Question

A student takes 2 mL acetic acid in a dry test tube and adds a pinch of sodium hydrogen carbonate to it. He makes the following observation:
I. A colourless and odourless gas evolves with a brisk effervescence.
II. The gas turns lime water milky when passed through it.
III. The gas burns 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 I
D. IV, I and II

Answer 1

Hint: Sodium hydrogen carbonate or Sodium bicarbonate (\[NaHC{{O}_{3}}\]) is commonly known as baking soda. The above reaction between sodium hydrogen carbonate and acetic acid is an example of acid-base reaction.

Complete step by step solution: When sodium hydrogen carbonate reacts with acetic acid, carbon dioxide is formed as a result. It is an example of Double displacement reaction.
First, let’s try to understand the basic double replacement reaction between sodium hydrogen carbonate and acetic acid,
Acetic acid reacts with sodium bicarbonate to form sodium acetate and carbonic acid:

\[\underset{sodium\text{ }bicarbonate}{\mathop{NaHC{{O}_{3}}}}\,+\underset{acetic\text{ }acid}{\mathop{C{{H}_{3}}COOH}}\,\to \underset{sodium\text{ }acetate}{\mathop{C{{H}_{3}}COONa}}\,+\underset{carbonic\text{ }acid}{\mathop{{{H}_{2}}C{{O}_{3}}}}\,\]

Carbonic acid is unstable and undergoes a decomposition reaction to produce the carbon dioxide gas:

\[\underset{carbonic\,\,acid}{\mathop{{{H}_{2}}C{{O}_{3}}}}\,\to \underset{water}{\mathop{{{H}_{2}}O}}\,+\underset{carbon\,\,dioxide}{\mathop{C{{O}_{2}}}}\,\]

The overall reaction can be given as:

\[\underset{sodium\text{ }bicarbonate}{\mathop{NaHC{{O}_{3}}}}\,+\underset{acetic\text{ }acid}{\mathop{C{{H}_{3}}COOH}}\,\to \underset{sodium\text{ }acetate}{\mathop{C{{H}_{3}}COONa}}\,+\underset{water}{\mathop{{{H}_{2}}O}}\,+\underset{carbon\,\,dioxide}{\mathop{C{{O}_{2}}}}\,\]

The brisk effervescence is caused due to the evolution of carbon dioxide gas. Carbon dioxide gas is colorless and odourless in nature. The carbon dioxide gas has also the property of extinguishing the burning splinter, brought near it as it doesn’t support combustion.
Further, if the formed \[C{{O}_{2}}\] gas is passed through lime water, it turns lime water milky due to formation of a white precipitate of calcium carbonate.
 \[\underset{\text{lime}\,\,\text{water}}{\mathop{Ca{{(OH)}_{2}}}}\,+\,\underset{carbon\,\,dioxide}{\mathop{C{{O}_{2}}}}\,\,\to \underset{calcium\,\,carbonate}{\mathop{CaC{{O}_{3}}}}\,+\underset{water}{\mathop{{{H}_{2}}O}}\,\]

So, the correct option is (d).

Note: \[S{{O}_{2}}\] can also turn lime water milky due to formation of calcium sulfite (\[CaS{{O}_{3}}\]). Calcium sulfite is white precipitate which changes the overall color of the lime water to milky white. The reaction can be given as:
\[\underset{\text{lime}\,\,\text{water}}{\mathop{Ca{{(OH)}_{2}}}}\,+\,\underset{sulphur\,\,dioxide}{\mathop{S{{O}_{2}}\,}}\,\to \underset{calcium\,\,sulphite}{\mathop{CaS{{O}_{3}}}}\,+\underset{water}{\mathop{{{H}_{2}}O}}\,\]