Acids, Alkalis & Neutralisation (Edexcel IGCSE Chemistry)

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Acids & alkalis

  • 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
    • It is a measure of the amount of the hydrogen ions present in solution

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
  • 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
  • For example, when hydrochloric acid is neutralised a sodium chloride and water are produced:

Equilibria Neutralisation Reaction of HCl and NaOH, downloadable AS & A Level Chemistry revision notes

  • 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(aq) OH(aq)⟶ H2O (l)

  • Neutralisation is very important in the treatment 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

Examiner Tip

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.

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Stewart

Author: Stewart

Expertise: Chemistry Lead

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 Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.