Question

What will be the action of the following substances on litmus paper? Dry $HCl$ gas, moistened $N{H_3}$ gas, lemon juice, carbonated soft drink, curd, soap solution.

Answer 1

Litmus paper is used to determine the pH of aqueous solution. The use of litmus is to test whether the solution is acidic or basic in nature. Light blue litmus paper turns red under acidic conditions and red litmus paper turns blue under basic or alkaline conditions.

Complete answer:
Now $(i)$ Dry $HCl$ gas: No change on litmus paper. This is because litmus paper changes color only if ions such as hydrogen $({H^ + })$ or hydronium ions $({H_3}{O^ + })$ are present in the solution. Can only furnish ions when it reacts with water. Thus, dry gas does not change the color of litmus paper.
$(ii)$ Moistened $N{H_3}$ gas: Red litmus will turn blue as it is basic in nature. A base is a molecule that accepts protons, in ammonia the nitrogen atom has an electron pair that readily accepts a proton. However, ammonia is classified as a weak base.
$(iii)$ Lemon juice: Blue litmus turns red as it contains citric acid. Lemon juice in its natural state is acidic with a pH value of $2$ . but once metabolized it actually becomes alkaline with a pH well above $7$
$(iv)$ Carbonated soft drinks: Blue litmus turns red as it contains carbonic acid. Carbonic acid $({H_2}C{O_3})$ , a compound of the element hydrogen, oxygen and carbon has a pH value between $8 - 10$ .
$(v)$ Curd: Blue litmus turns red as it contains lactic acid. Lattice acid is an organic acid, it has a molecular formula $C{H_3}CH(OH)COOH$ it has a pH value of $2.4$ .
$(vi)$ Soap solution: Red litmus turns blue as it is basic in nature. Soap is basic in nature because esters of higher fatty acids give potassium or sodium salt or higher fatty acid and glycerol when treated with an alkali. The presence of a strong base makes soap basic.

Note:
Litmus paper is either red or blue. Red paper turns blue when the pH is alkaline, while blue paper turns red when the pH turns acidic. While litmus paper is most often used to test the pH of liquid, it can also be used to test gases if the paper is dampened with distilled water before exposure to the gas. The litmus test is quick and simple but it is not an accurate indicator of pH; it does not yield a numerical pH value.