What is the test for chlorine gas?

The test for chlorine gas involves using damp red litmus paper, which is known to change color when exposed to chlorine. Chlorine is a bleaching agent, and when it comes into contact with damp red litmus paper, it bleaches the dye, turning the paper white. This reaction is specific to chlorine and is utilized as an indicator of its presence.

In contrast, the other options involve reactions that do not occur with chlorine. For instance, damp blue litmus paper turning red would indicate the presence of an acid, which does not pertain to chlorine. Similarly, if damp blue litmus paper is unaffected, this would imply the absence of any basic or alkaline substances, which does not provide specific information about chlorine. Therefore, option B accurately describes the characteristic bleaching effect of chlorine gas on litmus paper, confirming its presence.