How is litmus paper used in gas testing?

Litmus paper is used in gas testing to identify whether the gas is acidic or alkaline in nature.

Litmus paper is a type of filter paper that has been treated with a natural water-soluble dye obtained from lichens. The dye reacts to changes in pH, allowing it to be used as a pH indicator. When it comes into contact with an acidic or alkaline substance, it changes colour, providing a visual indication of the substance's pH level.

In gas testing, a strip of litmus paper is often moistened before it is exposed to the gas. This is because gases need to dissolve into a solution to react with the litmus paper. The moistened litmus paper is then held in the path of the gas. If the gas is acidic, blue litmus paper will turn red. Conversely, if the gas is alkaline, red litmus paper will turn blue.

For example, if you were testing for the presence of chlorine gas, you would use moist blue litmus paper. Chlorine is an acidic gas, so if it is present, the blue litmus paper will turn red. On the other hand, if you were testing for the presence of ammonia gas, you would use moist red litmus paper. Ammonia is an alkaline gas, so if it is present, the red litmus paper will turn blue.

It's important to note that while litmus paper can identify whether a gas is acidic or alkaline, it cannot provide a precise pH value. For more accurate results, other methods such as pH meters or universal indicator solutions may be used. However, for quick and simple testing, litmus paper is a valuable tool.