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

Dry $\mathrm{HCl}$ gas, Moistened ${\mathrm{NH}}_3$ gas, Lemon juice, Carbonated soft drink, Curd, Soap solution.

Litmus paper:

• Litmus paper is used to determine the acidic and basic nature of a substance.
• Blue litmus turns red when added to acid and red litmus turns blue when added to a base.
• It remains the same when added to a neutral solution.

Action of substances on litmus paper:

• Dry $\mathrm{HCl}$ gas - As both litmus paper and dry gas lack water, the paper will remain unaffected.
• Moistened ${\mathrm{NH}}_3$ gas - Red litmus turns blue as ammonia is basic in nature.
• Lemon juice - Blue litmus turns red as lemon juice is acidic in nature.
• Carbonated soft drink - Blue litmus turns red as the carbonated soft drink is acidic in nature.
• Curd - Blue litmus turns red as curd is acidic in nature.
• Soap solution - Red litmus turns blue as soap solution is basic in nature.