Question

Which of the following observations help us to determine whether a chemical reaction has taken place or not?

(i) Change in state.

(ii) Change in colour.

(iii) Change in shape

(iv) Evolution of gas.

(v) Change in temperature.

• A. (ii), (iii), (iv), (v)
• B. (i), (ii), (iv), (v)
• C. (i), (ii), (iii), (v)
• D. (iii), (v)

Answer 1

The correct option is B

(i), (ii), (iv), (v)

The observations that help us for determining a chemical reaction are:

(a) Change in state ( Electrolysis of water in a liquid state gives off oxygen and hydrogen gases )

(b) Change in colour ( When sugar is heated it turns brown )

(c) Evolution of gas ( Ferric sulphate on heating gives ferric oxide and sulphur dioxide and sulphur trioxide gases are evolved).

(d) Change in temperature ( When water is added to quicklime, temperature of the mixture increases )

Note: Change in shape is a physical reaction (ex: cutting of wood )