Attempt either option (A) or (B):

(A) (i) Give reasons for the following:

(I) Ionic compounds have generally high melting points and boiling points.

(II) Solder, an alloy of lead and tin, is used for welding electrical wires.

(III) Carbon cannot reduce the oxides of Na or Mg.

(ii) The reaction of compound 'X' with aluminium is used to join railway tracks:

(I) Identify the compound 'X'.

(II) Name the reaction.

(III) Write the balanced chemical equation of the reaction of compound 'X' with aluminium. [3+2]

OR

(B) (i) Write the balanced chemical equations when:

(I) A mixture of $Cu_2O$ and $Cu_2S$ is heated.

(II) $ZnS$ is heated in the presence of oxygen.

(ii) Give reasons for the following:

(I) The wires carrying current in homes have a coating of PVC.

(II) To make hot water tanks, copper is used and not steel.

(iii) Show the formation of ionic compound $CaO$ with electron dot structure.

[Atomic number: $Ca = 20$, $O = 8$] [2+2+1]

Attempt Option (A):

(i) Reasons:

(I) Ionic compounds have generally high melting and boiling points because the oppositely charged ions are held together by strong electrostatic (inter-ionic) forces of attraction throughout the crystal lattice. As the chapter states: "Ionic compounds have high melting and boiling points. This is because a considerable amount of energy is required to break the strong inter-ionic attraction."

(II) Solder (an alloy of lead and tin) is used for welding electrical wires because "the electrical conductivity and melting point of an alloy is less than that of pure metals," and specifically "Solder ... has a low melting point and is used for welding electrical wires together" — its low melting point lets it melt easily with a soldering iron to join wires without damaging them.

(III) Carbon cannot reduce the oxides of Na or Mg because these metals "have more affinity for oxygen than carbon" — i.e., Na and Mg are more reactive than carbon, so carbon cannot displace them from their oxides. Such highly reactive metals are instead obtained by electrolytic reduction of their molten compounds.

(ii)

(I) Compound X is iron(III) oxide, $Fe_2O_3$.

(II) The reaction is called the thermit reaction.

(III) Balanced equation:

$$Fe_2O_3(s) + 2Al(s) \rightarrow 2Fe(l) + Al_2O_3(s) + \text{Heat}$$

This highly exothermic displacement reaction produces molten iron directly at the joint, which is used to weld railway tracks and cracked machine parts.

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OR Attempt Option (B):

(i) Balanced equations:

(I) $$2Cu_2O(s) + Cu_2S(s) \xrightarrow{\text{Heat}} 6Cu(s) + SO_2(g)$$

(II) $$2ZnS(s) + 3O_2(g) \xrightarrow{\text{Heat}} 2ZnO(s) + 2SO_2(g)\quad(\text{roasting})$$

(ii) Reasons:

(I) The current-carrying wires in homes are coated with PVC (or a rubber-like material) because metals are good conductors of electricity, and a bare live wire would give an electric shock or cause a short circuit; PVC is an insulator that prevents current from passing to anything that touches the wire, as implied by the chapter's discussion of Activity 3.6 (metals conduct electricity) and the reason wires are insulated.

(II) Copper is used to make hot water tanks and not steel because copper is unreactive with water even on prolonged contact with hot water/steam — the chapter's reactivity series places copper below hydrogen among the metals "that do not react with water at all," whereas iron (in steel) reacts with steam ($3Fe + 4H_2O(g) \rightarrow Fe_3O_4 + 4H_2$) and rusts badly in the presence of moisture, which would damage the tank over time.

(iii) Formation of $CaO$ (electron dot structure):

Calcium ($Ca$, atomic number 20; configuration $2,8,8,2$) has 2 electrons in its outermost (valence) shell. Oxygen ($O$, atomic number 8; configuration $2,6$) has 6 electrons in its valence shell and needs 2 more to complete its octet.

Calcium transfers both its valence electrons to oxygen:

$$Ca \;(2,8,8,2) \longrightarrow Ca^{2+}\;(2,8,8) + 2e^-$$

$$O\;(2,6) + 2e^- \longrightarrow O^{2-}\;(2,8)$$

Electron-dot representation: $Ca$ with 2 dots is transferred completely to $O$'s outer shell (shown as crosses around O), giving $Ca^{2+}$ (no dots, stable octet in inner shell) and $O^{2-}$ surrounded by 8 dots/crosses (stable octet). The oppositely charged $Ca^{2+}$ and $O^{2-}$ ions then attract each other electrostatically to form the ionic compound calcium oxide, $CaO$.