How red litmus paper is used to test the presence of an acid?

                                         (OR)

The properties of acetic acid were studied by a student by conducting an experiment. Answer the following questions: 

(i) Which substance he must add to acetic acid to produce carbon dioxide? 

(ii) Mention the chemical reaction taking place. 

(iii) How would he test 𝐶𝑂₂ gas in the laboratory?

Dip the red litmus paper into an alkaline solution, and it will turn blue. Then dip this blue litmus paper into the acid, and it will turn red again. Thus indicating the presence of an acid. For example, dipping red litmus paper into sodium hydroxide solution turns it blue after that, dipping this blue litmus paper into hydrochloric acid turns litmus paper red again.

                                          (OR)

(i) The substance he must add to acetic acid is sodium carbonate $\left(N{a}_{2}C{O}_3\right)$ and sodium hydrogen carbonate $\left(NaHC{O}_3\right)$ to produce carbon dioxide.
(ii) The chemical reaction is as follows:

$2C{H}_{3}COOH+N{a}_{2}C{O}_3\to 2C{H}_{3}COONa+H_{2}O+C{O}_2$

$C{H}_{3}COOH+NaHC{O}_3\to C{H}_{3}COONa+H_{2}O+C{O}_2$

(iii) The test to identify carbon dioxide gas is a lime water test. Liberated $C{O}_2$ passes through lime water which turns it milky.