Topic 4 of 6
Period-3 chlorides in water
Distinguish dissolution, hydrated-ion hydrolysis and molecular hydrolysis.
A-Level 9476 (2026-2027)
Dissolving a chloride is not always the same as hydrolysing it
Compare ionic salts, hydrated metal ions and reactive molecular chlorides.
| Chloride and highest oxidation number | Bonding or structure | Behaviour with excess water |
|---|---|---|
| NaCl: Na +1 | Ionic solid. | Dissolves to Na+ and Cl-; approximately neutral solution. |
| MgCl2: Mg +2 | Ionic solid. | Dissolves to hydrated ions; weak acidity can arise from limited hydrolysis of hydrated Mg2+. |
| AlCl3: Al +3 | Substantial covalent character; Al2Cl6 occurs in dry liquid/vapour descriptions. | Forms strongly polarising hydrated Al3+ ions; their hydrolysis makes the solution acidic. |
| SiCl4: Si +4 | Simple molecular covalent liquid. | Hydrolyses to silica (often hydrated) and HCl; acidic solution and white material. |
| PCl5: P +5 | Molecular in the gas phase; solid has [PCl4]+ and [PCl6]- ions. | Hydrolyses to H3PO4 and HCl in excess water. |
The general ionic-to-covalent trend follows decreasing electronegativity difference between the period-3 element and chlorine. AlCl3 requires special care: the small, highly charged aluminium centre strongly polarises the chloride electron cloud, giving substantial covalent character. A simple electronegativity threshold is not a complete explanation.
When AlCl3 enters excess water, aluminium becomes hydrated. The highly charged central ion withdraws electron density from the O-H bonds of coordinated water, allowing proton release: [Al(H2O)6]3+ + H2O ⇌ [Al(H2O)5(OH)]2+ + H3O+. Chloride is not the source of the proton. Do not assume that dissolution automatically precipitates all aluminium as Al(OH)3.
| Conditions | Balanced overall reaction |
|---|---|
| SiCl4 in excess water; hydration of silica simplified | SiCl4 + 2H2O → SiO2 + 4HCl |
| PCl5 with limited water | PCl5 + H2O → POCl3 + 2HCl |
| PCl5 in excess water | PCl5 + 4H2O → H3PO4 + 5HCl |
In SiCl4 and PCl5, chlorine attracts the bonding electrons, leaving an electron-poor central atom. Water can donate an oxygen lone pair to that centre; replacement of chlorine by oxygen-containing groups and proton transfer release HCl. This is chemical change of the chloride, whereas dissolving NaCl separates existing ions. Mg2+ has lower charge density than Al3+, so it weakens coordinated O-H bonds less strongly and its hydrated ion releases protons less readily.
Low boiling temperature or volatility supports a discrete molecular structure, whereas high melting temperature and molten conduction support an ionic lattice. Water reactions provide different evidence: vigorous hydrolysis of a molecular chloride does not mean it was an ionic salt before water was added. Name the state when discussing PCl5 or AlCl3.
Worked example
Deduce structure before trying to name a compound
Oxide A has a high melting temperature, does not conduct when solid or molten, does not react with water, and reacts with hot concentrated alkali. Chloride B is a volatile liquid, does not conduct electricity when pure, and reacts vigorously with water to give an acidic solution. What structures do the observations support?
- For A, the high melting temperature suggests strong bonding throughout an extended solid. Lack of molten conduction argues against a simple ionic lattice: melting an ionic solid would free its ions to move.
- A giant covalent network fits the combined physical evidence. Reaction with alkali establishes acidic oxide behaviour; its failure to react with water does not make it a neutral oxide.
- For B, volatility suggests discrete molecules held together by relatively weak intermolecular attractions. Its pure liquid has no mobile ions, consistent with a simple molecular covalent structure.
- The acidic solution from B shows hydrolysis. The ions formed in water do not establish that the original liquid was ionic.
A is consistent with a giant covalent acidic oxide such as SiO2; B with a simple molecular chloride such as SiCl4. These observations support structure classes; naming a unique element would require the permitted element range or additional composition data.