Acids, Alkalis & Neutralisation (Edexcel IGCSE Chemistry: Double Science)

• 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

• 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:

• 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