Comparing Acids & Alkalis (Cambridge (CIE) IGCSE Combined Science): Revision Note

The pH scale and universal indicator

What is the pH scale?

• The pH scale goes from 1 – 14 

• All acids have pH values of below 7, all alkalis have pH values of above 7

• The lower the pH then the more acidic the solution is:

  • pH 0-2 = strong acid

    • Extremely acidic substances can have values of below 1

  • pH 3-6 = weak acid

• The higher the pH then the more alkaline the solution is:

  • pH 8-11 = weak alkali

  • pH 12-14 = strong alkali

• A solution of pH 7 is described as being neutral

The pH scale

The pH scale showing acidity, neutrality and alkalinity

How is universal indicator paper used?

• Universal indicator paper is used to measure the pH

• A small strip of universal indicator paper is dipped into the solution

• The colour is matched with a colour chart which indicates the pH which matches specific colours

• The colour and pH can be used to compare solutions:

  • Neutral solutions give a green colour (pH 7)

  • The more acidic the solution:

    • The lower the pH

    • The colour moves towards red/orange/yellow

  • The more alkaline the solution:

    • The higher the pH

    • The colour moves towards blue/purple

The pH scale with the Universal Indicator colours which can be used to determine the pH of a solution