What observation indicates the presence of chloride ions using a damp blue litmus test?

• A. Bleaches damp litmus paper.
• B. Turns litmus red.
• C. Turns litmus blue.
• D. No effect.

Answer: (A)

Chloride ions can be detected by producing chlorine gas when a sample is treated with concentrated sulfuric acid. Chlorine gas is a strong oxidizer and, when it dissolves in the moisture on damp blue litmus paper, it bleaches the paper, turning it colorless. So seeing the damp blue litmus paper bleaching indicates chlorine gas is evolved, which in turn signals the presence of chloride ions in the sample.

If the paper turned red, that would point to acidity rather than chlorine gas. If the blue litmus stayed blue or there was no change, that would mean no chlorine gas was produced.