Acids & Bases (Edexcel GCSE Chemistry: Combined Science)

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Defining 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 0 – 14 (extremely acidic substances can have values of below 0)
  • 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

Indicators

  • Two colour indicators are used to distinguish between acids and alkalis
  • Many plants contain substances that can act as indicators and the most common one is litmus which is extracted from lichens
  • Synthetic indicators are organic compounds that are sensitive to changes in acidity and appear different colours in acids and alkalis
  • Phenolphthalein and methyl orange are synthetic indicators frequently used in acid-alkali titrations

Two Colour Indicators Table

Two Colour Indicators Table, downloadable IGCSE & GCSE Chemistry revision notes

  • Synthetic indicators are used to show the endpoint in titrations as they have a very sharp change of colour when an acid has been neutralised by an alkali and vice-versa
  • Litmus is not suitable for titrations as the colour change is not sharp and it goes through a purple transition colour in neutral solutions making it difficult to determine an endpoint
  • Litmus is very useful as an an indicator paper and comes in red and blue versions, for dipping into solutions or testing gases

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Stewart

Author: Stewart

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.