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Acids, Alkalis & Neutralisation (Edexcel IGCSE Chemistry)
Revision Note
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:
- 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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