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.
phenolphthalein
D.
thymol Blue

Answer 1

To detect the presence of a strong acid, Andrea needs an indicator that changes color at a low pH, which corresponds to acidic conditions.

From the table provided:

• Thymol Blue has a pH range of 1.2 - 2.8, making it suitable for detecting strong acids.
• Phenol Red (pH range 6.4 - 8.0) is for neutral to slightly basic conditions.
• Phenolphthalein (pH range 8.0 - 10.0) is for basic conditions.
• Trinitrobenzoic acid (pH range 12.0 - 13.4) is for very basic conditions.

Given this information, Andrea should use D. thymol Blue, as it is suitable for detecting strong acids with a pH range well below 7.