Question

Sam blows air into limewater using a straw. He notices that the limewater turns milky. Identify the gas responsible for the change and the type of change involved.

• A. Gas - oxygen
  Change - chemical
• B. Gas - oxygen
  Change - physical
• C. Gas - carbon dioxide
  Change - chemical
• D. Gas - carbon dioxide
  Change - physical

Answer 1

The correct option is C Gas - carbon dioxide

Change - chemical
Exhaled air contains carbon dioxide and water vapour. Limewater is calcium hydroxide solution. When air is blown into limewater, calcium hydroxide $\left({\text{Ca(OH)}}_2\right)$ reacts with carbon dioxide $\left({\text{CO}}_2\right)$ to form calcium carbonate and water. The reaction involved is:

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

Calcium carbonate forms a white precipitate which is responsible for the milky appearance. Since new substances are formed in the reaction, a chemical change is exhibited.