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?

Answer 1

To determine the pH of the unknown solution based on its reactions with the three indicators, we need to analyze the provided information carefully:

1. Litmus Paper:

   • The unknown liquid turns litmus paper blue, which indicates that the pH is above 8.

2. Phenolphthalein:

   • The unknown liquid is not affected by phenolphthalein, meaning it remains unchanged (i.e., no pink color). Phenolphthalein does not change to pink until the pH reaches about 8.2. Therefore, we can conclude that the pH of the unknown liquid is less than 8.2.

3. Thymol Blue:

   • The unknown liquid turns thymol blue to yellow. According to the table, thymol blue turns yellow in the pH range of about 6 to 8 (specifically, it turns from red to yellow between pH 4 and 6, and remains yellow from pH 6 to 8).

Combining all the clues:

• From the litmus paper, we know the pH is greater than 8.
• From phenolphthalein, we know the pH is less than 8.2.
• From thymol blue, we affirm that the pH is in the range of 6 to 8.

Putting these conditions together, since the value must be above 8 and less than 8.2, the only possible conclusion is that the pH of the unknown solution is approximately 8.0.