Explain why silicon dioxide has a high melting point and does not conduct electricity.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Silicon dioxide is used to line furnaces because it can withstand very high temperatures. It is a giant covalent structure.
Model answer (4 marks)
Silicon dioxide has a giant covalent network, so every atom is bonded by strong covalent bonds. A large amount of energy is required to break these bonds, giving a very high melting point. Because all the electrons are involved in covalent bonds, there are no free or delocalised electrons to carry charge, so it does not conduct electricity.
Examiner tips
- Use the term "giant covalent network" to show structure. Mention "strong covalent bonds" and "large energy required" for the melting point. State "no free/delocalised electrons" for lack of conductivity. Keep answer concise to fit 4 marks.
Common mistakes
- Confusing silicon dioxide with ionic compounds and claiming it conducts electricity. Using vague phrases like "strong bonds" without linking to energy or conductivity. Forgetting to mention the absence of free electrons.
Mark scheme (4 marks)
- Silicon dioxide has many strong covalent bonds
- A large amount of energy is needed to break these bonds / overcome these bonds
- There are no free/delocalised electrons (in silicon dioxide)
- So there are no charged particles free to move / carry charge
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →