OCR A-Level Chemistry AS Breadth in chemistry (01), November 2021: Question 1

✅ Correct Answer

C (sodium chloride)

Sodium chloride is a giant ionic lattice. To melt or boil it, you must overcome strong ionic bonds acting in all directions throughout the lattice, requiring a massive amount of thermal energy.

💡 Key Knowledge

• Sodium chloride (NaCl): Giant ionic structure with strong electrostatic forces between oppositely charged ions (Na⁺ and Cl⁻).
• Ethanol (C₂H₅OH) & Water (H₂O): Simple molecular structures held together by hydrogen bonds (a type of intermolecular force).
• Heptane (C₇H₁₆): Simple molecular structure held together by weak London forces (induced dipole-dipole interactions).
• Rule of thumb: Intermolecular forces (hydrogen bonds, London forces) are significantly weaker than ionic or covalent bonds, meaning simple molecular substances generally have much lower boiling points than giant structures.

🧠 Exam Technique

Always categorise the bonding type of every option before looking at the specific forces within them. As soon as you spot a giant ionic compound mixed with molecular options, it will almost always have the highest melting and boiling point.

❌ Common Errors

• Mistaking hydrogen bonding for a true intramolecular bond. Students often see "hydrogen bonding" in water or ethanol and assume it's strong enough to rival ionic bonding.
• Forgetting that boiling a molecular substance only requires breaking intermolecular forces, whereas boiling an ionic substance breaks ionic bonds.