Explain why simple molecular covalent substances, such as water and carbon dioxide, have low melting points, whereas giant covalent structures, such as diamond, have very high melting points.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Simple molecular covalent substances such as H₂O and CO₂ are held together by weak intermolecular forces (dipole–dipole, hydrogen bonding, London dispersion). These forces are easily overcome, so only a small amount of energy is required to melt the substance, giving a low melting point.
In contrast, giant covalent structures such as diamond consist of a continuous network of strong covalent bonds. A large amount of energy is required to break these bonds throughout the structure, so the melting point is very high.
In contrast, giant covalent structures such as diamond consist of a continuous network of strong covalent bonds. A large amount of energy is required to break these bonds throughout the structure, so the melting point is very high.
Examiner tips
- Use the words "weak intermolecular forces" and "strong covalent bonds". Include the idea that energy to break bonds determines melting point. Keep the answer concise and directly linked to the marks.
- Remember to mention both types of substances (simple molecular and giant covalent) in the same answer.
Common mistakes
- Confusing intermolecular forces with covalent bonds. Using vague terms like "strong forces" without specifying "covalent bonds". Omitting the energy requirement explanation for melting points.
Mark scheme (4 marks)
- Simple molecular substances have weak intermolecular forces between molecules
- Little energy is needed to overcome these weak intermolecular forces, so the melting point is low
- Giant covalent structures contain many strong covalent bonds throughout the structure
- A large amount of energy is needed to break these strong covalent bonds, so the melting point is very high
Key terms in this question
simple molecular covalent substance · giant covalent structure · melting point
Related
- All Pearson Edexcel International GCSE Chemistry (4CH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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