The pH Scale

  • When acids are added to water, they lose electrons to form positively charged hydrogen ions (H+).
  • The presence of H+ ions is what makes a solution acidic.
  • When alkalis are added to water, they gain electrons to 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 ion present in solution.
  • 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 neutrality, since water has a pH of 7:

H+ + OH ⟶ H2O

  • 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 consist of a probe connected to an electronic device which measures the pH based on the solutions conductivity (since it contains ions, then it can conduct electricity) and provides an accurate result.
  • An indicator is a substance which changes colour depending on the pH of the solution to which it is added.
  • They are usually made from special types of dyes that are sensitive to pH changes.
  • Some indicators have very narrow pH ranges at which they operate.
  • Wide range indicators contain a mixture of different indicators and so can operate over a broader range of pH values.
  • Indicators have sharp colour changes meaning they change colour quickly and abruptly as soon as a pH specific to that indicator is reached.
  • Most indicators are actually themselves slightly acidic so only a few drops are needed and it is usually added using a teat pipette.

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 indicators which operate across a broad pH range (usually pH 2/3 – 14) 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-and-the-pH-scale2, IGCSE & GCSE Chemistry revision notes

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

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.