Andrea is performing an experiment in which she needs to know when a strong acid is present. She is not sure which indicator to use, so she looks up the following table in her science textbook.

Question

Indicator pH range
thymol Blue 1.2 - 2.8
phenol Red 6.4 - 8.0
phenolphthalein 8.0 - 10.0
trinitrobenzoic acid 12.0 - 13.4

According to the textbook, which indicator should Andrea use?

Hint: Neutral substances have a pH of 7. Acids have a pH that is less than 7, and bases have a pH that is greater than 7.
A.
trinitrobenzoic acid
B.
phenol Red
C.
thymol Blue
D.
phenolphthalein

Answer 1

Since Andrea is looking for an indicator that can detect a strong acid, she should choose one that has a pH range that includes values less than 7.

From the table:

• Thymol Blue has a pH range of 1.2 - 2.8 (suitable for strong acids)
• Phenol Red has a pH range of 6.4 - 8.0 (not suitable for strong acids)
• Phenolphthalein has a pH range of 8.0 - 10.0 (not suitable for strong acids)
• Trinitrobenzoic acid has a pH range of 12.0 - 13.4 (not suitable for strong acids)

Therefore, the best choice for detecting a strong acid is:

C. thymol Blue