Physical changes and chemical changes 0.1

Question 1

Physical changes can be reversible as well as irreversible. Explain with an example.

Answer 1

Physical changes are changes which involve changes only in physical property.These changes are reversible as well as irreversible changes.

For example : Salt dissolved in water can be recovered by allowing water to evaporate. This physical change is reversible. But when an apple is cut into pieces, no new substance is formed but we cannot get the whole apple back. This physical change is irreversible.

Question 2

Why lighting a cracker is a chemical change?

Answer 2

Lighting a cracker is a chemical change because here there is a formation of new compounds along with heat, light and sound which cannot be reversed.This reaction is called combustion reaction.

Question 3

Which of the following are reversible reactions?

Answer 3

Broken chalk piece to whole chalk piece
Baby to child
Cold water to hot water
Fresh bread to rotten bread
Ice to water
Paper to burnt paper
Unripe mango to ripe mango
Ans. The reversible reaction are

Broken chalk pieces ↔ whole chalk piece
Cold water ↔️hot water
Ice ↔️ water

Question 4

When a candle burns,both physical and chemical change take place. How?

Answer 4

When a candle burns, both physical and chemical changes occur.

Physical Changes: On heating, candle wax gets melted. Since it again turns into solid wax on cooling. So, the melting of wax and vapourisation of melted wax are physical changes.
Chemical Changes : The wax near flame burns and gives new substances like carbon dioxide, carbon soot, water vapour, heat and light.