Route choice — the second reactant decides
The acid is one reactant; the OTHER fixes the method. Insoluble second reactant (a metal, an insoluble base such as copper(II) oxide, or a carbonate): warm the acid, add the solid in excess so all the acid is used up, then filter off the unreacted solid and crystallise the filtrate. Soluble second reactant (an alkali): nothing is left to filter, so titrate with an indicator such as methyl orange, then repeat with no indicator before crystallising.
Hydrated vs anhydrous — water of crystallisation
Hydrated means chemically combined with water in a fixed, whole-number ratio, written after a dot: hydrated copper(II) sulfate, , forms blue crystals. Anhydrous means no water at all — , a white powder. Heating drives the water of crystallisation off; adding water back turns the powder blue again, the chemical test for water. The fixed ratio gives each form a calculable : 160 anhydrous, 250 hydrated.
(Extended) Precipitation makes an insoluble salt
The Core routes cannot make an insoluble salt — they need it dissolved until crystallisation. Instead mix two soluble salt solutions whose ions combine: lead(II) nitrate plus sodium sulfate gives a white lead(II) sulfate precipitate at once; the sodium nitrate stays dissolved. Filter to collect it, wash with distilled water to remove the by-product, then dry. No evaporation. Solubility rules are not required — the question says which salt is insoluble.
Drawn from real examiner reports.
Hydrated ≠ wet; anhydrous ≠ dry
These are precise terms, not everyday ones. Hydrated means the water is chemically combined inside the crystal in a fixed ratio — part of the formula, not moisture on the surface — and only heating drives it off. Anhydrous means no water at all, not merely "dry to the touch": a solid can feel dry and still be hydrated, with the water locked inside the crystal.
Filtering off an excess alkali
An alkali is already dissolved, so "add sodium hydroxide in excess and filter it off" removes nothing — the excess simply stays in solution and contaminates the salt. Filtration separates an undissolved solid from a liquid, and there is no such solid here. When both reactants are soluble the volumes must instead be matched exactly using a titration end point.
Evaporating to dryness, not saturation
Heat the filtrate only until it is saturated — test by dipping a cold glass rod in and looking for crystals on it — then cool it slowly so crystals grow. Boiling straight to dryness drives the water of crystallisation out of a hydrated salt (or decomposes it) and leaves a powder rather than well-formed crystals, so the product is no longer the salt that was asked for.
Sulfate salts written as sulfite or oxide
The sulfate ion survives the reaction intact, so sulfuric acid with potassium hydroxide gives potassium sulfate, — never a sulfide or an oxide. The same slip appears as "CuSO3" or "Cu2SO4" for copper(II) sulfate. Build the formula from the ion charges rather than guessing, then check it balances the equation.
Flagged Jun 2022 P41 Q5aiii — the sulfate ion stays intact; the product is potassium sulfate, not sulfide or oxide
(Extended) Fe(II) hydroxide is green, not brown
When the precipitate identifies the insoluble salt formed, the exact shade carries the mark. Adding hydroxide to a solution gives a pale-blue precipitate; to an solution it gives a green one. Orange/brown belongs to . "A coloured precipitate", or the wrong ion's colour, scores nothing even when the method is right.
Flagged Jun 2023 P43 Q5bi — pale-blue precipitate with Cu2+; Fe(II) hydroxide is green, not the orange/brown of Fe(III)
(Extended) Crystallising an insoluble salt
A soluble salt is recovered last, by evaporating and crystallising its solution. An insoluble salt appears first, the instant the two solutions meet, so it is only filtered, washed and dried. Evaporating that mixture would deposit the soluble by-product on top of the product instead of purifying it, and omitting the wash leaves the same by-product behind.
Name the burette, not a pipette
Both measure volume, but a burette delivers a variable, precisely measured volume during the titration, while a pipette delivers one fixed volume — the acid measured into the conical flask at the start. Name the specific instrument the question asks for.
Check solubility before choosing a method
Ask two questions, in order. Is the target salt soluble or insoluble? Soluble means evaporate and crystallise; insoluble means precipitate and filter. Then, for a soluble target, is the second reactant soluble or insoluble? That choice alone picks titration or excess-and-filter.
Finish the method: wash and dry
Every route ends the same way — collect the solid by filtration, wash it with a little cold distilled water, then dry it between filter papers or in a warm place. A method that stops at the crystals or the precipitate has not given the "pure, dry sample" asked for.
Filtration diagrams: residue vs filtrate
Draw the filter paper folded into a cone lining the funnel, the unreacted excess solid labelled as residue on the paper, and the clear filtrate collected below — the two are routinely swapped. Leave the evaporating basin open, never sealed, or the water vapour cannot escape.
Cambridge 0654 spec reference: Section C7 "Acids, bases and salts", sub-topic C7.3. Core: preparation, separation and purification of soluble salts from an acid + (an alkali by titration / excess metal / excess insoluble base / excess insoluble carbonate); the hydrated/anhydrous definitions. Supplement (Ext): preparation of insoluble salts by precipitation. The general solubility rules for salts are NOT required.
| Term | Mark-scheme-precise meaning |
|---|---|
| Hydrated | A substance chemically combined with water (in a fixed ratio, shown by a dot in the formula, e.g. ) |
| Anhydrous | A substance containing no water at all |
| Excess | More solid is added than can react -- ensures ALL the acid has reacted, leaving unreacted solid that can be filtered off |
| Titration | Adding one solution from a burette to a measured volume of another until an indicator shows the exact reacting (end-point) volume |
| Precipitation (Ext) | Mixing two soluble-salt solutions so their ions combine to form an insoluble solid (the precipitate) |
| Filtration | Separating an insoluble solid from a liquid using filter paper in a funnel |
| Crystallisation | Recovering solid crystals from a saturated solution by slow cooling |
Define a hydrated substance.
Write the balanced symbol equation for the reaction between copper(II) oxide and dilute sulfuric acid used in the preparation described above. (2 marks)
