Explain why silicon dioxide (SiO₂) has a much higher melting point than carbon dioxide (CO₂), despite both compounds having the same empirical formula type.

IB DP Chemistry Standard Level (2023 syllabus) — S2.4 From models to materials · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

CO₂ is a simple molecular substance, consisting of discrete CO₂ molecules.
CO₂ molecules are held together only by weak London/van der Waals dispersion forces, which require little energy to overcome.
SiO₂ has a giant covalent (network) structure in which each Si atom is covalently bonded to four O atoms (and each O to two Si atoms) throughout the entire lattice.
Melting SiO₂ requires breaking many strong covalent bonds throughout the entire network, requiring a very large amount of energy.

Examiner tips

  • Use the word "molecular" for CO₂ and "network covalent" for SiO₂. Mention the type of forces (London/van der Waals vs covalent).
  • Show the energy difference by stating that many covalent bonds must be broken.

Mark scheme (4 marks)

  1. CO₂ is a simple molecular substance / consists of discrete CO₂ molecules
  2. CO₂ molecules are held together only by weak London/van der Waals dispersion forces, which require little energy to overcome
  3. SiO₂ has a giant covalent / macromolecular / network covalent structure in which each Si atom is covalently bonded to four O atoms (and each O to two Si atoms) throughout the entire lattice
  4. Melting SiO₂ requires breaking many strong covalent bonds throughout the entire network, requiring a very large amount of energy

Key terms in this question

melting point

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