International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

What happens to moist litmus paper when chlorine gas is present?

1. Turns blue

2. Turns red

3. Bleaches white

4. No change

The correct answer is: Bleaches white

When moist litmus paper is exposed to chlorine gas, it undergoes a bleaching effect, turning white. This occurs because chlorine is a strong oxidizing agent and reacts with the dyes in the litmus paper. The dye is decolorized as chlorine attacks its molecular structure, leading to the loss of color. Chlorine gas is not only used for its disinfectant properties but also for its ability to react with various organic and inorganic substances, causing a bleaching effect. In the case of litmus paper, which typically contains organic dyes, the bleaching action results in the paper becoming white, demonstrating the presence of chlorine. This reaction is important in identifying chlorine gas in laboratory and industrial settings, where it can indicate the presence of chlorine for safety or analytical purposes. The lack of color change with the litmus paper points to the ineffectiveness of chlorine in altering the basic or acidic state of the indicator, emphasizing its bleaching properties instead.