Physical and Chemical Changes — Question 4
Back to all questionsQuestion 4
Give reasons:
(a) Freezing of water to ice and evaporation of water are physical changes.
(b) Burning of a candle is both a physical and a chemical change.
(c) Burning of paper is a chemical change.
(d) Cutting of a cloth piece is a physical change, though it cannot be reversed.
(a) Freezing of water to ice and evaporation of water are physical changes because the changes are reversible and the chemical composition of the substance remains the same. No new substance if formed. Only change of state occurs i.e. water (liquid) is converted to ice (solid) in freezing and in evaporation, water (liquid) is changed to vapour (gaseous) state.
(b) When a candle is lighted it produces carbon dioxide and water vapour as the new substances and along with it there is evolution of heat and light. This shows a chemical change. When a candle burns wax melts that falls on floor and again it solidifies which shows a physical change. So burning of a candle is both a physical and a chemical change.
(c) When a piece of paper is burnt a new substance ash is produced which cannot be changed back into paper. This change is permanent and irreversible, so it is a chemical change.
(d) Cutting a cloth piece does not produce any new substances. There is no change in the chemical composition of the cloth piece after cutting. Hence, even though it is irreversible, still it is a physical change.