CBSE Class 7 Science Physical and Chemical Changes Assignment Set 03

Answer: (c) chemical. Formation of a new substance is the main sign of a chemical change.

Teacher's Note:
1. In a physical change only shape, size or state changes.
2. The substance itself stays the same.

Answer: (a) white. Magnesium burns with a dazzling white flame to form magnesium oxide.

Teacher's Note:
1. A white ash of magnesium oxide is left behind.
2. The change is chemical and irreversible.

Answer: (c) reversible change. Sugar can be recovered by evaporating the water, so the change is physical and reversible.

Teacher's Note:
1. No new substance is formed.
2. Only the sugar mixes with the water.

Answer: A chemical change (it is also a biological process), because new substances are formed and the seed cannot return to its original form.

Teacher's Note:
1. Stored food is converted into new substances for growth.
2. The change cannot be reversed.

Answer: Heating limestone: CaCO₃ → CaO + CO₂. Heat is absorbed from the surroundings for the change to take place.

Teacher's Note:
1. An endothermic change takes in heat.
2. Chemical changes that give out heat are called exothermic.

Answer: The presence of oxygen (air) and moisture (water).

Teacher's Note:
1. Iron kept in dry air or in boiled water without air does not rust.
2. Both conditions must be present together.

Answer: 1. One or more new substances with different properties are formed.
2. Heat, light or other energy is given out or taken in.
3. Most chemical changes are permanent (irreversible).
4. There may be colour change, a gas, a precipitate or a sound.

Teacher's Note:
1. Burning and cooking are examples.
2. These signs help us decide whether a change is chemical.

Answer: Rusting is the slow formation of reddish-brown hydrated iron oxide on iron when it is exposed to air and moisture.
Prevention: painting, oiling or greasing the surface, galvanisation (coating with zinc), or using stainless steel.

Teacher's Note:
1. Rust weakens iron objects.
2. Prevention works by keeping air and water away from the iron.

Answer: 1. Physical change: no new substance forms; chemical change: a new substance forms.
2. Physical changes are usually reversible; chemical changes are usually irreversible.
3. Physical change involves little energy change; chemical change involves a noticeable absorption or release of energy.
4. Examples: melting of ice (physical); burning of wood (chemical).

Teacher's Note:
1. The main test is whether a new substance appears.
2. Melting, freezing and dissolving are physical.

Answer: Both. Melting of wax is a physical change, but burning of the wax vapour to form carbon dioxide and water vapour is a chemical change.

Teacher's Note:
1. Part of the candle only changes state.
2. Burning forms new substances.

Answer: A gas (carbon dioxide) is released with fizzing. It is a chemical change, because a new substance is formed.

Teacher's Note:
1. The gas can be passed through lime water, which turns milky.
2. Gas formation is a sign of a chemical change.

Answer: (a) Physical. (b) Chemical. (c) Physical. (d) Chemical.

Teacher's Note:
1. Cutting and melting do not form new substances.
2. Souring and cooking do.

Answer: It is the coating of iron or steel with a thin layer of zinc. It is done to protect the iron from rusting.

Teacher's Note:
1. Zinc keeps air and moisture away from the iron.
2. It also protects the iron even if the layer is scratched.

Answer: The salt in sea water speeds up rusting.

Teacher's Note:
1. This is why ship hulls are painted and given extra protection.
2. Iron near coasts also rusts faster.

Answer: Heat a saturated solution of copper sulphate, filter it and leave it to cool slowly without disturbance. Blue crystals separate out.

Teacher's Note:
1. Slow cooling gives larger crystals.
2. The process is called crystallisation, and it is a physical change.

Answer: (c) Rusting of iron. Rusting forms a new substance, iron oxide.

Teacher's Note:
1. The others only change the form or state of the substance.
2. Rust is a different substance.

Answer: (b) Melting of wax. Melted wax can be cooled and solidified again.

Teacher's Note:
1. The other changes form new substances.
2. They cannot be undone.