1Acid + alkali
Adding an alkali to an acid raises the pH. This reaction is neutralisation. When exactly enough alkali has been added, the solution is neutral.
neutralisationburette
Lesson 14.3 notes
Neutralisation
Acid plus alkali makes a salt and water; hydrogen and hydroxide ions; and neutralisation in everyday life.
2The pH graph
The pH stays low, then shoots up through 7 once all the acid has been used up. Extra alkali makes the solution alkaline.
3Ions in acids and alkalis
All acids contain hydrogen ions, H⁺. All alkalis contain hydroxide ions, OH⁻.
hydrogen ionhydroxide ion
4Making water
In neutralisation, H⁺ and OH⁻ ions join to make water: H⁺ + OH⁻ → H₂O. The ions left over make the salt.
water
5The salt
hydrochloric acid + sodium hydroxide → sodium chloride + water. Evaporate the water and crystals of the salt are left.
salt
6Indigestion
Too much stomach acid causes indigestion. An antacid tablet contains a base that neutralises some of the acid.
antacid
7Farming
Many crops grow badly in acidic soil. Farmers spread lime, a base made from limestone, to neutralise the acid and raise the pH.
lime
Explore: Neutralisation lab
Add alkali from the burette a little at a time. Can you stop at exactly pH 7?
Common mistake: Neutralisation does not always make a neutral solution
Neutralisation is the reaction between an acid and a base, such as an alkali. The solution is only neutral if exactly the right amounts react. Add too much alkali and it ends up alkaline.
Remember
- acid + alkali → salt + water. This is neutralisation.
- Acids contain hydrogen ions, H⁺. Alkalis contain hydroxide ions, OH⁻. They join to make water: H⁺ + OH⁻ → H₂O.
- Antacids, lime on acidic soil and toothpaste all work by neutralising acid.
Going further at GCSE