(a)(i) List two physical properties used as criteria for purity of substances (ii) describe how you would prepare a pure, dry sample of sodium chloride crys...

(a)(i) Two physical properties used as criteria for purity

• Melting point (a pure solid melts sharply at a fixed temperature).
• Boiling point (a pure liquid boils at a fixed temperature). Impurities lower the melting point and raise the boiling point, and both then occur over a range.

(a)(ii) Preparation of pure, dry sodium chloride by neutralization

1. Using a pipette, place a known volume of dilute hydrochloric acid in a conical flask and add a few drops of an indicator (e.g. methyl orange).
2. Titrate with sodium hydroxide solution from a burette until the indicator just changes colour (neutral point). Record the volume of alkali used.
3. Repeat the experiment using the same volumes of acid and alkali but without the indicator, so that the salt is not contaminated by the dye.
4. Evaporate the resulting neutral solution over a water bath to the point of crystallization (until a saturated solution is obtained).
5. Allow to cool so that crystals of sodium chloride form; filter off the crystals and dry them between filter papers.

\[HCl_{(aq)} + NaOH_{(aq)} \to NaCl_{(aq)} + H_2O_{(l)}\]

(a)(iii) Two other general methods of preparing soluble salts

• Reacting a dilute acid with excess insoluble base or metal oxide/carbonate, then filtering off the excess and crystallizing.
• Reacting a dilute acid with excess reactive metal, then filtering off the excess metal and crystallizing.

(b) Explanations

(i) Iron is more reactive (higher in the electrochemical series) than copper, so it displaces copper from copper(II) tetraoxosulphate(VI) solution. The reddish-brown copper metal is deposited on the iron sheet:

\[Fe_{(s)} + CuSO_{4(aq)} \to FeSO_{4(aq)} + Cu_{(s)}\]

(ii) The white gelatinous precipitate is aluminium hydroxide, \(Al(OH)_3\). Because aluminium hydroxide is amphoteric, it dissolves in excess sodium hydroxide to form a soluble aluminate (tetrahydroxoaluminate):

\[Al(OH)_3 + NaOH \to NaAl(OH)_4\]

(iii) Chlorine is a stronger oxidizing agent and oxidizes the pale-green iron(II) ions to yellow-brown iron(III) ions:

\[2FeCl_{2(aq)} + Cl_{2(g)} \to 2FeCl_{3(aq)}\]

(iv) Reaction of dilute hydrochloric acid with marble (calcium trioxocarbonate(IV)):

\[CaCO_{3(s)} + 2HCl_{(aq)} \to CaCl_{2(aq)} + H_2O_{(l)} + CO_{2(g)}\]

Two industrial processes in which limestone is used as a raw material: manufacture of cement, and the extraction of iron in the blast furnace (as a flux). (Also acceptable: manufacture of glass and manufacture of lime.)