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

What color change indicates the presence of ammonia gas during a litmus test?

1. Red litmus paper turns blue

2. Blue litmus paper turns red

3. Red litmus paper remains red

4. Blue litmus paper remains blue

The correct answer is: Red litmus paper turns blue

The presence of ammonia gas during a litmus test is indicated by the color change where red litmus paper turns blue. This occurs because ammonia is a basic compound. When ammonia gas comes into contact with red litmus paper, it causes the paper to undergo a chemical reaction that results in a color change to blue. This is characteristic of basic solutions, as they cause red litmus paper to change color, reflecting the fundamental property of acids and bases in the context of pH indicators. The other options describe scenarios that do not occur when ammonia is present. Blue litmus paper remains blue in a neutral or basic environment, and red litmus paper remains red in an acidic environment, indicating that no significant change occurs in those cases. Therefore, the transition of red to blue in the presence of ammonia is a clear indication of its basic nature.