Indicators and the pH Scale

The pH scale is a numerical scale that measures how acidic or alkaline a solution is. The scale ranges from 0 (very acidic) to 14 (very alkaline).

Diagram of a pH scale, displaying a range of colours and numbers with corresponding examples of substances. The scale is labelled 'acidic' on the left, 'neutral' in the middle and 'alkaline' on the right, visually representing the varying pH levels of substances.

  • Acidic substances have a pH value lower than 7
  • Alkaline solutions have a pH value greater than 7
  • Neutral solutions have a pH value equal to 7

If you add the right amount of an acid to an alkali, it will form a neutral solution. The same holds true in reverse, if you add the right amount of an alkali to an acid solution, it will form a neutral solution.

This means that if we had something from the left side of the pH scale (an acid) and added the right volume of something from the right side of the pH scale (an alkali), we would eventually form a substance in the middle. We would form a neutral solution that has a pH of 7.


An indicator is a substance that changes colour in response to changes in pH. The pH of a solution can be measured using an indicator or a pH meter.

Universal indicator

A universal indicator is an acid-base indicator that can be used to determine the pH of a solution. When added to a solution, it produces a range of colours that can be compared to the pH indicator chart. This provides an accurate measurement of the solution’s pH.

  • On the left side of the scale, between pH 0-6, are acidic solutions. These solutions will range in colour from dark red (strong acid) to yellow (weak acid).
  • In the middle, at pH 7, you get neutral solutions, which is indicated by a colour change to green. This is where you find pure water.
  • On the right side of the scale, between pH 8-14, are alkaline solutions. These solutions will range in colour from dark green (weak alkali) to purple (strong alkali).

When performing acid-base reactions, we can use universal indicators to determine when the reaction has reached a neutral state. If neutralisation has occurred, the solution should change to a green colour.

Litmus Paper

Litmus paper is paper that has been treated with a mixture of dyes. The two types of litmus paper are blue and red.

  • Blue litmus paper turns red when placed in an acidic solution
  • Red litmus paper turns blue when placed in an alkaline solution

Illustration demonstrating the litmus test, showing how acidic solutions turn blue litmus paper red, and alkaline solutions turn red litmus paper blue. Accompanying text outlines the benefits of litmus tests, such as simplicity and quick results, and limitations, including the inability to determine pH value, and that other substances in the solution can interfere with the colour change of the litmus paper. It provides a comprehensive overview of the process and its utility.

pH Meter

A pH meter is a more precise and accurate way to determine the acidity or basicity of a solution compared to using a universal indicator. When placed in a solution, it gives a numerical value representing the pH of the solution.

For example, if the reading is 2, the solution is acidic. The numerical data provided by the pH meter is more accurate than the colour-based method of using a universal indicator.

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