When hydrochloric acid is added to sodium ethanoate solution, the hydrogen ions join the ethanoate ions and form ethanoic acid: \mathrm{CH_3COONa + HCl \longrightarrow CH_3COOH + NaCl}. The solution starts to smell of vinegar.
Ethanoic acid is a weak acid, so once an ethanoate ion takes a hydrogen ion, the molecule it forms hardly ionizes again. Hydrochloric acid, a strong acid, stays fully ionized, and its hydrogen ions end up on the anion of the weaker acid.
Sodium and chloride ions take no part and remain in the solution. Any acid stronger than ethanoic acid, such as nitric acid or sulfuric acid, would set ethanoic acid free in the same way.
Carbonic acid is also a weak acid. When hydrochloric acid is added to a carbonate, each carbonate ion takes two hydrogen ions, one after the other, and becomes carbonic acid.
Carbonic acid is unstable and breaks down into water and carbon dioxide, which escapes as a gas. Marble, which is calcium carbonate, therefore gives off carbon dioxide in dilute hydrochloric acid: \mathrm{CaCO_3 + 2HCl \longrightarrow CaCl_2 + H_2O + CO_2}.
A hydrogencarbonate ion needs only one hydrogen ion, so sodium hydrogencarbonate reacts in the same way: \mathrm{NaHCO_3 + HCl \longrightarrow NaCl + H_2O + CO_2}. The carbon dioxide can be recognized because it turns limewater milky.
When sodium chloride is heated with concentrated sulfuric acid, hydrogen chloride comes off as a gas: \mathrm{NaCl + H_2SO_4 \longrightarrow NaHSO_4 + HCl}. It is denser than air and very soluble in water, so it is collected by downward delivery.
Hydrochloric acid and sulfuric acid are both strong acids, so strength does not explain this reaction. Hydrogen chloride boils at -85 °C, while sulfuric acid boils at about 340 °C, so on heating only hydrogen chloride leaves the mixture.
Acids that escape easily as gases, such as hydrogen chloride and nitric acid, are called volatile acids, and sulfuric acid is a non-volatile acid. Nitric acid can be made in the same way, by heating sodium nitrate with concentrated sulfuric acid.
The same idea works for bases. Heating ammonium chloride with calcium hydroxide, a strong base, sets free ammonia, a weak base: \mathrm{2NH_4Cl + Ca(OH)_2 \longrightarrow CaCl_2 + 2H_2O + 2NH_3}.
The hydroxide ions take hydrogen ions from the ammonium ions. The test tube is held with its mouth slightly lower, so that the water formed does not run back onto the hot glass.
Ammonia is lighter than air and very soluble in water, so it is collected by upward delivery. Damp red litmus paper held at the mouth of the collecting tube turns blue once the tube is full.
These reactions follow three patterns. A strong acid sets a weak acid free from its salt, as with sodium ethanoate and carbonates, and a strong base sets a weak base free, as with ammonium salts.
The third pattern is a non-volatile acid heated with the salt of a volatile acid, which drives the volatile acid off as a gas.
Several gases are made in the laboratory in these ways: carbon dioxide from marble and dilute hydrochloric acid, hydrogen chloride from sodium chloride and concentrated sulfuric acid, and ammonia from ammonium chloride and calcium hydroxide.