Question

How can a geologist confirm whether a sample of rock is limestone?

• A. By adding drops of aqueous $\mathrm{HCl}$over the rock surface and observing the color change.
• B. By adding the drops of aqueous $\mathrm{NaOH}$ over the rock surface and observing the color change.
• C. By adding drops of aqueous $\mathrm{NaCl}$ over the rock surface and observing the fizzing.
• D. By adding drops of aqueous $\mathrm{HCl}$over the rock surface and observing the fizzing.

Answer 1

The correct option is D

By adding drops of aqueous $\mathrm{HCl}$over the rock surface and observing the fizzing.

Explanation for the correct option:

• A geologist always carry a small bottle of $\mathrm{HCl}$(Hydrochloric acid) to perform a quick test, which is used to distinguish the most common carbonate rocks, dolomite and limestone. A few drops of aqueous $\mathrm{HCl}$ acid is put on the rock, and limestone responds by fizzing vigorously.
• Lime stone means Calcium carbonate $\left({\mathrm{CaCO}}_3\right)$, when treated with $\mathrm{HCl}$(Hydrochloric acid), it Carbon dioxide $\left({\mathrm{CO}}_2\right)$ is evolved, due to which fizzing is experienced.
• Reaction: $\underset{(\mathrm{Calcium}\mathrm{carbonate})}{\underbrace{{\mathrm{CaCO}}_3\left(\mathrm{s}\right)}}+\underset{(\mathrm{Hydrochloric}\mathrm{acid})}{\underbrace{\mathrm{HCl}\left(\mathrm{aq}\right)}}\to \underset{(\mathrm{Calcium}\mathrm{chloride})}{\underbrace{{\mathrm{CaCl}}_2\left(\mathrm{aq}\right)}}+\underset{\left(\mathrm{Water}\right)}{\underbrace{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}}+\underset{(\mathrm{Carbon}\mathrm{dioxide})}{\underbrace{{\mathrm{CO}}_2\left(\mathrm{g}\right)\uparrow }}$

Explanation for the incorrect options:

• (A) When Calcium carbonate $\left({\mathrm{CaCO}}_3\right)$, when treated with $\mathrm{HCl}$(Hydrochloric acid), there is no change in color is observed.
• (B) When aqueous Sodium hydroxide $\mathrm{NaOH}$ is dropped over rock surface, Sodium carbonate $\left({\mathrm{Na}}_2{\mathrm{CO}}_3\right)$and Calcium hydroxide $\left(\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\right)$ is formed and there no change in color is observed.
• (C) Aqueous Sodium chloride will react with Calcium carbonate $\left({\mathrm{CaCO}}_3\right)$ to give Sodium carbonate $\left({\mathrm{Na}}_2{\mathrm{CO}}_3\right)$ and Calcium chloride $\left({\mathrm{CaCl}}_2\right)$ , where no evolution of any gas is seen and hence, no fizzing is experienced,

Hence, the option (d) is correct. i.e., A geologist confirm whether a sample of rock is limestone, by adding drops of aqueous $\mathrm{HCl}$ over the surface of rock and observing the fizzing.