Question

(1) Give an example for each of the following with proper explanation:

a) A physical change which is reversible.
b) A chemical change in which change of colour is observed.

(2) How would you show that setting of curd is a chemical change? [3 MARKS]

Answer 1

(1) Each part: 1 Mark
(2) Answer: 1 Mark


(1) a) In case of water, when it is cooled up to zero degree Celsius it becomes ice. However, when the ice is left in a warm room, it becomes liquid again. So, the change occurs only in its phase which is a physical change and reversible.

b) Rusting of iron is an example of a chemical change in which colour change is observed. If we leave a piece of iron in open for some time, it acquires a film of brownish substance.This substance is called rust and the process is called rusting.

$Iron\left(Fe\right)+Oxygen(O_2,fromtheair)+water\left(H_{2}O\right)\to Rust\left(ironoxide\right)(F{e}_2{O}_3.H_{2}O)$


(2) After the formation of curd, milk cannot be re-obtained from it. Also, both milk and curd have different properties. Since these are the properties of a chemical change, setting of curd is a chemical change.