Acids and alkalis

This lesson covers: 

  1. The pH scale
  2. Using indicators
  3. Comparing acid and alkali strength

The pH Scale

The pH scale measures the acidity or alkalinity of a solution on a scale from 0 to 14.

Diagram of the pH scale showing acidity and alkalinity from 0 to 14 with neutral at 7.
  • pH 7 is neutral.
  • Below pH 7 is acidic .
  • Above pH 7 is alkaline.

Using Indicators

Indicators are special dyes that change colour depending on the pH.

Litmus paper and universal indicator are two common indicators.

Litmus paper as an indicator

Diagram showing blue litmus paper turning red in acid and red litmus paper turning blue in alkali.
  • Blue litmus paper turns red in acids.
  • Red litmus paper turns blue in alkalis.

Universal indicator

Illustration showing universal indicator changing colour in acid and alkali solutions.

Universal indicator solution can be added to substances.

  • It contains a mixture of indicators that change across pH range.
  • The colours match up with different pH levels on the pH scale, telling us if a substance is acid, alkali or neutral.

Acid and Alkali Strength

The pH scale also shows us the relative strength of acids and alkalis.

  • The lower the pH, the stronger the acid.
  • The higher the pH, the stronger the alkali.
Illustration of the pH scale showing the strength of acids and alkalis with examples including stomach acid, tomato juice, baking soda, and bleach.

For example:

  • Stomach acid = pH1, so it is a strong acid.
  • Tomato juice = pH 4 , so it is a weak acid.
  • Baking soda = pH 9, so it is a weak alkali.
  • Bleach = pH 12, so it is a strong acid.