Did you know? Sodium chloride mainly dissolves to hydrated ions in water, while silicon tetrachloride hydrolyses and releases acidic hydrogen chloride. The chemistry is different even though both contain chlorine.
This original Hougang JC1 H2 Chemistry guide follows the 2027 syllabus 9476 Period 3 chloride comparison: NaCl, MgCl₂, AlCl₃, SiCl₄ and PCl₅. It distinguishes ionic dissolution, hydrated-cation acidity and covalent chloride hydrolysis with carefully balanced, paper-only examples.
EDUKATESG · HOUGANG · JC1 H2 CHEMISTRY
11 chapters · Comparison table · Transfer questions · FAQs · Syllabus
Full chapter index
- 1. Start with the critical distinction: dissolution or hydrolysis?
- 2. Represent sodium chloride as an ionic lattice
- 3. Know why NaCl solution is approximately neutral
- 4. Read SiCl₄ as a covalent molecular compound
- 5. Balance the formal SiCl₄ hydrolysis model
- 6. Account for water’s chemical role
- 7. Explain magnesium chloride’s weak aqua-ion acidity
- 8. Understand hydrated aluminium-ion acidity
- 9. Include PCl₅ in the official Period 3 comparison
- 10. Avoid collapsing five water reactions into one rule
- 11. Use an unfamiliar chloride as a transfer test
1. Start with the critical distinction: dissolution or hydrolysis?
Sodium chloride and silicon tetrachloride both contain chlorine, but they do not experience the same transformation in water. Sodium chloride mainly separates from its solid ionic lattice into hydrated Na⁺ and Cl⁻. Silicon tetrachloride is a covalent molecular chloride that reacts chemically, allowing Si–Cl bonds to become silicon–oxygen-containing products and releasing acidic hydrogen chloride species.
The term dissolution means dispersal of a substance or its ionic constituents through a solvent. Hydrolysis instead means chemical reaction involving water. The fact that both processes occur on contact with water does not mean they are interchangeable. This is the basis of the 2027 H2 Chemistry 9476 comparison of the named Period 3 chlorides.
A tutor can ask students to list what species were present before water was added and what species exist afterward. If the original ions are simply hydrated, dissolution may describe the process. If covalent Si–Cl bonds are transformed, hydrolysis is the relevant category. No laboratory handling is needed to answer that question.
Student question: Do NaCl and SiCl₄ experience the same kind of water reaction?
Answer standard: No; ionic dissolution and covalent chloride hydrolysis are different.
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2. Represent sodium chloride as an ionic lattice
In solid NaCl, ions are held in an extended array by electrostatic attractions. The formula NaCl describes the simplest 1:1 ion ratio rather than a collection of discrete covalent NaCl molecules. Each sodium ion interacts with more than one chloride neighbour in the crystal structure.
Water has a permanent molecular dipole and can hydrate cations and anions. When sodium chloride dissolves, the ions are dispersed through water and become mobile. The simple equation NaCl(s) → Na⁺(aq) + Cl⁻(aq) communicates this physical process without inventing another compound.
The resulting salt solution can conduct because its hydrated ions move, but it does not follow that sodium metal or molecular chlorine is produced. A student who writes NaCl + H₂O → NaOH + HCl under ordinary conditions is confusing dissolution with an imagined hydrolysis reaction.
Student question: What is the appropriate equation for ordinary NaCl dissolution?
Answer standard: NaCl(s) → Na⁺(aq)+Cl⁻(aq).
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3. Know why NaCl solution is approximately neutral
In the simple school comparison, Na⁺ is not an appreciable acid through hydrolysis of its coordinated water, and Cl⁻ is the conjugate base of strong HCl, making it a very weak proton acceptor in water. A pure aqueous NaCl sample is therefore normally treated as approximately neutral.
This is a qualitative idealisation. Actual measured pH depends on temperature, dissolved carbon dioxide, sample impurities and the measurement method. It is not scientifically defensible to assert one exact pH for every container of ordinary salt solution.
The distinction matters because conductivity measures mobile charge, whereas pH reflects acid-base equilibria. A solution can have plenty of mobile ions and remain near neutral. Students should resist guessing acidity merely from the presence of a chloride ion.
