A compound that changes color in the presence of an acid or base is known as what?

A compound that changes color in the presence of an acid or a base is known as an indicator. Indicators are substances that exhibit a distinct color change at specific pH levels, allowing them to signal the presence of acids or bases in a solution. Common examples include litmus, phenolphthalein, and bromothymol blue, each of which shifts to a different color depending on the acidity or basicity of the environment.

Indicators are essential tools in titrations and pH testing, providing visual cues that help determine the pH of a solution without the need for complex instrumentation. This property is based on the chemical structure of the indicator, which undergoes a structural change in response to the hydrogen ion concentration in acidic or basic solutions.

Other choices such as salt, buffer, and solution do not specifically relate to the property of color change in relation to acidity or basicity. A salt is the product of an acid-base reaction, a buffer is a solution that resists changes in pH when small amounts of acid or base are added, and a solution is a homogeneous mixture of substances. Therefore, the role of an indicator in detecting and signaling pH changes is what distinguishes it as the correct answer.