Question

Rhea added an oxide Y through water and obtained a compound, which turned blue litmus red. She passed the same oxide Y through limewater and observed that the colour of limewater changed to milky-white. Identify Y.

• A. Magnesium oxide
• B. Carbon dioxide
• C. Nitrous oxide
• D. Sulphur dioxide

Answer 1

The correct option is B Carbon dioxide

Carbon dioxide is an acidic oxide as it produces carbonic acid when passed through water and hence, turns blue litmus red.

${\mathrm{CO}}_2\left(g\right)+H_{2}O\left(l\right)\to H_2{\mathrm{CO}}_3\left(\mathrm{aq}\right)\mathrm{Carbon}\mathrm{Water}\mathrm{Carbonic}\mathrm{dioxide}\mathrm{acid}$

Also, $C{O}_2$ when passed through limewater $\left(Ca\right(O{H}_2)$, changes the colour of limewater to milky-white. This is due to the formation of calcium carbonate $\left(CaC{O}_3\right)$ as shown below.

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

Although nitrous dioxide $\left(N{O}_2\right)$ and sulphur dioxide $\left(S{O}_2\right)$ are acidic oxides as, in water, they produce nitric acid $HN{O}_3$ and sulphuric acid $H_{2}S{O}_4$ respectively, they do not turn limewater milky.
Magnesium oxide is a basic oxide and does not turn limewater milky.