The colour of the phenolphthalein indicator in basic solution is:

• (A) Yellow
• (B) Green
• (C) Pink
• (D) Orange

The color of the phenolphthalein indicator in a basic solution is (C) Pink. This is an important concept in acid-base chemistry, and understanding why this happens can help you grasp the behavior of indicators in different pH environments.

What is Phenolphthalein?

Pheolphthalein is a common acid-base indicator used in titrations and other chemical analyses. It is colorless in acidic solutions and turns pink in basic solutions. The transition occurs at a pH range of approximately 8.2 to 10.0.

How Does It Work?

The color change of phenolphthalein is due to its molecular structure, which can exist in different forms depending on the pH of the solution. In acidic conditions, the phenolphthalein molecule is protonated, meaning it has extra hydrogen ions attached, which keeps it colorless. As the pH increases and the solution becomes more basic, these hydrogen ions are released, leading to a structural change that results in the pink color.

Understanding pH and Color Change

• Acidic Solutions: Below pH 8.2, phenolphthalein remains colorless.
• Neutral Solutions: Around pH 7, it is still colorless.
• Basic Solutions: Above pH 8.2, it turns pink, indicating the presence of hydroxide ions (OH^-).

Practical Applications

Phenolphthalein is widely used in titrations to determine the endpoint of a reaction. For example, when titrating a strong acid with a strong base, the appearance of a pink color signals that the solution has reached a neutralization point, allowing chemists to calculate the concentration of the acid or base involved.

Visualizing the Change

Think of phenolphthalein like a light switch. In an acidic environment, the switch is off (colorless), and as you add a base, it’s like flipping the switch on (turning pink). This visual cue is incredibly useful in laboratory settings.

In summary, when you encounter phenolphthalein in a basic solution, you can confidently identify it as pink. This color change not only serves as a visual indicator but also reflects the underlying chemistry of acid-base reactions.