(a) Explain why transition metals (i) have high melting points (ii) have variable oxidation states, (iii) exhibit paramagnetism (b) (i) Name the impurities ...

(a) Properties of transition metals

• (i) High melting points: transition metals have strong metallic bonds because both the outer s electrons and some inner d electrons take part in bonding, giving a large number of delocalised electrons and a strong lattice that needs much energy to break.
• (ii) Variable oxidation states: the energies of the 3d and 4s electrons are close, so different numbers of these electrons can be lost during bonding, giving several stable oxidation states.
• (iii) Paramagnetism: they have unpaired electrons in their d orbitals, and these unpaired electrons cause the atoms to be attracted into a magnetic field (paramagnetic).

(b) Bauxite and aluminium oxide

• (i) Impurities in bauxite: iron (III) oxide (Fe₂O₃) and silicon (IV) oxide (SiO₂).
• (ii) Removal of impurities: the powdered bauxite is treated with hot concentrated sodium hydroxide solution. The amphoteric aluminium oxide dissolves to form sodium aluminate, while the insoluble impurities (iron (III) oxide) are filtered off. Pure aluminium oxide is then precipitated and heated.
• (iii) Amphoteric nature of aluminium oxide: aluminium oxide is amphoteric because it reacts with both acids and bases to form a salt and water. With acid: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O. With base: Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄.

(c) Electrolysis of copper (II) tetraoxosulphate (VI) using copper electrodes

(i) At the cathode, copper ions are discharged and copper is deposited: Cu²⁺_(aq) + 2e^- → Cu_(s). At the anode, the copper electrode itself dissolves (goes into solution) rather than oxygen being evolved: Cu_(s) → Cu²⁺_(aq) + 2e^-. Copper thus dissolves from the anode and is deposited on the cathode. This is the principle used in the electrolytic purification of copper.

(ii) No, the blue colour does not change. For every Cu²⁺ ion removed at the cathode, one Cu²⁺ ion enters the solution from the anode. The concentration of copper (II) ions therefore remains constant, so the blue colour of the solution is unchanged.