Hydrogen Chloride (HCl)

Summary: Hydrogen chloride (HCl) is a simple covalent molecule in which hydrogen and chlorine share one pair of electrons, leaving three lone pairs on chlorine; a colourless gas that dissolves in water to form hydrochloric acid and gives white fumes of ammonium chloride with ammonia. Tags: igcse chemistry Created: 2026-07-18

Hydrogen chloride (HCl) is a Compound of hydrogen and chlorine and a key IGCSE example of Covalent Bonding between two different non-metal atoms. The hydrogen atom (1 electron) needs one more electron to reach the helium duplet, and the chlorine atom (2,8,7) needs one more to reach the argon octet (see Group 0 Noble Gases); both needs are met by sharing a single pair of electrons, forming one H—Cl covalent bond and leaving three lone pairs on the chlorine atom. Although the Molecule itself is covalent, hydrogen chloride behaves very differently when dissolved: in water it splits completely into H⁺ and Cl⁻ ions, producing hydrochloric acid, a strong acid central to Acids and Bases chemistry. As a simple molecular substance, HCl is a colourless gas at room temperature because only a weak Intermolecular Force holds neighbouring molecules together. Its most famous laboratory behaviour is the formation of dense white fumes of ammonium chloride when it meets Ammonia gas — the classic demonstration of Diffusion.

Bonding: Dot-and-Cross Description

In the Dot-and-Cross Diagram of HCl, hydrogen’s 1 electron is drawn as a cross and chlorine’s 7 outer electrons as dots:

• Hydrogen contributes 1 electron and chlorine contributes 1 electron to a single shared (bonding) pair — one dot and one cross between the atoms.
• Chlorine’s remaining 6 outer electrons form 3 lone pairs; hydrogen has no lone pairs.
• Counting the shared pair, hydrogen has 2 outer electrons (duplet) and chlorine has 8 (octet).
• Only outer-shell electrons are drawn; chlorine’s inner shells (2,8) are omitted.

Displayed formula: H—Cl (a single covalent bond).

Key Data

Dissolving in Water: Hydrochloric Acid

Hydrogen chloride is extremely soluble in water. On dissolving, the covalent molecules fully dissociate into ions:

HCl(g) → H⁺(aq) + Cl⁻(aq)

• The solution is hydrochloric acid, a strong acid: complete dissociation gives a high concentration of H⁺(aq), so it has a very low pH (see Indicators and pH and Acids and Bases).
• The aqueous solution conducts electricity and shows all typical acid reactions — with metals (giving hydrogen), with bases in neutralisation, and with Carbonates (giving carbon dioxide). It is widely used in Making Salts to prepare chlorides.
• Dissolved in methylbenzene (a dry organic solvent), HCl stays as covalent molecules, so the solution shows no acidic properties and does not conduct — evidence that acidity requires H⁺ ions in water.

Reaction with Ammonia and Uses

When hydrogen chloride gas meets Ammonia gas, dense white fumes of solid ammonium chloride form:

NH₃(g) + HCl(g) → NH₄Cl(s)

In the classic sealed-tube experiment, the white ring forms nearer the HCl end because the lighter NH₃ molecules (Mᵣ = 17) diffuse faster than the heavier HCl molecules (Mᵣ = 36.5) — see Diffusion and Kinetic Particle Theory. The white-fume reaction also serves as a gas test for ammonia (see Chemical Tests).

Uses:

• Manufacture of hydrochloric acid for cleaning (“pickling”) steel, in metal processing, and for pH control in industry.
• Hydrochloric acid in the laboratory for salt preparation, titrations and Qualitative Analysis.
• Made industrially by burning hydrogen in chlorine: H₂ + Cl₂ → 2HCl.

Sources

• Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025) — Section 2.5, Cambridge Assessment International Education

Common Misconceptions