AQA GCSE Chemistry

Revision Notes

4.2.6 The pH Scale & Neutralisation

Acids & Bases

  • When acids are added to water, they form positively charged hydrogen ions (H+)
  • The presence of H+ ions is what makes a solution acidic
  • When alkalis are added to water, they form negative hydroxide ions (OH)
  • The presence of the OH ions is what makes the aqueous solution an alkali
  • The pH scale is a numerical scale which is used to show how acidic or alkaline a solution is, in other words it is a measure of the amount of the ions present in solution

The pH Scale

  • The pH scale goes from 1 – 14 (extremely acidic substances can have values of below 1)
  • 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
  • The higher the pH then the more alkaline the solution is
  • A solution of pH 7 is described as being neutral

The pH scale, IGCSE & GCSE Chemistry revision notes

The pH scale showing acidity, neutrality and alkalinity

Measuring pH

  • pH can be measured using an Indicator  or a digital pH Meter
  • pH meters contain a special electrode with a thin glass membrane that allows hydrogen ions to pass through; the ions alter the voltage detected by the electrode
  • An indicator is a substance which changes colour depending on the pH of the solution to which it is added
  • There are natural indicators and synthetic indicators which have different uses
  • Generally, natural indicators are wide range indicators contain a mixture of different plant extracts and so can operate over a broad range of pH values
  • Synthetic indicators mostly have very narrow pH ranges at which they operate
    • They have sharp colour changes meaning they change colour quickly and abruptly as soon as a pH specific to that indicator is reached
  • Indicators are intensely coloured and very sensitive so only a few drops are needed

Neutralisation

  • A neutralisation reaction occurs when an acid reacts with an alkali
  • When these substances react together in a neutralisation reaction, the H+ ions react with the OH ions to produce water.
  • This is the net ionic equation of all acid-base neutralisations and is what leads to a neutral solution, since water has a pH of 7:

H+ OH– ⟶ H2O

  • Not all reactions of acids are neutralisations
    • For example when a metal reacts with an acid, although a salt is produced there is no water formed so it does not fit the definition of neutralisation
  • Neutralisation is very important in the treament of soils to raise the pH as some crops cannot tolerate pH levels below 7
  • This is achieved by adding bases to the soil such as limestone and quicklime

Universal Indicator

  • Universal indicator is a wide range indicator and can give only an approximate value for pH
  • It is made of a mixture of different plant indicators which operate across a broad pH range and is useful for estimating the pH of an unknown solution
  • A few drops are added to the solution and the colour is matched with a colour chart which indicates the pH which matches with specific colours
  • Universal indicator colours vary slightly between manufacturer so colour charts are usually provided for a specific indicator formulation

Universal-indicator-and-the-pH-scale2, IGCSE & GCSE Chemistry revision notes

pH scale with the Universal Indicator colours used to determine the pH of a solution

Exam Tip

A common error is to suggest using Universal Indicator as a suitable indicator for an acid-base titration. This is incorrect as a sharp colour change is required to identify the end-point, which cannot be achieved with Universal Indicator.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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