Addition of phenolphthalein to an unknown colorless solution does not cause a color change. The addition of bromthymol blue to the same solution leads to a yellow color. (b) Which of the following can you establish about the solution: (i) A minimum pH, (ii) A maximum pH, or (iii) A specific range of pH values?

Verified step by step guidance

1. Identify the pH range where phenolphthalein changes color. Phenolphthalein is colorless in acidic solutions and turns pink in basic solutions, typically around a pH of 8.2 to 10.
2. Determine the pH range where bromthymol blue changes color. Bromthymol blue is yellow in acidic solutions and blue in basic solutions, with a transition range from pH 6.0 to 7.6.
3. Since phenolphthalein does not change color, the pH of the solution is likely below 8.2.
4. Since bromthymol blue turns yellow, the pH of the solution is likely below 6.0.
5. Combine these observations to establish that the pH of the solution is below 6.0, providing a specific range of pH values.

Key Concepts

Acid-Base Indicators

Acid-base indicators are substances that change color in response to changes in pH. Phenolphthalein is colorless in acidic solutions and turns pink in basic solutions, while bromthymol blue is yellow in acidic conditions and blue in basic ones. The lack of color change with phenolphthalein suggests the solution is either neutral or acidic, while the yellow color from bromthymol blue indicates an acidic environment.

pH Scale

The pH scale measures the acidity or basicity of a solution, ranging from 0 (very acidic) to 14 (very basic), with 7 being neutral. A solution with a pH less than 7 is considered acidic, while a pH greater than 7 is basic. The behavior of the indicators in the given scenario helps to establish the pH characteristics of the unknown solution.

Determining pH Range

Based on the observations with the indicators, we can infer a range for the pH of the unknown solution. Since phenolphthalein does not change color, the pH must be less than 8.2 (the transition point for phenolphthalein), and since bromthymol blue is yellow, the pH must be less than 6. Therefore, we can conclude that the solution has a pH range of less than 6 to 8.2.