3.9 - How bases react with acids
Use reactions with acids to recognise bases, explain how water forms, and distinguish the salt-and-water pattern from acid reactions that also release a gas.
Recognising the salt-and-water pattern
The working definition used in specification point 3.9 is that a base reacts with an acid to form a salt and water only. Metal oxides such as copper(II) oxide and metal hydroxides such as sodium hydroxide show this pattern:
acid + base → salt + water
hydrochloric acid + sodium hydroxide → sodium chloride + water
sulfuric acid + copper(II) oxide → copper(II) sulfate + water
A salt contains positive ions, such as metal ions, and the negative ions supplied by the acid. For example, sodium chloride contains Na⁺ and Cl⁻.
In an acid–hydroxide reaction, H⁺ ions react with OH⁻ ions to make water:
H⁺(aq) + OH⁻(aq) → H₂O(l)
The total charge is zero on each side and atoms are conserved. The other ions remain in solution as the dissolved salt. An oxide can also consume hydrogen ions to form water. This explains why a base reduces the acidity even if the base was initially an insoluble solid.
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