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?

Name: Chemistry: The Central Science
Author: Brown
ISBN: 9780134414232

검증된 단계별 안내

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.

주요 개념

질문에 올바르게 답하기 위해 반드시 이해해야 하는 핵심 개념들은 다음과 같습니다.

pH Indicators

pH indicators are substances that change color in response to changes in pH levels. Phenolphthalein is colorless in acidic and neutral solutions but turns pink in basic solutions, while bromthymol blue is yellow in acidic conditions and blue in basic conditions. Understanding how these indicators function helps determine the acidity or basicity of a solution.

Acid-Base Theory

Acid-base theory explains the behavior of acids and bases in terms of proton (H+) transfer. According to the Brønsted-Lowry theory, acids donate protons while bases accept them. The color change observed with bromthymol blue indicates that the solution is acidic, as it turns yellow in the presence of excess H+ ions.

pH Scale

The pH scale quantifies the acidity or basicity of a solution, ranging from 0 (very acidic) to 14 (very basic), with 7 being neutral. A pH less than 7 indicates an acidic solution, while a pH greater than 7 indicates a basic solution. The behavior of the indicators in the unknown solution suggests that its pH is below 7, confirming its acidic nature.