Question

What do you understand by physical change and chemical change? Classify the following as physical or chemical changes:

1. Dissolving salt in water
2. Electric current is passed through water and it is broken down into oxygen and hydrogen
3. Evaporation of alcohol
4. Digestion of food

Answer 1

Physical change: A change in which no new substance is formed is known as a physical change. It is a temporary change and can be reversed easily.

Chemical change: A change in which a new substance is formed is referred to as a chemical change. It is a permanent change and cannot be reversed easily.

1. Dissolving salt in water: It is a chemical change because dissolving salt in water results in the dissociation of sodium chloride into sodium cation and chloride anion, thus a new substance is formed.
2. Electric current is passed through water and it is broken down into oxygen and hydrogen: This process is known as electrolysis and it is a chemical change as it results in the formation of new substances like hydrogen and oxygen.
3. Evaporation of alcohol: It is a physical change as evaporation just leads to change in the physical state and no new substance is formed.
4. Digestion of food: It is a chemical change as digestion involves breaking down of complex food material into simpler one which have entirely different composition and thus results in formation of a new substance.