Question

How does an acid-base indicator work?

Short answer

Acid-base indicators work by changing their color in response to the pH of the solution they are in. In acidic solutions, they donate protons and display the color of the undissociated molecules (acidic form). In basic solutions, they accept protons and display the color of the dissociated ions (basic form). In a neutral solution, a color intermediate between the two extremes is displayed.

Step-by-step solution

Understanding acid-base indicators

Acid-base indicators are substances that change their color based on the pH level of the solution they are in. These indicators are usually weak acids or bases themselves. This characteristic color change allows them to be used to determine whether a substance is acidic or basic.

Working of an Indicator in Acidic Solution

In an acidic solution, an acid-base indicator donates protons (H+) due to the presence of excess H+ ions in the solution. Hence, more undissociated indicator molecules are present which display the color of the acidic form of the indicator.

Working of an Indicator in Basic Solution

In a basic solution, an acid-base indicator accepts protons (H+) due to the presence of excess OH- ions in the solution. Therefore, more dissociated indicator ions are present and they display the color of the basic form of the indicator.

Neutral Solution

In a neutral solution (pH = 7), the amount of undissociated indicator molecules and dissociated indicator ions are equal. Therefore, the color observed in a neutral solution is the intermediate between the colors observed in acidic and basic solutions.