Preparation of Soluble and Insoluble Salts
Which method you use depends entirely on whether the salt is soluble. Learn the solubility rules first — everything else follows from them.
1. Naming salts
The metal (or ammonium) comes from the base; the rest of the name comes from the acid.
| Acid | Salt formed |
|---|---|
| Hydrochloric (HCl) | chloride |
| Sulfuric (H₂SO₄) | sulfate |
| Nitric (HNO₃) | nitrate |
| Ethanoic | ethanoate |
Example: copper oxide + sulfuric acid → copper sulfate + water
2. The solubility rules
ALL sodium, potassium and ammonium salts are SOLUBLE. ALL nitrates are SOLUBLE. Most chlorides are soluble — EXCEPT silver and lead. Most sulfates are soluble — EXCEPT barium, calcium and lead. Most carbonates are INSOLUBLE — EXCEPT sodium, potassium and ammonium. Most hydroxides are INSOLUBLE — EXCEPT sodium, potassium, ammonium and (partly) calcium.
Learn these before anything else — not remembering the solubility chart was a recorded error, and the rules decide both the method and the products of precipitation.
3. Method 1 — soluble salt from an INSOLUBLE base
Use when the base is an insoluble metal oxide, hydroxide or carbonate.
Example: copper oxide + sulfuric acid → copper sulfate
- Warm the acid gently
- Add the solid base in EXCESS, stirring until no more dissolves
- FILTER to remove the unreacted excess solid
- Evaporate the filtrate to the point of crystallisation (until crystals just appear)
- Leave to crystallise slowly
- Dry the crystals between filter paper
Add the base IN EXCESS to ensure ALL the acid reacts. That is the reason the examiner wants — leftover acid would contaminate the salt.
Filtering removes the EXCESS solid. The excess is the point of the method, and filtering is how it is removed.
Do NOT evaporate to dryness — that would decompose the crystals and lose the water of crystallisation. Heat until crystals just start to form, then leave to cool.
How you know there is excess: solid remains undissolved after stirring, and effervescence stops (with a carbonate).
4. Method 2 — soluble salt from a SOLUBLE base (titration)
Use when the base is a soluble alkali — you cannot add excess, because you could not filter it off.
Example: sodium hydroxide + hydrochloric acid → sodium chloride
- Titrate to find the exact volume of acid needed, using an indicator
- Repeat WITHOUT the indicator, using that exact volume
- Evaporate and crystallise
The second run without indicator is essential — otherwise the salt is contaminated with dye. This is a frequently examined point.
5. Method 3 — INSOLUBLE salt by precipitation
Mix two SOLUBLE solutions that contain the required ions. The insoluble salt forms as a precipitate.
Example: barium sulfate
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
- Mix the two soluble solutions
- FILTER to collect the precipitate as the residue
- WASH with distilled water
- DRY in a warm oven or between filter paper
Both starting solutions must be SOLUBLE — choose them using the solubility rules. Nitrates and sodium/potassium salts are always safe choices.
Wash the precipitate to remove the soluble by-product, which would otherwise contaminate it.
The ionic equation shows only the ions that react:
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
Include state symbols, and leave out the spectator ions.
6. Choosing the method
Is the salt SOLUBLE?
- Yes, and the base is insoluble → acid + excess solid, filter, crystallise
- Yes, and the base is soluble → titration
- No → precipitation
7. The four reactions that make salts
acid + metal → salt + HYDROGEN acid + base (metal oxide/hydroxide) → salt + WATER acid + carbonate → salt + WATER + CARBON DIOXIDE acid + alkali → salt + WATER (neutralisation)
Only the carbonate reaction produces carbon dioxide — the effervescence is how you recognise it.
8. Mistakes that cost marks
Not knowing the solubility rules.
Using the wrong method for the salt’s solubility.
Not adding the base in excess, or not explaining why.
Forgetting to filter off the excess.
Evaporating to dryness.
Leaving indicator in a titration-prepared salt.
Choosing insoluble starting materials for a precipitation.
Not washing the precipitate.
Including spectator ions in an ionic equation.
Wrong products for acid + carbonate.
Frequently asked questions
How do I name a salt? The metal comes from the base; the ending comes from the acid — sulfuric gives sulfate.
Which salts are always soluble? All sodium, potassium, ammonium salts and all nitrates.
Which carbonates are soluble? Only sodium, potassium and ammonium.
How do I make a soluble salt from an insoluble base? Add the base in excess to warm acid, filter, then crystallise.
Why add the base in excess? To ensure all the acid reacts, so no acid contaminates the salt.
Why not evaporate to dryness? It would decompose the crystals and drive off the water of crystallisation.
How do I make a salt from a soluble base? By titration — then repeat without indicator.
How do I make an insoluble salt? Precipitation — mix two soluble solutions, then filter, wash and dry.
Why wash the precipitate? To remove the soluble by-product.
Which acid reaction gives carbon dioxide? Acid + carbonate.
Quick revision checklist
- I can name the salt from the acid and base
- I know all the solubility rules
- I can choose the correct method for any salt
- I know the excess-solid method in order
- I can explain why excess is used and why we filter
- I know not to evaporate to dryness
- I know the titration method needs a second run without indicator
- I can plan a precipitation using soluble starting materials
- I know to wash and dry the precipitate
- I can write ionic equations with state symbols
- I know all four salt-forming reactions and their products
These notes cover the preparation of soluble and insoluble salts in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus for your own exam series.
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