📖 In-Depth Theory
Methods of Making Salts
The method for making a salt depends on whether it is SOLUBLE or INSOLUBLE.
METHOD 1 — Acid + excess metal/metal oxide/carbonate (for soluble salts):
1. Add excess solid (metal, metal oxide or carbonate) to the acid — ensures all acid is used up.
2. FILTER off the excess unreacted solid (the salt remains in solution).
3. EVAPORATE the filtrate to crystallise the salt, or leave to crystallise slowly.
4. Filter and dry the crystals.
Example — copper sulfate from copper oxide:
CuO + H₂SO₄ → CuSO₄ + H₂O
Add excess black CuO powder to warm sulfuric acid → blue solution forms.
Filter → blue CuSO₄ solution passes through.
Evaporate → blue CuSO₄ crystals form.
METHOD 2 — Titration (for soluble salts from soluble starting materials):
Used when both reactants are soluble (e.g. NaOH + HCl → NaCl + H₂O).
Cannot use Method 1 — cannot filter off excess solid.
Instead, use a TITRATION to find the exact volume of acid needed to neutralise the alkali.
Then repeat without indicator → evaporate to get pure salt crystals.
Making Insoluble Salts — Precipitation
INSOLUBLE SALTS cannot be made from evaporation — they would never crystallise from solution.
They are made by MIXING two solutions that each contain one of the ions needed.
The insoluble salt PRECIPITATES immediately.
Example — making barium sulfate (BaSO₄):
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
Mix barium chloride solution + sodium sulfate solution.
BaSO₄ precipitates as a white solid immediately.
FILTER to collect the precipitate.
WASH with distilled water to remove soluble impurities.
DRY in an oven.
Precipitation reactions are also used to:
Remove unwanted ions from solution (e.g. water treatment).
Test for specific ions (e.g. adding AgNO₃ to test for chloride ions — white AgCl precipitate forms).
Neutralisation in Detail
NEUTRALISATION is the reaction between an ACID and a BASE (or alkali) to form a SALT and WATER.
The IONIC EQUATION for neutralisation is always the same:
H⁺(aq) + OH⁻(aq) → H₂O(l)
This shows the ESSENTIAL reaction — a hydrogen ion from the acid combines with a hydroxide ion from the alkali to form water.
TITRATION is the technique for precisely determining how much acid neutralises a given volume of alkali:
1. Pipette a known volume (e.g. 25.00 cm³) of alkali into a conical flask.
2. Add a few drops of INDICATOR (e.g. phenolphthalein — pink in alkali, colourless in acid).
3. Slowly add acid from a BURETTE until the indicator just changes colour (the END POINT).
4. Record the volume of acid used (the TITRE).
5. Repeat to get concordant (consistent) results.
6. Calculate mean titre from concordant results.
To make a PURE SALT by titration:
Repeat without indicator — the indicator would contaminate the salt.
Use the known volume from the titration.
⚠️ Common Mistake
When making a salt by adding excess solid to acid — you MUST use EXCESS solid to ensure ALL the acid reacts. If you don't use excess, some acid remains and the final product will be contaminated with acid. Then filter off the excess unreacted solid — the salt is in the filtrate.