Here's why:
* Litmus paper is designed to detect changes in pH (acidity or alkalinity).
* Gases themselves don't directly interact with litmus paper. They are in their gaseous state, which means their molecules are far apart and not readily interacting with the litmus paper molecules.
* Water acts as a solvent. When a gas dissolves in water, it forms ions that can interact with the litmus paper. This interaction causes the litmus paper to change color.
For example:
* Carbon dioxide gas (CO2) is acidic. When it dissolves in water, it forms carbonic acid (H2CO3), which lowers the pH. This causes red litmus paper to turn blue.
* Ammonia gas (NH3) is basic. When it dissolves in water, it forms ammonium hydroxide (NH4OH), which raises the pH. This causes blue litmus paper to turn red.
Therefore, moist litmus paper allows for the gas to dissolve and interact with the litmus paper, leading to a visible color change that indicates the gas's acidity or alkalinity.
