When hydrochloric acid is mixed with sodium hydroxide solution, the hydrogen ions from the acid and the hydroxide ions from the base combine to form water, \mathrm{H^+ + OH^- \longrightarrow H_2O}.
The sodium ions and chloride ions take no part in this change and stay in the solution, so evaporating the water leaves sodium chloride, a salt.
The whole reaction is written \mathrm{HCl + NaOH \longrightarrow NaCl + H_2O}, and every neutralization of an acid by a metal hydroxide forms water from these two ions in the same way.
An acid and a base neutralize each other exactly when the amount of hydrogen ions the acid can give equals the amount of hydroxide ions the base can give.
Sulfuric acid is diprotic, so 1 mol of it gives 2 mol of hydrogen ions and needs 2 mol of sodium hydroxide, \mathrm{H_2SO_4 + 2NaOH \longrightarrow Na_2SO_4 + 2H_2O}.
At this point, called the equivalence point, the valency of the acid multiplied by its amount equals the valency of the base multiplied by its amount.
Ethanoic acid is a weak acid and only a small fraction of its molecules are ionized, yet a given amount of it needs as much sodium hydroxide as the same amount of hydrochloric acid.
As hydroxide ions remove hydrogen ions, more ethanoic acid molecules ionize, until all of them have reacted, \mathrm{CH_3COOH + NaOH \longrightarrow CH_3COONa + H_2O}.
Ammonia neutralizes hydrogen chloride by taking its hydrogen ion directly, \mathrm{NH_3 + HCl \longrightarrow NH_4Cl}, so this neutralization forms a salt but no water.
The stomach makes hydrochloric acid, and an antacid tablet containing a substance such as calcium carbonate or magnesium hydroxide neutralizes some of it when there is too much.
Many crops grow poorly in strongly acidic soil, so farmers spread slaked lime, which is calcium hydroxide, to neutralize the acid and raise the pH of the soil.
Sulfur dioxide in the waste gas from power stations is acidic, and passing the gas through a slurry of limestone neutralizes it before it reaches the air.
When a strong acid neutralizes a strong base, the solution at the equivalence point contains only a salt such as sodium chloride and is neutral, at pH 7 at 25 °C.
When 0.1 mol/L ethanoic acid is neutralized by 0.1 mol/L sodium hydroxide solution, the sodium ethanoate solution at the equivalence point is basic, at about pH 8.7, and BTB turns blue.
When ammonia is neutralized by hydrochloric acid in the same way, the ammonium chloride solution is acidic, at about pH 5.3, so neutralization means that the acid and the base have reacted exactly, not that the solution is neutral.