The table below shows the color of three indicators at different pH values.

Question

Indicator pH=0 pH=2 pH=4 pH=6 pH=8 pH=10 pH=12 ph=14
Litmus paper Red Red Red Red Blue Blue Blue Blue
Phenolphthalein No change No change No change No change No change Pink Pink Pink
Thymol blue Red Red Yellow Yellow Yellow Blue Blue Blue

An unknown liquid is tested with three indicators:
• The unknown liquid turns litmus paper blue.
• The unknown liquid is not affected by phenolphthalein indicator.
• The unknown liquid turns yellow when tested with thymol blue indicator.

What is the pH of the unknown solution?
A.
2

B.
6

C.
8

D.
12

Answer 1

To determine the pH of the unknown solution based on the behavior of the indicators, let's analyze each indicator's result:

1. Litmus paper: The unknown liquid turns litmus paper blue. This indicates that the solution is basic. Litmus paper turns blue at a pH of 8 and above.

2. Phenolphthalein: The unknown liquid is not affected by the phenolphthalein indicator, which means it does not turn pink. Phenolphthalein changes to pink in a basic solution starting around pH 8, so the unknown cannot be in the pH range that produces a pink color (pH 8 and above). Thus, the pH must be below 8.

3. Thymol blue: The unknown liquid turns yellow when tested with thymol blue. Thymol blue changes from red to yellow between pH 4 and pH 6 in basic solutions, and it turns blue after pH 8, indicating the solution is neutral at pH 6 or slightly basic (around pH 7-8) but not fully basic.

From this, we can conclude that the unknown liquid meets the following criteria:

• It is basic enough to turn litmus paper blue (indicating pH ≥ 8).
• However, it cannot be basic enough for phenolphthalein to turn pink (indicating pH < 8).
• It turns thymol blue yellow, which indicates a pH range around 6.

Given these analyses, the only viable pH under 8 is 6. Therefore, the pH of the unknown solution is:

B. 6.