Predict pH and products when Period 3 chlorides meet water
Compare the specified chloride reactions with water and explain the likely acidity of the resulting mixtures.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Period 3 trends are qualitative unless supplied data are given. Equations show the stated products and conditions; observations and pH depend on concentration and experimental conditions. Written practice is self-checked
Before you begin
- Use electron configurations, oxidation numbers, balanced equations and ionic charges.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens to NaCl in water in the syllabus model?
What you will learn
- Describe the specified chloride–water reactions
- Write balanced hydrolysis equations
- Compare resulting solutions qualitatively by pH
Ionic chlorides dissolve; their cations can acidify water
NaCl dissolves to form hydrated Na⁺ and Cl⁻ ions; neither ion hydrolyses appreciably, so its solution is approximately neutral. MgCl₂ dissolves and gives a weakly acidic solution because hydrated Mg²⁺ can hydrolyse water slightly; Cl⁻ is a spectator ion.
AlCl₃ also gives an acidic aqueous solution. The small, highly charged Al³⁺ ion polarises water molecules strongly; a simplified hydrated-ion equation is [Al(H₂O)₆]³⁺ + H₂O ⇌ [Al(H₂O)₅OH]²⁺ + H₃O⁺. Aqueous pH depends on concentration, so do not invent an exact value.
Covalent chlorides hydrolyse to acidic products
SiCl₄ reacts with water to produce silica and hydrochloric acid: SiCl₄ + 2H₂O → SiO₂ + 4HCl. PCl₅ reacts with excess water to give phosphoric acid and hydrochloric acid: PCl₅ + 4H₂O → H₃PO₄ + 5HCl.
Both reactions produce acidic mixtures. Identify the specified amount of water when writing phosphorus chloride products; limited water can give different products. Do not describe these reactions as simple dissolution of unchanged molecules.
Pause and explain
Why is AlCl₃(aq) acidic?
Worked example
Compare the likely pH of NaCl(aq) and AlCl₃(aq), explaining the difference.
Show the worked solution
- NaCl dissolves to ions that do not appreciably react with water.
- Hydrated Al³⁺ hydrolyses water and produces H₃O⁺.
- NaCl solution is approximately neutral; AlCl₃ solution is acidic.
Answer NaCl(aq) is approximately neutral; AlCl₃(aq) is acidic because hydrated Al³⁺ hydrolyses water to form H₃O⁺.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the SiCl₄ hydrolysis equation and predict the solution's pH class.
Give me a hint
Hydrolysis forms silica and four HCl per SiCl₄ formula unit.
Compare my reasoning
- One Si atom gives one SiO₂.
- Four Cl atoms form four HCl; two H₂O balance O and H.
- HCl makes the mixture acidic.
SiCl₄ + 2H₂O → SiO₂ + 4HCl; the mixture is acidic.
Look for these in your work
- Products selected
- Equation balanced
- pH direction correct
Write the equation for PCl₅ reacting with excess water and name the acids formed.
Give me a hint
Use the specified excess-water product set.
Compare my reasoning
- PCl₅ + 4H₂O provides one P, four O and eight H.
- Products H₃PO₄ + 5HCl contain the same atoms.
- Both products are acids in aqueous mixture.
PCl₅ + 4H₂O → H₃PO₄ + 5HCl; phosphoric acid and hydrochloric acid are formed.
Look for these in your work
- Excess-water condition retained
- Atoms balanced
- Acid products identified
Rank NaCl(aq), MgCl₂(aq) and AlCl₃(aq) qualitatively from least acidic to most acidic in the usual comparison, and explain the limits of the ranking.
Give me a hint
Compare cation hydrolysis, and remember that solution concentrations affect pH.
Compare my reasoning
- Na⁺ does not appreciably hydrolyse, so NaCl is approximately neutral.
- Mg²⁺ hydrolyses weakly; Al³⁺ hydrolyses more strongly due to higher charge density.
- At comparable concentrations, acidity tends to increase NaCl < MgCl₂ < AlCl₃; exact pH requires concentration and conditions.
At comparable concentrations, NaCl is approximately neutral, MgCl₂ weakly acidic and AlCl₃ more acidic; exact pH depends on solution concentration and conditions.
Look for these in your work
- Neutral salt identified
- Cation hydrolysis compared
- Concentration limit stated
Common mistakes
- Calling every chloride solution neutral because it contains Cl⁻
- Treating AlCl₃ hydrolysis as simple dissolution
- Using the SiCl₄ equation for PCl₅
- Assigning exact pH without concentration
What can you explain now?
What happens to NaCl in water in the syllabus model?
Compare with the explanation
It dissolves and gives an approximately neutral solution
Na⁺ and Cl⁻ do not hydrolyse appreciably.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Which chloride merely dissolves, which hydrated ions acidify water, and which covalent chlorides hydrolyse?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.