Question

While researching on various components of air, a scholar Rakesh passed a gas through limewater.
He observed that the limewater turned milky. Identify the gas from the following.

• A. Methane
• B. Carbon dioxide
• C. Oxygen
• D. Nitrogen

Answer 1

The correct option is B

Carbon dioxide

• Limewater is a diluted solution of calcium hydroxide $\left(Ca\right(OH{)}_2)$, which is colourless.
• Of all the given options, only carbon dioxide when passed through limewater $\left(Ca\right(OH{)}_2)$, turns it milky.
• Therefore, the gas should be carbon dioxide.
• The limewater turns milky due to the formation of calcium carbonate $\left(CaC{O}_3\right)$. The reaction for the same is shown below:

$\mathrm{Ca}\left(\mathrm{OH}{)}_2\right(\mathrm{aq})+{\mathrm{CO}}_2(g)\to {\mathrm{CaCO}}_3(s)+H_{2}O(l)\mathrm{Calcium}\mathrm{Carbon}\mathrm{Calcium}\mathrm{Water}\mathrm{hydroxide}\mathrm{dioxide}\mathrm{carbonate}$