Student question: Does a solution containing ions have to be strongly acidic?
Answer standard: No. NaCl can supply mobile ions without substantial hydrolytic acidity.
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4. Read SiCl₄ as a covalent molecular compound
Silicon tetrachloride consists of molecular units in which silicon is bonded to four chlorine atoms. In the ideal VSEPR model, the molecule is tetrahedral. Its Si–Cl bonds are polar covalent bonds, not the same extended ionic arrangement found in NaCl crystals.
SiCl₄ reacts with water in chemical transformations that substitute silicon–chlorine connections with silicon–oxygen-containing bonds. The formation of Si–O species and HCl is different from simple hydration of existing sodium and chloride ions. The relevant reaction is a change of chemical connectivity.
It is not necessary to claim that ’empty silicon 3d orbitals cause the entire reaction’ as a universal mechanistic explanation. The H2 syllabus names the chloride water reactions and expects bonding-related chemical reasoning; a detailed modern mechanism is more complex than that historical shortcut.
Student question: What makes SiCl₄ different from an NaCl-type ionic solid?
Answer standard: It is a covalent molecule whose Si–Cl bonds can be chemically transformed by water.
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5. Balance the formal SiCl₄ hydrolysis model
A formal initial product can be represented as orthosilicic acid Si(OH)₄ with four HCl equivalents: SiCl₄ + 4H₂O → Si(OH)₄ + 4HCl. The equation has one silicon and four chlorines on each side, with four oxygen atoms and eight hydrogen atoms balanced.
The Si(OH)₄ species can condense and form hydrated silica or silica-like polymeric products. For a simplified net formula, SiCl₄ + 2H₂O → SiO₂ + 4HCl is also balanced. It represents an idealised silica end product rather than a guarantee that the actual solid is pure crystalline SiO₂.
Students should not write SiCl₄ + H₂O → SiO₂ + 4HCl, which is unbalanced for H and O. The correct equation must use a product description consistent with the question and must preserve every atom. These equations are for paper analysis only; SiCl₄ is moisture-reactive and dangerous to handle.
Student question: What is the balanced net silica equation?
Answer standard: SiCl₄+2H₂O→SiO₂+4HCl, with hydrated-product caveats.
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6. Account for water’s chemical role
Water supplies oxygen to silicon-containing hydrolysis products and hydrogen equivalents associated with chloride-derived acid. It is therefore a reactant in SiCl₄ hydrolysis rather than only an inert solvent surrounding molecules that stay chemically unchanged.
A comparison of reactant and product formulas reveals why the reaction is not just SiCl₄(aq) followed by an unchanged SiCl₄ molecule. Chlorine leaves the silicon centre, silicon binds oxygen-containing groups, and acidic species form. The elementary pathway may include several steps, but the net chemical transformation is clear.
Students should avoid calculating an exact hydrolysis enthalpy without suitable bond, state and solvation data. A stable Si–O product helps explain the favourable chemistry, but a paper reaction formula alone does not supply a measured rate or a precise thermodynamic free energy.
Student question: What oxygen-containing change indicates true SiCl₄ hydrolysis?
Answer standard: Water-derived oxygen becomes part of new silicon-containing products.
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7. Explain magnesium chloride’s weak aqua-ion acidity
Magnesium chloride is largely ionic in the school structural model, and its aqueous solution contains hydrated Mg²⁺ and Cl⁻. Mg²⁺ has higher charge density than Na⁺ and polarises the O–H bonds of coordinated water molecules more strongly.
A limited proton-transfer equilibrium may occur: [Mg(H₂O)₆]²⁺ + H₂O ⇌ [Mg(H₂O)₅(OH)]⁺ + H₃O⁺. This gives a chemically sensible account of weak acidity in a typical aqueous magnesium salt without claiming that two HCl molecules are directly produced when MgCl₂ dissolves.
The equation is a simplified first hydrolysis step. Actual pH depends on ion concentrations and solution conditions; a formula alone does not provide an exact number. Tutors should distinguish hydration of the cation from the subsequent limited acid-base hydrolysis of its coordinated water.
