Salt preparation is one of those Combined Science Chemistry topics that looks harmless in the notes and then quietly eats four or five marks in Paper 3. The chemistry itself is not hard. The difficulty is that the question gives you a salt, and you have to decide which of three methods to use before you write a single word.
This post gives you the decision process we use ourselves, plus the exact phrasing examiners reward.
Start With Solubility, Always
Every salt preparation question is answered in the same order:
- Is the salt soluble or insoluble in water?
- If soluble, is it a sodium, potassium or ammonium salt?
- Then, and only then, choose your method.
You must know the solubility rules cold. For the Singapore/Cambridge syllabus:
- All nitrates are soluble.
- All sodium, potassium and ammonium salts are soluble.
- Chlorides are soluble, except silver chloride and lead(II) chloride.
- Sulfates are soluble, except barium sulfate, lead(II) sulfate and calcium sulfate (slightly soluble).
- Carbonates are insoluble, except sodium, potassium and ammonium carbonates.
If you can recite that list in twenty seconds, you have already secured most of the marks in this topic.
The Three Methods
Method 1: Titration (Soluble Salts of Sodium, Potassium or Ammonium)
These salts come from soluble bases (NaOH, KOH, aqueous ammonia). Because both reactants are soluble, you cannot use excess and filter off the leftovers, so you must measure the exact volumes.
Example: preparing potassium sulfate.
- Pipette 25.0 cm³ of potassium hydroxide into a conical flask and add a few drops of indicator.
- Titrate with dilute sulfuric acid from a burette until the indicator just changes colour. Record the volume.
- Repeat without the indicator, using the same volumes of acid and alkali.
- Evaporate the solution to the point of crystallisation, then allow it to cool and crystallise.
- Filter the crystals and dry them between filter papers.
The mark that most students drop is the "repeat without indicator" step. Say why: the indicator would contaminate the salt.
Method 2: Reacting With Excess Insoluble Substance, Then Filtering
Use this for soluble salts that are not sodium, potassium or ammonium salts, for example copper(II) sulfate, zinc chloride or magnesium nitrate.
Here the acid reacts with an insoluble metal, insoluble base (metal oxide or hydroxide) or insoluble carbonate. Because the solid does not dissolve on its own, you can safely add it in excess.
Example: preparing copper(II) sulfate crystals from copper(II) oxide.
- Warm dilute sulfuric acid in a beaker.
- Add copper(II) oxide a little at a time, stirring, until some solid remains unreacted (this shows the acid is used up).
- Filter to remove the excess copper(II) oxide.
- Heat the filtrate to concentrate the solution to the point of crystallisation.
- Leave to cool so crystals form, then filter and dry between filter papers.
Note the two different reasons for filtering and for the final drying. Examiners want "to remove unreacted copper(II) oxide", not just "to filter".
One caution: copper metal does not react with dilute sulfuric acid, so you must start from the oxide, hydroxide or carbonate. Reactive metals such as sodium are also unsuitable because the reaction is too violent.
Method 3: Precipitation (Insoluble Salts)
For barium sulfate, lead(II) iodide, silver chloride, calcium carbonate and friends, mix two soluble solutions that contain the right ions.
Example: preparing barium sulfate.
- Mix aqueous barium nitrate (or chloride) with aqueous sodium sulfate.
- Filter to obtain the barium sulfate residue.
- Wash the residue with distilled water to remove soluble impurities such as sodium nitrate.
- Dry between filter papers or in a warm oven.
The washing step is a real mark. Also, you never evaporate to crystallise here, because the product is the residue, not the filtrate.
Phrasing That Earns Marks
- "Heat to the point of crystallisation" beats "evaporate to dryness", which would decompose or dehydrate hydrated crystals.
- "Add excess until no further reaction / some solid remains" shows the acid is fully used up.
- "Wash with distilled water" for precipitates, not tap water.
- Name the reagents. "Dilute sulfuric acid" scores; "acid" does not.
- If asked for an ionic equation, include state symbols, for example Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
A Two-Line Checklist for the Exam
Write this in the margin before answering:
- Insoluble salt? Precipitation, then filter, wash, dry.
- Soluble Na/K/NH₄ salt? Titration, then crystallise.
- Any other soluble salt? Excess solid plus acid, filter, then crystallise.
Three branches, and you will never freeze on this question again.
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