Iron has a melting point of 1538 °C, whereas oxygen has a boiling point of −183 °C. Both substances consist of particles, but their physical properties are very different. Explain, in terms of particles, why iron is a solid at room temperature whilst oxygen is a gas at room temperature.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Iron has a melting point of 1538 °C. Oxygen has a boiling point of −183 °C. Room temperature is approximately 25 °C.
Model answer (4 marks)
Iron particles are held together by strong metallic forces of attraction.
A large amount of energy is required to overcome these forces, so iron remains solid at 25 °C (well below its 1538 °C melting point).
Oxygen molecules are held together by weak van der Waals forces.
Only a small amount of energy is needed to overcome these forces, so oxygen is a gas at 25 °C (well above its –183 °C boiling point).
A large amount of energy is required to overcome these forces, so iron remains solid at 25 °C (well below its 1538 °C melting point).
Oxygen molecules are held together by weak van der Waals forces.
Only a small amount of energy is needed to overcome these forces, so oxygen is a gas at 25 °C (well above its –183 °C boiling point).
Examiner tips
- Use the term ‘metallic forces’ for iron and ‘van der Waals forces’ for oxygen.
- Show the energy comparison (high vs low) to justify the state at 25 °C.
- Keep the answer short and to the point – 4 marks only.
- Mention the temperature comparison to the melting/boiling points.
Common mistakes
- Confusing metallic with ionic forces for iron.
- Using ‘electrostatic’ instead of ‘metallic’ for iron.
- Failing to mention the temperature relative to the melting/boiling point.
Mark scheme (4 marks)
- Iron particles are held together by strong (metallic/electrostatic) forces of attraction
- A large amount of energy is needed to overcome the forces in iron, so it remains solid at 25 °C (well below 1538 °C)
- Oxygen molecules have weak intermolecular forces between them
- Only a small amount of energy is needed to overcome the intermolecular forces in oxygen, so oxygen is a gas at 25 °C (well above −183 °C)
Key terms in this question
gas · solid · particles · melting point · boiling 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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