A blue litmus paper turns red when dipped into a solution. What does this indicate about the solution?

Step 1: Analyze the question The question asks what it means when blue litmus paper turns red in a solution. This is about how indicators work.

Step 2: Explain the concept Litmus is a natural indicator (extracted from lichens) that changes colour depending on the pH of a solution:

| Solution type | Effect on blue litmus | Effect on red litmus | |---|---|---| | Acidic (pH < 7) | Turns red | Stays red | | Basic/alkaline (pH > 7) | Stays blue | Turns blue | | Neutral (pH = 7) | No change | No change |

Acids release H⁺ ions in solution, and these make the litmus dye change from blue to red.

Step 3: Apply to the question

• Blue litmus turned red, so the solution must be acidic.
• A (basic): a base would leave blue litmus blue, so this is wrong.
• B (neutral): a neutral solution would cause no colour change, so this is wrong.
• D (faulty paper): the colour change is the expected behaviour of litmus in an acid, so nothing suggests the paper is faulty.

Step 4: Answer The correct option is C. The solution is acidic.

Exam tip: Remember "Acid turns Blue to Red". Both start with the letters A-B-R: Acid, Blue → Red. Also, only an acid can change blue litmus, and only a base can change red litmus. A neutral solution changes neither.