Here's why:
* Ammonia is a base: Ammonia (NH₃) is a weak base. It readily reacts with water to form ammonium ions (NH₄⁺) and hydroxide ions (OH⁻):
* NH₃(g) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)
* Indicator paper changes color: The indicator paper contains a chemical that changes color depending on the pH of the solution. Since the ammonia solution becomes basic due to the formation of hydroxide ions, the indicator paper turns a specific color indicating the presence of a base.
* Moistness is crucial: The indicator paper must be moist because ammonia gas dissolves in water. The dissolved ammonia then reacts with the water, forming the basic solution that triggers the color change in the indicator.
Common indicator paper used for ammonia:
* Red litmus paper: Red litmus paper turns blue in the presence of a base, indicating the presence of ammonia gas.
In summary:
Moist indicator paper is used to test for ammonia gas because the gas dissolves in water to form a basic solution, which causes the indicator paper to change color due to the change in pH.
