CBSE Class 7 Science Question Paper Set 31 with Solutions

Answer: (d) m/s. Speed is distance divided by time, and the SI units are metre for distance and second for time.

Teacher's Note:
1. Always link the unit of a quantity to the units of the quantities in its formula.
2. Many students pick km/h because it is common in daily life, but that is not the SI unit.

Answer: Neutralisation. The acid and base cancel each other's effect and form salt and water.

Teacher's Note:
1. Neutralisation gives a salt and water as products.
2. Do not confuse it with displacement or decomposition reactions.

Answer: Fertilisation. The male gamete joins the female gamete (egg) to form a zygote.

Teacher's Note:
1. Fertilisation forms the zygote, which grows into the embryo.
2. Do not confuse it with pollination, which is only the transfer of pollen to the stigma.

Answer: Arteries. They carry oxygenated blood away from the heart (the pulmonary artery is the exception).

Teacher's Note:
1. Remember: arteries take blood Away from the heart.
2. Veins bring blood back to the heart, usually carrying less oxygen.

Answer: Oscillatory (periodic) motion. The bob moves to and fro about its mean position repeating after equal intervals of time.

Teacher's Note:
1. A to-and-fro motion that repeats is oscillatory, and it is also periodic.
2. A pendulum is not circular or rectilinear motion.

Answer: Fragmentation (a type of asexual reproduction). Algae such as Spirogyra break into pieces, and each piece grows into a new algal filament.

Teacher's Note:
1. Rapid spread from a single parent without gametes points to asexual reproduction.
2. Algae can also spread by spores, but fragmentation is the standard textbook answer.

Answer: Yeast (also some bacteria). Yeast can respire without oxygen and produces alcohol and carbon dioxide.

Teacher's Note:
1. Anaerobic means living or respiring without oxygen.
2. Yeast is the classic example used in making bread and alcohol.

Answer: True. Gills take in the oxygen dissolved in water and give out carbon dioxide.

Teacher's Note:
1. Gills are the breathing organs of fishes.
2. Animals living in water do not always have gills; dolphins and whales breathe with lungs.

Answer: False. A speedometer measures the speed of a vehicle; the odometer measures the distance moved.

Teacher's Note:
1. Speedometer is for speed and odometer is for distance travelled.
2. Learn the two instruments as a pair to avoid mixing them up.

Answer: False. Potato reproduces by vegetative propagation, as new plants grow from the buds (eyes) of the tuber.

Teacher's Note:
1. Potato is a stem tuber and its eyes are buds that grow into new plants.
2. Most plants grown by seeds are sexual reproduction; vegetative parts give asexual reproduction.

Answer: 1. (a) Physical change: only the shape changes and no new substance forms (aluminium is malleable).
2. (b) Chemical change: food is broken down into new substances.
3. (c) Physical change: only the size and shape of the wood change.
4. (d) Chemical change: burning produces new substances such as gases, heat and light.

Teacher's Note:
1. A chemical change forms a new substance and is usually not easily reversed.
2. Burning and digestion are chemical, while changes in shape or size are physical.

Answer: 1. Distance = 54 km; time = 90 min = 1.5 h.
2. Speed = distance ÷ time = 54 ÷ 1.5 = 36.
3. Speed of the bus = 36 km/h (= 10 m/s).

Teacher's Note:
1. Convert the time to hours first so that the units match km/h.
2. Writing the unit in the final answer is part of getting the marks.

Answer: 1. Aerobic respiration takes place in the presence of oxygen, while anaerobic respiration takes place without oxygen.
2. In aerobic respiration glucose is fully broken down into carbon dioxide and water, releasing a large amount of energy; in anaerobic respiration glucose is broken down incompletely (for example into alcohol and carbon dioxide in yeast, or lactic acid in muscles), releasing much less energy.

Teacher's Note:
1. Compare on at least two points: oxygen needed and amount of energy released.
2. Mention an example of each, such as muscles during heavy exercise for anaerobic respiration.

Answer: 1. (a) Unisexual flowers (only stamens or only pistil): papaya, cucumber, corn.
2. (b) Bisexual flowers (both stamens and pistil): rose, mustard, lady's finger (petunia and pea are also correct).

Teacher's Note:
1. A unisexual flower has either stamens or pistil, but a bisexual flower has both.
2. Any three correct plants from each group earn full marks.

Answer: 1. (a) Xylem carries water and minerals from the roots to the leaves.
2. (b) Phloem carries the food made in the leaves to other parts of the plant.

Teacher's Note:
1. Both tissues together form the transport system of the plant.
2. Memory trick: Phloem carries food (P and F go together).

Answer: 1. The image is virtual (it cannot be obtained on a screen) and erect (upright).
2. The image is of the same size as the object.
3. The image is formed as far behind the mirror as the object is in front of it.
(Also: the image is laterally inverted, i.e. left and right appear interchanged.)

Teacher's Note:
1. Think of your own image in a mirror: same size, upright, and as far behind as you stand in front.
2. Do not forget the term laterally inverted, which is often asked.

Answer: 1. An electric fuse is a safety device containing a thin wire of a material with a low melting point. It melts and breaks the circuit when too much current flows, protecting appliances from damage and fire.
2. Closed circuit: the path is complete, so current flows and the bulb glows.
3. Open circuit: the path has a break (for example, the switch is off), so current cannot flow and the bulb does not glow.

Teacher's Note:
1. A fuse works by the heating effect of current, and it is always connected in series.
2. In an open circuit the break can be anywhere, not only at the switch.

Answer: 1. Animal A is a frog (an amphibian): lungs on land and moist skin in water.
2. Animal B is a fish: it has gills to take dissolved oxygen from water.
3. Animal C is a whale (or dolphin): it is a mammal that breathes air through nostrils (blowhole) into lungs.

Teacher's Note:
1. Link each animal to its breathing organ: skin and lungs, gills, or lungs only.
2. A whale lives in water but is not a fish, since it breathes air.

Answer: 1. (a) Acetic acid is found in vinegar.
2. (b) Lactic acid is found in curd (sour milk).
3. (c) Calcium hydroxide is found in lime water (slaked lime, used in whitewash).
4. (d) Sodium hydroxide is found in caustic soda, used in making soap.

Teacher's Note:
1. Link each acid or base to a daily-life substance; this is a very commonly asked pattern.
2. Sour taste means acid and bitter or soapy feel means base, but never taste unknown chemicals.

Answer: 1. (a) Rusting needs both oxygen (air) and moisture (water). If either is absent, iron does not rust. Rusting is a chemical change in which iron forms hydrated iron oxide.
2. (b) Rusting is prevented by keeping iron away from air and water: (i) painting it, (ii) greasing or oiling it, (iii) galvanisation (coating with zinc), (iv) making alloys such as stainless steel, and (v) keeping it dry.

Teacher's Note:
1. Both oxygen and water must be present together, so mention both.
2. Give at least three prevention methods, all based on cutting off air or water.