Explain why silicon dioxide has a very high melting point.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Silicon dioxide is used to line the inside of furnaces because it can withstand extremely high temperatures.
Model answer (4 marks)
Silicon dioxide has a giant covalent structure.
In this structure every silicon atom is covalently bonded to four oxygen atoms and each oxygen is bonded to two silicon atoms, giving a continuous network.
The bonds in the network are very strong covalent bonds.
To melt SiO₂ a large amount of energy must be supplied to break these bonds throughout the whole structure.
Hence a very high temperature is required to melt silicon dioxide.
In this structure every silicon atom is covalently bonded to four oxygen atoms and each oxygen is bonded to two silicon atoms, giving a continuous network.
The bonds in the network are very strong covalent bonds.
To melt SiO₂ a large amount of energy must be supplied to break these bonds throughout the whole structure.
Hence a very high temperature is required to melt silicon dioxide.
Examiner tips
- Use the word "giant covalent structure" first. Explain the network of Si–O bonds. State that many strong covalent bonds must be broken. Finish with the consequence: high melting point.
Common mistakes
- Saying only that it is a covalent compound, not a giant network. Forgetting to mention the strength of the covalent bonds. Using vague terms like "strong bonds" without linking to energy required.
Mark scheme (4 marks)
- Silicon dioxide has a giant covalent structure
- There are many strong covalent bonds throughout the structure
- A large amount of energy is required to break these bonds
- Therefore a very high temperature is needed to melt it
Key terms in this question
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →