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

What is the pH and universal indicator color of a weak acid?

1. 1-3, red

2. 4-6, yellow

3. 7, green

4. 8-10, blue

The correct answer is: 4-6, yellow

A weak acid typically has a pH range between 4 and 6. This indicates that while the acid is capable of donating protons, the concentration of hydrogen ions in solution is not as high as that of strong acids, which would have a much lower pH. When using a universal indicator, a weak acid would yield a yellow color, corresponding to this pH range. The color change observed in a universal indicator reflects the acidity or alkalinity of the solution according to its pH value, with colors changing through various stages from red (for strong acids) to orange, yellow, green (neutral), and through blue to purple (for strong bases). The other ranges and their associated colors do not align with the characteristics of a weak acid. A pH of 1-3 with a red hue indicates a strong acid, while a neutral pH of 7 results in a green color suitable for pure water or neutral solutions. Similarly, a pH of 8-10 would indicate a basic solution, which is not applicable here. Thus, the identification of a weak acid as having a pH of 4-6 and a yellow color is accurate and consistent with the properties of weak acids in solution.