Question

Acetic acid was added to a solid X kept in a test tube. A colorless and odorless gas was evolved. The gas was passed through lime water which turned milky. It was concluded that

• A. Solid X is sodium hydroxide and the gas evolved is ${CO}_2$
• B. Solid X is sodium bicarbonate and the gas evolved is ${CO}_2$
• C. Solid X is sodium acetate and the gas evolved is ${CO}_2$
• D. Solid X is sodium chloride and the gas evolved is ${CO}_2$

Answer 1

The correct option is B

Solid X is sodium bicarbonate and the gas evolved is ${\mathrm{CO}}_2$

The explanation for the correct option (B):

• Carbon dioxide gas (${\mathrm{CO}}_2$) is produced when Acetic acid (${\mathrm{CH}}_3\mathrm{COOH}$) is added to Sodium bicarbonate (${\mathrm{NaHCO}}_3$). This ${\mathrm{CO}}_2$ gas is colorless and odorless.
• The reaction between ${\mathrm{CH}}_3\mathrm{COOH}$ and ${\mathrm{NaHCO}}_3$ is:

$$\begin{gathered} {\mathrm{CH}}_3\mathrm{COOH}\left(\mathrm{aq}\right)+{\mathrm{NaHCO}}_3\left(\mathrm{s}\right)\to {\mathrm{CH}}_3\mathrm{COONa}\left(\mathrm{aq}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ \mathrm{Sodium}\mathrm{Water} \\ \mathrm{acetate} \end{gathered}$$

• When ${\mathrm{CO}}_2$ gas is passed over Lime water or an aqueous solution of Calcium hydroxide ($\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$), a white precipitate called Calcium carbonate (${\mathrm{CaCO}}_3$) forms.
• The Lime Water turns milky due to the formation of ${\mathrm{CaCO}}_3$.
• The reaction between $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$ and ${\mathrm{CO}}_2$ is:

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

The explanation for the incorrect options:

(A)

• When Acetic acid is added to Sodium hydroxide ($\mathrm{NaOH}$), then Sodium acetate salt and Water are formed. ${\mathrm{CO}}_2$ gas is not evolved in this reaction.
• The reaction between ${\mathrm{CH}}_3\mathrm{COOH}$ and $\mathrm{NaOH}$ is:

${\mathrm{CH}}_3\mathrm{COOH}\left(\mathrm{aq}\right)+\mathrm{NaOH}\left(\mathrm{aq}\right)\to {\mathrm{CH}}_3\mathrm{COONa}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

(C)

• The chemical formula of Sodium acetate is ${\mathrm{CH}}_3\mathrm{COONa}$. It is a salt of Acetic acid. Sodium acetate does not react with Acetic acid.

(D)

• Sodium chloride ($\mathrm{NaCl}$) is salt and it does not react with Acetic acid.

Final Answer: Hence, option (B) is correct. Solid X is sodium bicarbonate and the gas evolved is ${\mathrm{CO}}_2$.