Question

A student takes about 2mL of ethanoic acid in a dry test tube and adds a pinch of sodium hydrogen carbonate to it. He reports the following observations:

I. Immediately a colorless and odorless gas evolves with brisk effervescence.

II. The gas turns lime water milky when passed through it.

III. The gas bums with an explosion when a burning splinter is brought near it.

IV. The gas extinguishes the burning splinter that is brought near it.

The correct observations are

• A. I, II, and III
• B. II, III, and IV
• C. III, IV, and III
• D. I, II, and IV

Answer 1

The correct option is D

I, II, and IV

Explanation for correct option:

(D) Ethanoic acid $\left({\mathrm{CH}}_3\mathrm{COOH}\right)$ reacts with Sodium hydrogen carbonate $\left({\mathrm{NaHCO}}_3\right)$ to liberate carbon dioxide gas. In this reaction, Acetic acid reacts with sodium bicarbonate to form sodium acetate and carbonic acid that is unstable.
$\underset{\left(\mathrm{Sodium}\mathrm{bicarbonate}\right)}{\mathrm{NaHC}{\mathrm{O}}_3}+\underset{\left(\mathrm{Acetic}\mathrm{acid}\right)}{\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}}\to \underset{\left(\mathrm{Sodium}\mathrm{acetat}\right)\mathrm{e}}{\mathrm{C}{\mathrm{H}}_3\mathrm{COONa}}+\underset{\left(\mathrm{Carbonic}\mathrm{acid}\right)}{{\mathrm{H}}_2\mathrm{C}{\mathrm{O}}_3}$
Carbonic acid is unstable and undergoes a decomposition reaction to produce the carbon dioxide gas:

$\underset{\left(\mathrm{Carbonic}\mathrm{acid}\right)}{{\mathrm{H}}_2\mathrm{C}{\mathrm{O}}_3}\to \underset{\left(\mathrm{Water}\right)}{{\mathrm{H}}_2\mathrm{O}}+\underset{\left(\mathrm{Carbon}\mathrm{dioxide}\right)}{\mathrm{C}{\mathrm{O}}_2}$
The complete reaction can be given as:

$\underset{\left(\mathrm{Sodium}\mathrm{bicarbonate}\right)}{{\mathrm{CH}}_3\mathrm{COOH}\left(\mathrm{aq}\right)}+\underset{\left(\mathrm{Sodium}\mathrm{bicarbonate}\right)}{{\mathrm{NaHCO}}_3\left(\mathrm{aq}\right)}\to \underset{\left(\mathrm{Sodium}\mathrm{acetate}\right)}{{\mathrm{CH}}_3\mathrm{COONa}\left(\mathrm{aq}\right)}+\underset{\left(\mathrm{Water}\right)}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}+\underset{\left(\mathrm{Carbon}\mathrm{dioxide}\right)}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}$

(I) Therefore, when 2 mL acetic acid is combined with a pinch of Sodium hydrogen carbonate in a dry test tube, a colorless and odorless gas with a rapid effervescence is produced, which is CO₂.

(ii) When CO₂ is injected into lime water, it turns the liquid milky.

$\underset{\mathrm{Lime}\mathrm{Water}}{\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)}+\underset{}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}\to \underset{\mathrm{White}\mathrm{ppt}}{{\mathrm{CaCO}}_3\left(\mathrm{s}\right)}+{\mathrm{H}}_2\mathrm{O}$

(iii) The gas not bums with an explosion Mien a burning splinter is brought near it

(iv) When CO₂ is brought close to a burning splinter, it also extinguishes it.

Final answer: Hence, the correct option is (D), i.e I, II, and IV.