A reaction is the reaction between an acid and a base (or an alkali) that produces a and water
For an alkali, the central chemistry is that H+ ions from the acid combine with OH− ions from the alkali to form water:
H+(aq) + OH−(aq) → H2O(l)
This is the net ionic equation for every acid–alkali neutralisation; the spectator ions (e.g. Na+ and Cl−) move from reactants to products unchanged
Example: hydrochloric acid neutralised by sodium hydroxide
Full equation: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
Net ionic equation: as above
Example: sulfuric acid neutralised by potassium hydroxide
H2SO4(aq) + 2 KOH(aq) → K2SO4(aq) + 2 H2O(l)
Not every acid reaction is a neutralisation
A metal reacting with an acid gives a salt and hydrogen gas, not water, so it does not count as a neutralisation (see topic 13 section 1)
Neutralisation specifically requires that water be produced alongside the salt — so it is the acid + base / alkali / metal carbonate type reactions that qualify
A practical use: treating acidic soil
Many crops grow poorly when the soil pH drops below 7
Farmers spread bases such as crushed limestone (calcium carbonate) or quicklime (calcium oxide) on acidic soil
These bases neutralise the soil acidity and raise the pH back into the range crops tolerate