International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

What color does a universal indicator show for a weak alkali?

1. Red

2. Pink

3. Blue

4. Green

The correct answer is: Blue

A universal indicator is a pH indicator that changes color at different pH levels, providing a spectrum based on acidity or alkalinity of a solution. In the case of a weak alkali, the pH typically falls in the range of around 8 to 10. In this range, the universal indicator will display a color that corresponds to a mild alkaline condition, which is typically blue. The pH scale runs from 0 to 14, with 0 being highly acidic (red) and 14 being highly alkaline (purple). Therefore, weak alkalis, which are less strong than strong alkalis and do not fully dissociate in water, will yield a blue color on the indicator, indicating a basic but not excessively alkaline environment. Other potential colors, such as red, typically indicate strong acidity; pink usually denotes a stronger alkaline solution, while green represents a neutral pH around 7. Hence, blue is the expected color for a weak alkali using a universal indicator.