food chains, harming birds and fish and human health.