(a)i) How would an aqueous solution of iron (II) tetraoxosulphate (VI) be converted into an aqueous solution of magnesium tetraoxosulphate (VI)? (ii) Write ...

(a)(i) Add excess magnesium (turnings/powder) to the aqueous iron(II) tetraoxosulphate(VI) solution and warm. Magnesium is more reactive than iron, so it displaces iron from the solution; the grey iron is deposited while magnesium tetraoxosulphate(VI) forms in solution. Filter off the excess magnesium and the displaced iron to obtain the \(MgSO_4\) solution.

(a)(ii) \[Mg + FeSO_4 \to MgSO_4 + Fe\]

(b)(i) Compounds are recrystallised after preparation in order to purify them, that is, to remove soluble impurities and obtain pure, well-formed crystals.

(b)(ii) Steps in recrystallisation

1. Dissolve the impure solid in the minimum volume of a hot suitable solvent to make a hot saturated solution.
2. Filter the hot solution to remove insoluble impurities.
3. Allow the filtrate to cool so that pure crystals separate out while soluble impurities remain in solution.
4. Filter off the crystals, wash with a little cold solvent, and dry them.

(c)(i) Two gases that change the colour of acidified potassium heptaoxodichromate(VI): sulphur(IV) oxide, \(SO_2\), and hydrogen sulphide, \(H_2S\) (both are reducing gases).

(c)(ii) The colour changes from orange to green (the orange \(Cr_2O_7^{2-}\) is reduced to green \(Cr^{3+}\)).