Indicators and the pH Scale

IGCSE Edexcel Chemistry
2.28-2.30 Indicators and pH
Key Concept: Indicators change color with pH. The pH scale runs from 0 to 14.

Section A: Indicators

1. State the color of litmus in acid and in alkali. [2]
2. State the color change of phenolphthalein in acid and alkali. [2]
3. What is the pH of a neutral solution? [1]

Section B: pH Values

4. Classify each pH value as acidic, neutral or alkaline: 2, 6, 7, 9, 13. [3]
5. Explain why universal indicator is useful in the lab. [2]

Total marks: 10

Mark Scheme

1. Acid: red; alkali: blue [2]
2. Acid: colorless; alkali: pink [2]
3. pH 7 [1]
4. 2 acidic, 6 weakly acidic, 7 neutral, 9 alkaline, 13 strongly alkaline [3]
5. It gives an approximate pH value with a color scale [2]