Salts which are soluble in water can be prepared by the following two methods.
(a) Reaction of water soluble acid with water soluble base
The solution present in the conical flask now contains the soluble salt and water. Transfer the solution to an evaporating dish. If the salt being prepared is stable towards heat, then evaporate the solution to dryness to get the salt. Alternatively, heat the solution gently until a thin film of crystals is formed on the surface. Cool this solution slowly (now saturated). Filter to get pure crystals. Dry between folds of filter paper.
Interesting Information: Copper sulphate is used as a fungicide. Other salts are used in water softening and glass manufacture.
This is also called titration method. Acid and base selected depending on salt. e.g., potassium nitrate from KOH + HNO₃.
Prepare pure crystals of sodium sulphate.
➤ Add 50 cm³ 1 mol/dm³ NaOH in conical flask + few drops phenolphthalein (pink).
➤ Run 1 mol/dm³ H₂SO₄ from burette until colourless. Note volume.
➤ Repeat without indicator using same volumes. Transfer to dish, heat gently until ⅓ left. Dip glass rod – if cloudy, concentrate enough.
➤ Cool, crystallize. Filter, wash with cold distilled water, dry in filter paper/oven.
(b) Reaction of an acid with an insoluble base (or metal)
Neutralisation between soluble acid and insoluble base (metal oxide, carbonate or hydroxide). Acid and base selected depending on salt. Also formed when acid reacts with a metal.
Procedure: Mix acid with selected base/metal with constant stirring. Add insoluble base/metal in excess to ensure all acid reacted. Filter off excess. Evaporate filtrate to saturated solution. Cool to crystallize. Filter and dry crystals.
Examples:
Prepare pure crystals of zinc sulphate.
➤ Take ~25 cm³ dilute sulphuric acid in beaker, warm gently.
➤ Add small granules of zinc metal slowly with stirring until excess settles.
➤ Filter to remove unreacted zinc. Collect filtrate.
➤ Evaporate in dish by gentle heating to concentrate (no boiling).
➤ Cool slowly, crystals appear. Filter, dry on filter paper.
🔍 18.2 Quick Check!
1. Which salts of barium and calcium are soluble in water? (from table: most chlorides are soluble? but BaCl₂, CaCl₂ are soluble; BaSO₄, CaSO₄ insoluble. So chlorides of Ba & Ca are soluble, nitrates etc.)
2. How barium sulphate is prepared in the laboratory? (By reacting a soluble barium salt like BaCl₂ with a soluble sulphate like Na₂SO₄ – precipitation, but that’s for insoluble salts. However from context: insoluble salt prep uses precipitation.)
📌 key notes: Two main preparative routes for soluble salts: titration (soluble acid + soluble base, e.g., NaOH + H₂SO₄ → Na₂SO₄) and excess insoluble base/metal (e.g., Zn + H₂SO₄ → ZnSO₄). In titration, indicator used to find volume, then repeated without indicator. In excess method, add solid until no more reacts, filter, evaporate, crystallize. Always dry crystals carefully (filter paper or oven).
🧠 memory tips & tricks
🔹 TITRATION → soluble acid + soluble base → need indicator (phenolphthalein) → colour change then repeat without indicator.
🔹 INSOLUBLE BASE METHOD → add excess solid (metal / carbonate / hydroxide) to acid → filter off excess → filtrate contains salt.
🔹 Zn + H₂SO₄ → ZnSO₄ + H₂ (effervescence shows reaction).
🔹 Evaporation: heat gently until saturated (glass rod test: dip → cloudy means ready).
🔹 Crystals washed with cold distilled water to remove impurities.
📅 lesson planner
⬇️ click answer: true green, others red.
📚 student guidelines: Focus on two main routes – titration (soluble acid + soluble base) and excess insoluble base/metal. Understand the role of indicator and why experiment is repeated without it. For excess method, remember to filter off unreacted solid. Crystallization: concentrate until saturated, cool slowly. Use mnemonics. Quiz yourself.