Acids, Alkalis & Neutralisation (Edexcel IGCSE Chemistry (Modular))

Revision Note

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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 neutralization.

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