Question

Identify a natural chemical change which gives out a gas that turns lime water milky.

• A. Respiration
• B. Photosynthesis
• C. Combustion of petrol
• D. Softening of hard water

Answer 1

The correct option is A Respiration

Respiration and photosynthesis are natural chemical changes whereas combustion of petrol and softening of hard water are man-made chemical changes.

During respiration, glucose in the cells reacts with the oxygen inhaled from the air and forms carbon dioxide and water. The reaction involved is:

${\text{C}}_6{\text{H}}_{12}{\text{O}}_6\text{(s)}+{\text{6O}}_2\text{(g)}\to {\text{6CO}}_2\text{(g)}+{\text{6H}}_2\text{O(l)}$

Carbon dioxide reacts with limewater (${\text{Ca(OH)}}_2$) to form calcium carbonate (${\text{CaCO}}_3$) and water. The calcium carbonate formed is responsible for milky appearance. The reaction involved is:

${\text{CO}}_2\text{(g)}+{\text{Ca(OH)}}_2\text{(aq)}\to {\text{CaCO}}_3{\text{(s) + H}}_2\text{O(l)}$

During photosynthesis, plants use carbon dioxide and water in presence of sunlight to form glucose and oxygen.