Question

When u add sodium hydrogen carbonate to acetic acid in a test tube, a gas liberates immediately with brisk effervescence. Name this gas. Describe the method of testing this gas.

Answer 1

Hint: Sodium hydrogen carbonate is also known as sodium bicarbonate. When the aqueous solution of sodium hydrogen carbonate is added to the organic compound and if brisk effervescence is observed then this is confirmation of the presence of carboxylic acid in that compound.

Complete step by step answer:
Carboxylic acid has a tendency to donate a proton therefore acts as acids. And they are weaker acids than inorganic acids. As they do not completely dissociate in solution. This acidic property is useful in the detection of this acid group.
When acetic acid solution is added to the solution of sodium hydrogen carbonate carbon dioxide gas is evolved with brisk effervescence along with sodium acetate form. The reaction is given as
\[C{H_3}COOH + NaHC{O_3} \to C{H_3}COONa + {H_2}O + C{O_2} \uparrow \]
Carbon dioxide is evolved with brisk effervescence. It can be observed with eyes. But to confirm the evolution of this gas there is another test known as lime water test. When carbon dioxide gas is passed through the solution of lime water the solution turns milky because of the formation of calcium carbonate. The reaction is
\[Ca{(OH)_2} + C{O_2} \to CaC{O_3} + {H_2}O\]
Lime water $\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;$ white color

Note
When carbon dioxide passes through that lime water for a long time, the calcium carbonate that has formed previously converts to calcium hydrogen carbonate. The solution of calcium hydrogen carbonate is clear. Hence the solution will now become clear again. The reaction is given as
\[CaC{O_3} + {H_2}O + C{O_2} \to Ca{(HC{O_3})_2}\]