Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• Topic 1.1 — Solids, liquids and gases
• Topic 2.4 — Ions and ionic bonds
• Topic 2.5 — Simple molecules and covalent bonds
• Topic 2.6 — Giant covalent structures
(a)(i)
For the correct answer:
B
Explanation: At $25^{\circ}C$, the temperature is well above substance B’s boiling point of $-183^{\circ}C$, meaning it has completely vaporised into a gas.
(a)(ii)
For the correct answer:
F
Explanation: Substance F has a melting point of $44^{\circ}C$, so at $25^{\circ}C$ it remains a solid. Its relatively low melting/boiling points and poor electrical conductivity indicate it is held together by weak intermolecular forces between simple molecules.
(a)(iii)
For the correct answer:
D
Explanation: Substance D melts at $-39^{\circ}C$ and boils at $357^{\circ}C$, so at $25^{\circ}C$ it is in the liquid state. Because it is a good electrical conductor in both states, it is a metal (specifically, Mercury).
(b)(i)
For the correct answer:
Substance: A
Reason 1: High melting point and/or high boiling point.
Reason 2: Poor conductor of electricity when solid AND molten.
Explanation: Giant covalent structures (like diamond or silicon dioxide) contain billions of atoms joined by strong covalent bonds, requiring immense thermal energy to break, hence the very high melting point ($1600^{\circ}C$). They generally lack delocalised electrons or mobile ions, making them poor electrical conductors in any state.
(b)(ii)
For the correct answer:
Substance: C
Reason: It only conducts electricity when molten (poor when solid).
Explanation: In a solid ionic lattice, the charged ions are fixed in place and cannot carry a current. Upon melting (which requires significant energy, hence the high melting point of $770^{\circ}C$), the ionic lattice breaks down, freeing the ions to move and conduct electricity.
