Silicon dioxide (SiO₂) is used to make glass and has a very high melting point. Explain why silicon dioxide has a very high melting point in terms of its giant covalent structure and the bonds present.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Silicon dioxide is a giant covalent structure used in many industrial applications, including the manufacture of glass and ceramics, because of its exceptional thermal stability.
Model answer (5 marks)
Silicon dioxide is a giant covalent structure in which each silicon atom is covalently bonded to four oxygen atoms and each oxygen atom is bonded to two silicon atoms, forming a continuous three‑dimensional network. A covalent bond is a shared pair of electrons between two atoms. In SiO₂ there are a very large number of such covalent bonds throughout the crystal lattice. These covalent bonds are very strong, requiring a large amount of energy to break. Consequently, a large amount of energy is needed to disrupt the network and melt the solid, giving silicon dioxide a very high melting point.
Examiner tips
- Mention the 3‑D network of covalent bonds
- Explain that covalent bonds are shared electron pairs
- State that many strong bonds exist
- Link the energy required to break bonds with the high melting point
Common mistakes
- Confusing SiO₂ with ionic compounds
- Failing to describe the 3‑D network
- Using vague terms like "strong bonds" without explaining they are covalent
Mark scheme (5 marks)
- Silicon dioxide is a giant covalent structure containing many atoms covalently bonded together
- A covalent bond is a shared pair of electrons between atoms
- There are very many / a large number of covalent bonds throughout the structure
- These covalent bonds are strong
- A large amount of energy is needed to break these bonds, so the melting point is very high
Key terms in this question
giant covalent structure · melting point
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
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- Decode the mark scheme abbreviations →
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