Science, about class 7

Mei cuts a thin slice of carrot and looks at one of its cells under a microscope. Which parts does the cell have?

1. AA cell wall, a nucleus and chloroplasts
2. BA nucleus and a membrane, but no cell wall
3. CA cell wall and a nucleus, but no chloroplasts
4. DA cell wall and chloroplasts, but no nucleus

In a pond, snails and tadpoles both eat algae. Fish eat the snails and the tadpoles. A disease kills all the fish. What is most likely to happen next?

1. AAt first there are more snails and tadpoles. Later they may eat the algae down and go hungry.
2. BThe snails and tadpoles are better off from now on, because nothing eats them any more.
3. CThe snails and tadpoles die out, because the fish were feeding them with their food.
4. DNothing changes, because snails and tadpoles do not eat fish at all.

In one lung illness, the walls of the air sacs grow thick. Why does a person with this illness get out of breath sooner?

1. AThe thick walls are muscles that squeeze the air out too soon.
2. BOxygen cannot cross any wall, so the blood gets no oxygen at all.
3. CThe blood takes in carbon dioxide from the air sacs instead of oxygen.
4. DGases cross thick walls slowly, so less oxygen gets into the blood.

For 3 weeks Ravi has felt sad on most days. He sleeps badly and no longer enjoys games he used to like. What should he do?

1. AWait, because moods in puberty change fast and this will pass by itself.
2. BTell a trusted adult or a health worker now. If one does not help, ask another.
3. CKeep it to himself, because these feelings are private and nobody can help.
4. DAsk a stranger online, because a stranger will not tell his family.

Sperm are made in a testis. Which list gives, in order, the parts they pass through on the way out of the body?

1. ASperm duct, bladder, urethra
2. BEpididymis, oviduct, urethra
3. CUreter, sperm duct, urethra
4. DEpididymis, sperm duct, urethra

Wax X melts at 55 °C and boils at 330 °C. An adult melts some in a bowl over boiling water. The wax reaches 90 °C. What state is it in, and why?

1. AA liquid, because it is above its melting point but below its boiling point.
2. BA gas, because it is heated by boiling water.
3. CA solid, because it is below its boiling point.
4. DA liquid, because it is above its boiling point.

Ana cycles along a flat, straight road at a steady speed. The forward force on her bike is 16 N. What is the total of friction and air resistance pulling back?

1. A0 N
2. B32 N
3. C16 N
4. D8 N

Kofi sits 2 m from a campfire on a cold night. His face feels warm, but the air around him is cool. How does the heat reach his face?

1. ABy conduction, along the air between him and the fire
2. BBy radiation, as infrared from the fire
3. CBy convection, as warm air rises from the fire to his face
4. DIt does not travel, because his face makes its own warmth

In a dark room, a lamp gives out only green light. Ana holds up a white scarf, a green scarf and a red scarf. Which scarf looks black?

1. AThe white scarf
2. BThe green scarf
3. CNone of them, because the lamp lights every scarf the same
4. DThe red scarf

Lucía lives close to the equator. She compares the length of the day in June with the length of the day in December. What does she find?

1. ABoth days are about the same length.
2. BThe June day is much longer than the December day.
3. CThe December day is much longer than the June day.
4. DOne of the two days has no night at all.

Layers of sandstone are buried very deep. Over millions of years, heat and pressure bake and squeeze them, but they do not melt. What do they become?

1. AIgneous rock
2. BMetamorphic rock
3. CSedimentary rock
4. DThey stay as sandstone, because rock never changes

Ravi has a jar of sand mixed with salt. He wants to get the sand and the salt back apart. Which steps work?

1. AAdd water and stir, filter out the salt, then let the water in the filtrate evaporate.
2. BFilter the dry mixture, then let the filtrate evaporate.
3. CAdd water and stir, filter out the sand, then let the water in the filtrate evaporate.
4. DAdd water and stir, then let all the water evaporate.

A drop of water sits on a clear plastic sheet that lies on a page of small print. The letters under the drop look bigger. Why?

1. AThe drop is curved and thicker in the middle, so it bends light like a converging lens.
2. BThe drop is thinner in the middle, so it spreads the rays like a diverging lens.
3. CThe water makes the light brighter, and brighter letters always look bigger.
4. DThe water soaks into the page and makes each letter swell up.

A copper pan is heated, and a dark layer forms on it. Copper joins with oxygen from the air and makes copper oxide. Which word equation is right?

1. Acopper oxide → copper + oxygen
2. Bcopper + oxygen → copper oxide
3. Ccopper + copper oxide → oxygen
4. Dcopper → oxygen + copper oxide

A plant has a fault that stops its root cells from growing long hairs. The plant wilts in dry soil much sooner than a normal plant. Explain why.

A pond food chain is algae, snail, fish and heron. The algae hold 8 000 kJ of energy. About a tenth of the energy passes on at each of the 3 steps. About how much energy reaches the heron?

Sam puts his hands on his ribs and breathes out slowly. His chest gets smaller. Explain how this pushes air out of his lungs.

Lucía marks the first day of each period on a calendar. One period starts on 2 May. May has 31 days. Her next period starts on 2 June. About which day of her cycle does an ovary let go of an egg?

Mei cuts an onion in a still kitchen. Someone at the far end smells it sooner on a hot day than on a cool day. Explain why, using particles.

A spring is 12 cm long with no load. Ana hangs 4 stones on it and it is 20 cm long. The spring stretches by equal amounts for each stone, and it is not overstretched. How long is it with 7 stones? In a real test, keep your feet clear of the stones.

A rover has a mass of 150 kg. On Earth each kilogram weighs about 10 N. On the Moon each kilogram weighs about 1.6 N. How much less does the rover weigh on the Moon than on Earth?

An electric kettle takes in 120 kJ of energy. The water in it gains 102 kJ. The rest warms the kettle and the air. What is the efficiency of the kettle?

Rain fills a crack in a boulder with 350 cm³ of water. When the water freezes, every 100 cm³ of it makes 109 cm³ of ice. By how much is the ice bigger than the water was?

The Earth is a little nearer the Sun in early January than in early July. Kofi lives in the northern half of the Earth, where January is cold. Explain why it is cold there in January.

(a)Ana joins 4 of these bulbs, each on its own branch across the battery. This is a parallel circuit. How many amps does the battery give? [2 marks]

(b)Ana takes the bulbs away. She joins one bulb of 10 Ω and a dimmer set to 20 Ω in one loop with the battery. What is the current? [2 marks]

(c)Ana now joins the same bulb and dimmer in parallel, each on its own branch across the battery. Explain why the bulb is brighter than in part b. [2 marks]