What will be the action of the following substances on litmus paper?

Dry HCl gas, moistened NH₃ gas, lemon juice, carbonated soft drink, curd, soap solution

(i) Dry HCl gas: No change on litmus paper. This is because litmus paper changes colour only if ions such as hydrogen (H⁺) or hydronium (H3O⁺) ions are present in a solution. HCl can only furnish ions when it reacts with water. Thus, dry HCl gas does not change the colour of litmus paper.

(ii) Moistened NH₃ gas: Red litmus turns blue as it is basic in nature

(iii) Lemon Juice: Blue litmus turns red as it contains citric acid

(iv) Carbonated soft drinks: Blue litmus turns red as it contains carbonic acid

(v) Curd: Blue litmus turns red as it contains lactic acid

(vi) Soap solution: Red litmus turns blue as it is basic in nature.