• 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