Lesson 4 of 6
Period-3 chlorides in water
Distinguish dissolution, hydrated-ion hydrolysis and molecular hydrolysis.
A-Level 9476 (2026-2027)
I can explain the specified oxide and chloride patterns.Syllabus
Syllabus 9476, 5(e). Explain the specified oxide and chloride patterns.
Dissolving a chloride is not always the same as hydrolysing it
Compare ionic salts, hydrated metal ions and reactive molecular chlorides.
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A few drops of silicon(IV) chloride are added to water. What happens?
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It reacts vigorously, giving fumes and a solution of about pH 2
SiCl4 is hydrolysed: SiCl4 + 2H2O -> SiO2 + 4HCl. The HCl formed gives steamy fumes and a strongly acidic solution.
| 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 exampleDeduce 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?
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- 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.
| Compound | Equation with water | Approximate pH |
|---|---|---|
| Na2O | Na2O(s) + H2O(l) → 2NaOH(aq) | 13-14 |
| MgO | MgO(s) + H2O(l) → Mg(OH)2(s); only slightly soluble | 9 (accept 8-10) |
| Al2O3 and SiO2 | No reaction; insoluble | 7 |
| P4O10 | P4O10(s) + 6H2O(l) → 4H3PO4(aq) | 2 (accept 1-3) |
| SO3 | SO3(g) + H2O(l) → H2SO4(aq) | 1 (accept 1-2) |
| NaCl | Dissolves: NaCl(s) → Na+(aq) + Cl-(aq); no hydrolysis | 7 |
| MgCl2 | Dissolves to [Mg(H2O)6]2+, which hydrolyses very slightly | 6.5 |
| AlCl3 | AlCl3(s) + 6H2O(l) → [Al(H2O)6]3+(aq) + 3Cl-(aq), then [Al(H2O)6]3+ ⇌ [Al(H2O)5(OH)]2+ + H+ | 3 |
| SiCl4 | SiCl4(l) + 2H2O(l) → SiO2(s) + 4HCl(aq); white solid and white fumes of HCl | 1-2 |
| PCl5 | PCl5(s) + 4H2O(l) → H3PO4(aq) + 5HCl(aq) | 1-2 |
Check your understanding
Separate samples of four solids are added to water. Which one gives a solution of pH about 3?
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AlCl3
Al3+ is small and highly charged. It polarises water molecules bonded to it, and [Al(H2O)6]3+ releases some H+, giving pH about 3.
Exam-style question
Magnesium chloride and silicon tetrachloride are added separately to water. For each, describe what you would see, write an equation and state the approximate pH of the solution formed. Explain the difference in behaviour. [4]
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