Student question: What can give MgCl₂ solution weak acidity?
Answer standard: Proton transfer from water coordinated to polarising Mg²⁺.
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8. Understand hydrated aluminium-ion acidity
Anhydrous aluminium chloride has substantial covalent character, while in water strongly hydrated aluminium ions play a major role. The cation [Al(H₂O)₆]³⁺ has high charge density and can weaken O–H bonds in its coordinated water molecules through polarisation.
The first simplified acid-base step is [Al(H₂O)₆]³⁺ + H₂O ⇌ [Al(H₂O)₅(OH)]²⁺ + H₃O⁺. More hydrolysis and formation of hydroxide species may follow depending on pH and concentration.
This is not an equation saying Al³⁺ magically liberates three HCl molecules from chloride ions. The acidic species are tied to hydrated metal-ion chemistry. A strong tutorial answer names the aqua ion and explains why its greater charge density matters.
Student question: Why can AlCl₃ solution be more acidic than NaCl solution?
Answer standard: Hydrated Al³⁺ can hydrolyse coordinated water, generating H₃O⁺.
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9. Include PCl₅ in the official Period 3 comparison
Phosphorus pentachloride is another covalent chloride named by the 2027 syllabus. A familiar initial water reaction can be represented as PCl₅+H₂O→POCl₃+2HCl. Further hydrolysis gives POCl₃+3H₂O→H₃PO₄+3HCl.
Combining those paper equations gives PCl₅+4H₂O→H₃PO₄+5HCl. That result should be interpreted as a complete hydrolysis model: partial reaction conditions may show different intermediate species.
This contrasts with NaCl dissolution because the phosphorus-chlorine connectivity changes and oxygen-containing products form. Actual PCl₅ chemistry involves corrosive, moisture-reactive substances; the balanced equations do not constitute a laboratory procedure.
Student question: What is a balanced complete paper reaction for PCl₅ with water?
Answer standard: PCl₅+4H₂O→H₃PO₄+5HCl.
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10. Avoid collapsing five water reactions into one rule
Across NaCl, MgCl₂, AlCl₃, SiCl₄ and PCl₅, the chemical effect of water depends on the species and bonding. Sodium chloride mainly forms hydrated ions; magnesium and aluminium aqua ions can undergo proton-transfer hydrolysis; SiCl₄ and PCl₅ undergo covalent chloride hydrolysis.
The periodic table trend is useful, but a phrase such as ‘chlorides get more acidic toward phosphorus’ is not a sufficient chemical explanation. It fails to distinguish why the pH changes and what covalent or ionic species actually form. The 2027 H2 outcome expects the reaction behaviour of each named chloride.
A high-quality answer uses a property table with columns for bonding, role of water and acid-base outcome. The student should be able to rewrite the correct equations and then describe the significance in ordinary language.
Student question: What three categories of water behaviour recur in the named chlorides?
Answer standard: Ionic hydration, aqua-ion proton transfer and covalent chloride bond hydrolysis.
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11. Use an unfamiliar chloride as a transfer test
Imagine a fictional solid X that dissolves and releases hydrated cations and anions, with no appreciable pH change. The evidence is consistent with ordinary ionic dissolution; the student should not invent acid formation merely from the presence of chlorine in the formula.
A second fictional chloride Y is tetrahedral and covalent, giving an oxygen-containing solid and HCl equivalents after reaction with water. Such evidence points toward a chemically reactive hydrolysis model, like the named SiCl₄ case, but the identity of Y requires additional information.
A third case produces a weakly acidic aqueous solution from a dissolved metal chloride. The student should consider whether the hydrated cation can act as an acid. Distinguishing these pathways is more useful than copying the same water-reaction equation for every substance.
Student question: What evidence separates hydration from hydrolysis in an unfamiliar problem?
Answer standard: Whether chemical species and bond connectivity change rather than simply dispersing hydrated ions.
