Soluble salts from acid + excess solid
Make a soluble salt by adding excess of an insoluble solid to a warm acid, so all the acid is used up. Route 1, acid + reactive metal → salt + hydrogen: Zn + 2HCl → ZnCl₂ + H₂. Route 2, acid + insoluble base/oxide → salt + water: CuO + H₂SO₄ → CuSO₄ + H₂O. Route 3, acid + carbonate → salt + water + CO₂: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Then filter off the excess solid, evaporate the filtrate to saturation, cool to crystallise, and dry with filter paper.
Titration, precipitation, solubility rules
Soluble reactant and salt (e.g. NaOH + HCl → NaCl + H₂O): excess cannot be filtered, so titration finds exact volumes; repeat without indicator and evaporate for crystals. Insoluble salts: precipitation — mix two solutions of the ions, e.g. Pb(NO₃)₂ + Na₂SO₄ → PbSO₄(s) + 2NaNO₃. Solubility: nitrates and all Na, K, ammonium salts soluble; chlorides except AgCl, PbCl₂; sulfates except BaSO₄, PbSO₄; most carbonates, hydroxides insoluble.
(Extended) Water of crystallisation
(Extended) Water of crystallisation is water molecules chemically combined in a salt's crystal lattice; the hydrated formula shows them after a dot, e.g. CuSO₄·5H₂O (blue), Na₂CO₃·10H₂O (washing soda), MgSO₄·7H₂O (Epsom salt). The anhydrous salt (white CuSO₄) has none. Find n from Mr data: n = [Mr(hydrated) − Mr(anhydrous)] / 18. For CuSO₄·nH₂O with Mr 249.5 and Mr(CuSO₄) = 63.5 + 32 + 64 = 159.5, n = (249.5 − 159.5)/18 = 5.
Drawn from real examiner reports.
Precipitation, not crystallisation
The method for making an insoluble salt is precipitation — mixing two solutions whose ions combine to a solid — not crystallisation. Crystallisation is the later step for a soluble salt. To make lead(II) sulfate: mix a soluble lead salt with a sulfate solution, filter the white precipitate, wash with distilled water and dry. Naming "crystallisation" here scores zero.
W22 P42 Q2(d)(i)
Evaporate to saturation, not to dryness
For good crystals of a soluble salt, evaporate only to saturation — test by dipping a cold glass rod and watching for crystals — then cool slowly so crystals grow, filter, and dry with filter paper at room temperature. Do not evaporate to dryness or heat in an oven: this drives off the water of crystallisation and can decompose the salt (blue CuSO₄·5H₂O → white CuSO₄).
W22 P42 Q4(d)(ii); W22 P41 Q4(e)(i)
Observations, not product names
Marks for "describe what you observe" go to what you can see, hear or feel — effervescence, a solid dissolving, a colour change — not product names. In a salt preparation, "hydrogen is produced" or "zinc chloride is formed" names products and scores zero; "bubbles of colourless gas" and "the solid dissolves" earn the marks. Keep names for "state the products" or an equation.
W22 P31 Q6(a); W22 P32 Q6(a)
Excess-solid route needs an insoluble base
The excess-solid method works only when the base is insoluble, because you rely on filtering off the leftover solid. If the base is soluble (NaOH, KOH, ammonia) you cannot filter any excess, so use titration to match exact volumes. "Add excess NaOH, then filter" for NaCl is wrong — dissolved NaOH cannot be filtered, leaving impurity.
(Extended) Hydrated is not dissolved
(Extended) "Hydrated" does not mean dissolved in water. A hydrated salt is a solid whose crystals contain water of crystallisation chemically combined in the lattice, e.g. blue CuSO₄·5H₂O. A salt dissolved in water is "aqueous" (aq) — different. Anhydrous means no water of crystallisation (white CuSO₄). Heating drives off this combined water and changes the colour.
(Extended) Use Mr(H₂O)=18 and Cu=63.5
(Extended) Two arithmetic traps spoil water-of-crystallisation answers. First, the Mr of water is 18 (2×1 + 16), not 16 — dividing by 16 gives the wrong n. Second, use the syllabus Ar: copper is 63.5, so Mr(CuSO₄) = 63.5 + 32 + 64 = 159.5, not 160. Then n = [Mr(hydrated) − Mr(anhydrous)]/18, counting all four oxygens of the sulfate.
Choose the preparation route
Decide the route in two questions. Is the salt insoluble? If yes → precipitation: mix two soluble solutions, then filter, wash, dry. If the salt is soluble, is the base soluble? Insoluble base (metal, oxide, carbonate) → excess-solid method; soluble base (alkali) → titration.
Getting pure dry crystals
For crystals from a soluble-salt solution: filter off excess solid, evaporate to saturation, cool slowly to crystallise, filter, then dry with filter paper. Never evaporate to dryness or heat in an oven, which loses the water of crystallisation.
Use SNAP to pick reagents
Recall solubility with SNAP: all Sodium, Nitrate, Ammonium, Potassium salts are soluble. Chlorides soluble except AgCl, PbCl₂; sulfates except BaSO₄, PbSO₄; carbonates and hydroxides mostly insoluble. Use it to choose precipitation reagents.
Salt — an ionic compound formed when the hydrogen ions of an acid are replaced by metal ions (or ammonium ions). The name has two parts: the metal (or ammonium) cation, and the anion from the acid (chloride from ; sulfate from ; nitrate from ).
Solubility rules (Core — essential rote recall):
| Salt type | Solubility rule |
|---|---|
| Nitrates | All soluble |
| Sodium, potassium, ammonium salts | All soluble |
| Chlorides | Soluble — except (white) and (white) |
| Sulfates | Soluble — except (white) and (white); slightly soluble |
| Carbonates | Insoluble — except , , |
| Hydroxides | Insoluble — except NaOH, KOH, ; slightly soluble |
Memory aid: "SNAP" — Sodium, Nitrate, Ammonium, Potassium are ALL soluble, no exceptions.
State the solubility of: (a) all nitrates, (b) all sodium/potassium/ammonium salts, (c) silver chloride () and lead(II) chloride (), (d) barium sulfate ().
Calcium carbonate is added to dilute hydrochloric acid to produce a salt.
(a) Write a balanced symbol equation for this reaction, including state symbols.
(b) Describe what you would observe during this reaction.
(c) State which step is carried out to remove excess calcium carbonate from the reaction mixture.
