Acids, Alkalis & Neutralisation (Edexcel IGCSE Chemistry)

• 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

• 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
• 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⁺  +  OH^– ⟶ H₂O

• 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