Question

On adding acetic acid to sodium hydrogen carbonate in a test tube, a student observed:
(a) No reaction.
(b) A colourless gas with pungent smell
(c) Bubbles of a colourless and odourless gas
(d) A strong smell of vinegar

Answer 1

when an acid reacts with a base, neutralisation reaction takes place. This neutralisation reaction is accompanied by the formation of salt and water along with the evolution of heat which is called enthalpy of neutralisation.

Complete step by step solution:
Acetic acid is a carboxylic acid with a ${ pK }_{ a }$ of 4.75. From this value of ${ pK }_{ a }$ it is clear that acetic acid is a weak acid. Sodium hydrogen carbonate is a white solid and has a melting point of $50\quad ^{ o }{ C }$. Its ${ pK }_{ a1 }$ is 10.3 and its ${ pK }_{ a2 }$ is 6.4. It is amphoteric in nature and when dissolved in water, forms a weak alkaline solution.

When acetic acid is added to an aqueous solution of sodium bicarbonate, an acid-base reaction takes place (neutralisation reaction), which leads to the formation of a salt of sodium acetate and water along with the evolution of carbon dioxide. The salt sodium acetate has a tendency to undergo hydrolysis. Therefore on complete neutralisation, the pH of the solution is not neutral i.e. not 7 but rather greater than 7.
When carbon dioxide is evolved, it appears as a brisk effervescence. Carbon dioxide gas is colourless and odourless. It can be identified by passing it through lime water. If on passing a gas through lime water it turns milky, the gas is carbon dioxide gas. The reaction between acetic acid and sodium hydrogen carbonate/sodium bicarbonate is shown below:

$\begin{matrix} { CH }_{ 3 }COOH(aq) \\\ Acetic\quad acid \end{matrix}+\begin{matrix} NaH{ CO }_{ 3 }(aq) \\\ Sodium\quad bicarbonate \end{matrix}\rightarrow \begin{matrix} C{ H }_{ 3 }COONa(aq) \\\ sodium\quad acetate \end{matrix}+\begin{matrix} { H }_{ 2 }O(l) \\\ Water \end{matrix}+\begin{matrix} { CO }_{ 2 }(g)\uparrow \\\ Carbon\quad dioxide \end{matrix}$

Hence the correct answer is (c) Bubbles of a colourless and odourless gas.

Note: When carbon dioxide is passed through lime water (aqueous solution of calcium hydroxide) it turns milky due to the formation of solid calcium carbonate which is white in colour and insoluble in water.