Addition of phenolphthalein to an unknown colorless solution does...

Question

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. (a) Is the solution acidic, neutral, or basic?

Accepted Answer

Understand the role of indicators: Phenolphthalein and bromthymol blue are pH indicators that change color at specific pH ranges. Phenolphthalein is colorless in acidic to neutral solutions and turns pink in basic solutions (pH > 8.2). Bromthymol blue is yellow in acidic solutions (pH < 6.0), green in neutral solutions, and blue in basic solutions (pH > 7.6).. Analyze the effect of phenolphthalein: Since the addition of phenolphthalein to the solution does not cause a color change, the solution is not basic. This means the pH is likely below 8.2.. Analyze the effect of bromthymol blue: The addition of bromthymol blue results in a yellow color, indicating that the solution is acidic, as bromthymol blue is yellow at pH values below 6.0.. Combine the observations: The lack of color change with phenolphthalein and the yellow color with bromthymol blue both suggest that the solution is acidic.. Conclude the solution's nature: Based on the behavior of both indicators, the solution is acidic.

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. (a) Is the solution acidic, neutral, or basic?

Key Concepts

pH Indicators

Acid-Base Theory

pH Scale