Carbon dioxide is a simple molecular compound. Explain why carbon dioxide is described as a compound rather than a mixture, and why it has a low melting point compared to diamond, which is also made up of non-metal atoms.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Carbon dioxide, CO₂, is produced during the combustion of fossil fuels. Diamond is a naturally occurring form of carbon. Both substances contain only non-metal atoms, yet their physical properties are very different.
Model answer (5 marks)
A compound is a substance made of atoms of different elements that are chemically bonded together, whereas a mixture contains separate components that are not bonded. Carbon dioxide consists of carbon and oxygen atoms joined by covalent bonds, so it is a compound.
CO₂ is a simple molecular compound; the molecules are held together only by weak intermolecular forces (London dispersion forces). Only a small amount of energy is required to overcome these forces, so CO₂ has a low melting point.
Diamond, in contrast, is a giant covalent network of carbon atoms. Every carbon atom is bonded to four others by strong covalent bonds, forming a rigid lattice. Breaking this network requires a large amount of energy, giving diamond a very high melting point.
CO₂ is a simple molecular compound; the molecules are held together only by weak intermolecular forces (London dispersion forces). Only a small amount of energy is required to overcome these forces, so CO₂ has a low melting point.
Diamond, in contrast, is a giant covalent network of carbon atoms. Every carbon atom is bonded to four others by strong covalent bonds, forming a rigid lattice. Breaking this network requires a large amount of energy, giving diamond a very high melting point.
Examiner tips
- Use the definition of a compound vs mixture; mention covalent bonds in CO₂
- Explain that CO₂ has only weak London forces, so melting point is low
- Contrast with diamond’s giant covalent network and high energy requirement
Common mistakes
- Saying CO₂ is a mixture because it contains only non‑metals
- Confusing the type of bonds in CO₂ with the type of bonds in diamond
- Failing to mention that CO₂’s low melting point is due to weak intermolecular forces rather than the strength of its covalent bonds
Mark scheme (5 marks)
- A compound contains atoms of different elements chemically bonded together (whereas a mixture does not involve chemical bonding)
- Carbon dioxide has covalent bonds between the carbon and oxygen atoms
- Carbon dioxide is a simple molecular substance with intermolecular forces between molecules
- Only a small amount of energy is needed to overcome these intermolecular forces, giving carbon dioxide a low melting point
- Diamond is a giant covalent structure with many strong covalent bonds throughout, requiring a large amount of energy to break them, giving diamond a high melting point
Key terms in this question
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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