Salt: ionic compound formed by electrostatic attraction between oppositely charged ions, the positive ions coming from a base and the negative ions from an acid (HCl + NaOH → NaCl + H₂O).
Lattice: a repeating three-dimensional arrangement of ions that maximises attraction and minimises repulsion.
Must-know points
Ionic compounds are solids at room conditions with high melting and boiling points, because the strong electrostatic forces between the ions in the lattice need a lot of energy to break. (This is a repeated question: give two reasons: strong electrostatic attraction between oppositely charged ions, and the giant lattice.)
Conductivity: solid salts do not conduct (ions fixed in the lattice). Molten and aqueous salts conduct because the ions are free to move and carry charge.
Hard but brittle: a blow shifts the layers so like charges meet and repel, and the crystal splits.
Solubility rules: all sodium, potassium and ammonium salts are soluble; all nitrates are soluble; chlorides are soluble except lead(II) chloride and silver chloride; carbonates are insoluble except sodium, potassium and ammonium; sulphates are soluble except lead, barium and calcium; hydroxides are insoluble except sodium, potassium, ammonium and (partly) calcium.
Preparing a soluble salt:
Acid + alkali (titration), e.g. NaCl: pipette 25 cm³ of acid, add indicator (phenolphthalein), add alkali from the burette to the end point, note the volume, then repeat WITHOUT indicator using that volume, evaporate to a saturated solution, cool, crystallise, filter.
Acid + excess metal, e.g. ZnSO₄: Zn + H₂SO₄ → ZnSO₄ + H₂. Add excess zinc until bubbling stops, filter off the extra zinc, evaporate the filtrate, crystallise.
The steps in every method: add excess solid, filter, evaporate to saturation, cool and crystallise, filter and dry.
Past-paper questions with model answers
Why do ionic compounds have high melting points, two reasons (B, D, M): strong electrostatic forces between oppositely charged ions; the ions are held in a rigid giant lattice, so a large amount of energy is needed to separate them.
What is the usual state of ionic compounds under normal conditions (B): solids (with a lattice structure).
2025 and 2026 MCQs: sodium chloride is soluble (silver chloride, lead chloride, calcium carbonate are not); a soluble salt from an acid and an alkali is made by titration; excess metal is removed by filtration.