Keep hydroxide and sulfate trends separate, and explain each reagent in an ion test rather than memorising colours alone.
Two families with opposite trends
From Mg to Ba, hydroxides become more soluble, whereas sulfates become less soluble. Learn the compound name with the direction: “Group 2 solubility increases” is incomplete.
Mg(OH)₂ is sparingly soluble. Ca(OH)₂ has limited solubility. Sr(OH)₂ and Ba(OH)₂ are more soluble. MgSO₄ is soluble, CaSO₄ is sparingly soluble, and SrSO₄ and especially BaSO₄ have very low solubility.
“Insoluble” is a practical description, not literally zero dissolved particles. A sparingly soluble hydroxide can still produce an alkaline solution. Do not call it a weak base merely because little dissolves.
| Down Mg → Ca → Sr → Ba | Hydroxides | Sulfates |
|---|---|---|
| Solubility trend | Increases | Decreases |
| Useful endpoint | Mg(OH)₂ sparingly soluble | BaSO₄ effectively insoluble for this test |
| A precipitate is… | Undissolved solid, typically white | Undissolved solid, typically white |
Adding hydroxide to Group 2 ion solutions
Add aqueous sodium hydroxide to a solution of a soluble Group 2 salt and look for a precipitate. At ordinary test concentrations Mg²⁺ forms white Mg(OH)₂; Ba²⁺ generally gives no visible change because Ba(OH)₂ is much more soluble. Results for intermediate members depend on concentrations.
Dissolved colourless ions do not make a white solution. “White precipitate” identifies a solid dispersed in the liquid; “colourless solution” identifies the liquid appearance.
Test for sulfate: acid first, then barium chloride
To a fresh portion of the unknown solution, add dilute hydrochloric acid, then aqueous barium chloride. A white precipitate indicates sulfate under these test conditions: Ba²⁺ and SO₄²⁻ form BaSO₄(s).
The acid removes interfering carbonate ions by converting them to carbon dioxide and water. Without this step, white BaCO₃ could be mistaken for BaSO₄. If carbonate causes fizzing on acid addition, let it finish before the precipitation test.
Do not acidify with sulfuric acid: it introduces the ion you are trying to detect. Barium chloride is useful because it is soluble and supplies Ba²⁺ ions, whose sulfate is very insoluble.
From the full equation to the ionic equation
Write correct salt formulae and balance the full equation. Split soluble aqueous ionic compounds into ions. Keep solids, liquids and gases intact. Cancel species that appear unchanged on both sides. These cancelled species are spectators.
For the example below, Na⁺ and NO₃⁻ cancel. Check both atoms and total charge: +2 and −2 combine to give the neutral solid. Do not write BaSO₄(aq), because precipitation is the formation of solid BaSO₄.
Preparing and collecting an insoluble salt
Mix suitable soluble reactants, then filter the suspension to collect the precipitate. Wash the residue with a small amount of distilled water to remove soluble contaminants, then dry it. Evaporating the whole mixture would leave soluble salts mixed with the desired solid.
In a gravimetric calculation, excess precipitating reagent helps ensure the target ion is collected completely. Wet residue gives an overestimated mass; loss of precipitate during transfer gives an underestimated mass.
For comparisons use matched volumes, concentrations and temperature. Record both a positive result and a genuine negative result. Wear eye protection and use the prescribed small quantities; soluble barium salts require appropriate handling and waste collection.
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Filtration of a sulfate precipitate
Labels to include:
- Filter funnel with filter paper touching its inside
- Solid residue on the paper, not below it
- Receiving flask and filtrate
- Distilled-water wash directed onto residue
- Optional separate vacuum setup: Büchner funnel, paper, side-arm flask and suction connection
The insoluble product stays on the paper. Soluble spectator ions leave in the filtrate and washings. Filtration and washing perform different jobs.
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. State both solubility trends down Mg–Ba.Show answer
Hydroxides become more soluble; sulfates become less soluble.
Q2. Why must dilute H₂SO₄ not be used to acidify a sulfate test?Show answer
It adds SO₄²⁻ ions, which produce BaSO₄ when BaCl₂ is added. That would create a false positive even if the original sample contained no sulfate.
Q3. Write the ionic equation for precipitating magnesium hydroxide and check its charge.Show answer
Mg²⁺(aq) + 2OH⁻(aq) → Mg(OH)₂(s). Total charge on the left is +2 − 2 = 0; the product is neutral.
Q4. How would you distinguish aqueous NaCl and BaCl₂ using one reagent?Show answer
Add aqueous sodium sulfate separately to each. BaCl₂ forms a white BaSO₄ precipitate; NaCl shows no visible change. Name the reagent as Na₂SO₄ solution, not just “sulfate ions”.
Q5. A sulfate precipitate is filtered but not washed or dried before weighing. Explain two possible effects.Show answer
Retained water adds to the measured mass. Soluble contaminants in the retained solution can also add mass. Both can make a mass-based sulfate result too high.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- Chemrevise — AQA 2.2 Group 2 (October 2025) — Primary coverage checklist, pp1–4; qualifications explained in the notes.
- AQA 7405 specification — 3.2.2 and relevant Required Practical 4 work.
- AQA 7404/1 2020 mark scheme — Q05.3, p16.
- AQA 7404/1 June 2019 mark scheme — Q08.1–08.2, pp14–15.
- AQA 7404/1 June 2019 examiner report — Q08, pp5–6: distinguish ions and consider the acid reaction.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.