Simple molecular structures
3 marks · EasyExamine the table below showing properties of some substances.
| Substance | Structure | Melting point (°C) | Boiling point (°C) |
|---|---|---|---|
| Oxygen | simple molecular | -219 | -183 |
| Graphite | giant covalent | 3930 | 4830 |
| Sulphur dioxide | simple molecular | -73 | -10 |
Which substance in the table is likely to have the hardest structure? Give a reason.
Explain why oxygen has a much lower melting point than graphite.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Graphite, because it has a giant covalent structure which is very strong and hard. · must show identification and correct reason
(b) Oxygen is simple molecular, so it has weak intermolecular forces that are easily overcome, giving it a low melting point. Graphite has a giant covalent structure held by strong covalent bonds, needing a lot of energy to break. · must show mention of weak intermolecular forces in O₂; strong covalent bonds in graphite
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Marking points
B1 (a) · 1 mark · Correctly identifies graphite and provides correct reason (giant covalent structure).
M1 (b) · 1 mark · Explains that oxygen has weak intermolecular forces/needs little energy to melt.
A1 (b) · 1 mark · Explains graphite has strong covalent bonds/needs a lot of energy to melt.
Worked solution
a) Graphite is likely to have the hardest structure because it has a giant covalent structure, making it very strong.
b) Oxygen is simple molecular with weak intermolecular forces, requiring little energy to melt, so it has a low melting point. Graphite is a giant covalent structure, where strong covalent bonds must be broken, so its melting point is very high.