Hydrochloric Acid

Hydrochloric acid is the acid 0620 uses most often, both in the lab and on the exam paper. It is a strong, monoprotic acid that makes chloride salts, and it is the classic case of a covalent compound that becomes an acid only once water lets it ionise.

Key facts

Property	Detail
Formula	HCl
Relative molecular mass (Mr)	36.5 (H 1, Cl 35.5)
Bonding (pure)	Simple covalent molecule (hydrogen chloride gas)
In water	Fully ionised: HCl -> H+ + Cl-
Acid strength	Strong and monoprotic (one H+ per molecule)
Salts produced	Chlorides (Cl-)

Where this compound appears in 0620

• Acids, bases and indicators (Topic 8): the three typical acid reactions.
• The pH scale (Topic 8): a strong acid with a low pH; contrasted with weak acids.
• Preparation of salts (Topic 8): making soluble chloride salts.
• Rate of reaction (Topic 6): Mg + HCl and CaCO3 + HCl are standard rate investigations.
• Tests for ions (Topic 11): a caution — never use HCl to acidify before the silver nitrate halide test.

Reactions you must know

With metals (salt + hydrogen):

Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g) Zn(s) + 2HCl(aq) -> ZnCl2(aq) + H2(g)

With bases / metal oxides (salt + water):

CuO(s) + 2HCl(aq) -> CuCl2(aq) + H2O(l) (black solid to blue solution) NaOH(aq) + HCl(aq) -> NaCl(aq) + H2O(l)

With carbonates (salt + water + carbon dioxide):

CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) + CO2(g)

Effervescence is seen, and the gas turns limewater milky.

Why it needs water to act as an acid

Pure hydrogen chloride is a covalent gas that does not conduct and shows no acidic behaviour. Only when it dissolves in water does it ionise completely:

• HCl(g) — covalent molecules, no mobile ions, not acidic
• HCl(aq) — fully ionised to H+ + Cl-, conducts, behaves as a strong acid

This is a Supplement point: dissolved in an organic solvent such as methylbenzene, HCl stays as molecules and is not acidic, showing that acidity depends on the H+ ions that water releases.

Worked exam question

Dilute hydrochloric acid is added separately to magnesium ribbon and to warm copper(II) oxide.

(a) Write the equation for the reaction with magnesium and name the gas produced. (2 marks)

(b) State what you would observe when hydrochloric acid is warmed with copper(II) oxide. (2 marks)

(c) Hydrochloric acid and ethanoic acid at the same concentration have different pH values, with hydrochloric acid the lower. Explain why. (2 marks)

Mark scheme

• (a) Mg + 2HCl -> MgCl2 + H2 [1]; the gas is hydrogen [1]
• (b) The black copper(II) oxide dissolves / disappears [1]; forming a blue solution (copper(II) chloride) [1]
• (c) Hydrochloric acid is a strong acid that is fully ionised, giving a higher concentration of H+ ions [1]; ethanoic acid is a weak acid, only partially ionised, so it has fewer H+ ions and a higher pH [1]

Examiner note: in (c) the comparison must be about the degree of ionisation and H+ concentration — “HCl is stronger” on its own scores nothing. In (b) name the colour of both the solid (black) and the solution (blue).

Common exam mistakes

• Making the wrong salt. Hydrochloric acid always gives chlorides — with zinc it is ZnCl2, never ZnCl or a sulfate.
• Forgetting the 2 in front of HCl. A 2+ metal ion needs two chloride ions (MgCl2, CaCl2, CuCl2), so the equations need 2HCl to balance.
• Acidifying with HCl before the silver nitrate test. That adds chloride ions and gives a false white precipitate — acidify with dilute nitric acid instead.
• Calling dry HCl gas acidic. Hydrogen chloride gas is covalent and shows no acidity; it must dissolve in water and ionise to H+ + Cl- first.