Worked evidence: five Period 3 chlorides
Each entry specifies a role for water and an ordinary school-level model. Some product species depend on hydration and concentration; actual laboratory chemistry may be more complex.
| Chloride / model | Bonding or reactant | Aqueous species / products | Difference |
|---|---|---|---|
| NaCl | Ionic lattice | Na⁺(aq)+Cl⁻(aq) | Dissolution, little hydrolytic acidity |
| MgCl₂ | Ionic salt | Hydrated Mg²⁺ and Cl⁻ | Limited aqua-ion hydrolysis |
| AlCl₃ | Anhydrous covalent character, hydrated Al species in water | [Al(H₂O)₆]³⁺ and hydrolysis products | Acidic aqua ion |
| SiCl₄ | Molecular covalent | Si(OH)₄ / hydrated silica and 4HCl | Covalent chloride hydrolysis |
| PCl₅ | Molecular chloride | POCl₃ intermediate, ultimately H₃PO₄+HCl | Stepwise hydrolysis |
| SiCl₄ formal equation | SiCl₄+4H₂O | Si(OH)₄+4HCl | Atom-balanced first product model |
| SiCl₄ silica net model | SiCl₄+2H₂O | SiO₂+4HCl | Real silica can be hydrated |
| PCl₅ full net model | PCl₅+4H₂O | H₃PO₄+5HCl | Fully hydrolysed paper representation |
Four transfer diagnostics
- Correct the false NaCl+H₂O→NaOH+HCl equation by writing the hydrated sodium and chloride ions.
- Balance SiCl₄+2H₂O→SiO₂+4HCl, while noting the silica product is a simplified representation.
- Explain acidity from AlCl₃ through hydrolysis of [Al(H₂O)₆]³⁺, not wholesale conversion of Cl⁻ to HCl molecules.
- Distinguish complete PCl₅ hydrolysis from initial POCl₃ formation and ordinary salt dissolution.
Tutor checklist
- Confirm H2 Chemistry 9476 Period 3 water-reaction outcomes and the learner’s examination year.
- Draw the ionic or covalent starting structures before interpreting the water reaction.
- Name whether water hydrates existing ions or participates in chemical bond transformation.
- Use hydrated cation acidity for Mg²⁺ and Al³⁺ with appropriate equilibrium qualifications.
- Balance at least one SiCl₄ and one PCl₅ hydrolysis model.
- Do not treat a reactive covalent chloride as a simple salt lattice.
- Avoid speculative mandatory empty-3d-orbital explanations.
- Keep the work as paper reasoning; SiCl₄ and PCl₅ are moisture-reactive hazards.
Frequently asked questions
Why does SiCl₄ hydrolyse but NaCl simply dissolve?
SiCl₄ has reactive covalent Si–Cl bonds; NaCl has an ionic lattice that separates into hydrated Na⁺ and Cl⁻.
What is the formal SiCl₄ hydrolysis equation?
SiCl₄+4H₂O→Si(OH)₄+4HCl, with silicon hydroxyl products able to condense.
What is the simplified silica net equation?
SiCl₄+2H₂O→SiO₂+4HCl, an idealisation of silica-forming chemistry.
Why can aluminium chloride solution be acidic?
Hydrated aluminium ions polarise coordinated water and transfer protons into solution.
Does sodium chloride make HCl when dissolved?
Not under the ordinary salt-dissolution model; it supplies hydrated ions.
Are all Period 3 chlorides identical in water?
No. Their structures, hydrated-ion chemistry and covalent hydrolysis reactions differ.
Are these hydrolysis outcomes in 2027 H2 Chemistry?
Yes. Syllabus 9476 explicitly names NaCl, MgCl₂, AlCl₃, SiCl₄ and PCl₅.
Connected Chemistry guides
Families can bring a current school Period 3 question and the student’s exact examination year for a tutorial discussion. Confirm teacher, class provision, venue and prices directly with a tuition provider. No seats or results are guaranteed.
Official syllabus and safety limits
- SEAB 2027 H2 Chemistry 9476 Section 5
- Official 2027 A-Level syllabus directory
- Immutable eduKateSG Clementi small-group tutorial reference
- Hougang Chemistry Tuition and Tutor Library
The official 2027 H2 syllabus covers reactions with water for all five named Period 3 chlorides. The equations are illustrative, and silica hydration and metal aqua-ion speciation depend on conditions. This article intentionally contains no instructions for handling moisture-reactive chlorides.