1. 1
   Silicon dioxide, SiO₂, the main component of sand, is used to make glass. It has a very high melting point of around 1710°C. A student tests a sample of solid silicon dioxide to see if it conducts electricity, connecting it in a circuit with a battery and ammeter.

   (a)

   Which statement explains why silicon dioxide has such a high melting point?

   [1 mark]

   • ASilicon dioxide is a giant covalent structure in which every atom is joined to its neighbours by strong covalent bonds throughout the whole structure
   • BSilicon dioxide consists of simple molecules held together by weak intermolecular forces
   • CSilicon dioxide is an ionic compound with strong electrostatic forces between ions
   • DSilicon dioxide contains delocalised electrons that hold the structure together

   (b)

   The student finds that the solid silicon dioxide does not conduct electricity. Which statement explains this observation?

   [1 mark]

   • ASilicon dioxide repels electrons from the battery
   • BSilicon dioxide contains ions that are too large to move
   • CSilicon dioxide has too high a melting point to conduct electricity
   • DAll the electrons in silicon dioxide are held in localised covalent bonds between atoms, with none free to move through the structure

   (c)

   Explain, in terms of structure and bonding, why silicon dioxide has such a high melting point and does not conduct electricity as a solid.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Silicon carbide, SiC, is a very hard, giant covalent macromolecule used to make cutting tools, with a melting point of around 2700°C. An engineer investigates why cutting tools made from silicon carbide can be used at extremely high temperatures and pressures without melting or breaking down.

   (a)

   Which statement best describes the bonding in silicon carbide?

   [1 mark]

   • ASilicon carbide molecules are held together by weak intermolecular forces
   • BEvery silicon and carbon atom is joined to its neighbours by strong covalent bonds throughout the whole structure
   • CSilicon carbide is held together by electrostatic forces between Si⁴⁺ and C⁴⁻ ions
   • DSilicon carbide consists of layers that can slide over each other

   (b)

   Which statement correctly explains why silicon carbide is extremely hard and resistant to melting?

   [1 mark]

   • ASilicon carbide is made of very heavy atoms that are difficult to move
   • BSilicon carbide contains free-moving delocalised electrons that make it rigid
   • CEvery atom in the giant structure is bonded to several neighbours by strong covalent bonds, which must all be broken to melt or break the structure
   • DSilicon carbide has weak forces between layers that resist melting

   (c)

   Explain why silicon carbide is a suitable material for cutting tools that must withstand very high temperatures without melting or breaking down.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A researcher compares two substances: boron nitride (a giant covalent structure, melting point over 2900°C) and carbon dioxide (a simple molecular substance, melting point −78°C, i.e. it sublimes as dry ice).

   (a)

   Explain, in terms of structure and bonding, why boron nitride has such a dramatically higher melting point than carbon dioxide.

   [3 marks]

   (b)

   The researcher also tests both substances for electrical conductivity and finds that neither boron nitride nor carbon dioxide conducts electricity, whether solid, liquid or gas. Explain why both substances fail to conduct electricity in any state, despite having very different structures and bonding, and describe how this is different from an ionic compound.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A materials engineer is selecting a substance to use as a heat-resistant, electrically insulating component. She is choosing between quartz (a giant covalent structure of silicon dioxide, melting point 1710°C) and paraffin wax (a simple molecular substance, melting point around 60°C).

   (a)

   Describe, in terms of structure and bonding, why quartz has such a high melting point compared with paraffin wax, and explain why quartz is a good electrical insulator.

   [6 marks]

   (b)

   The engineer also considers using a sample of table salt, sodium chloride (an ionic compound, melting point 801°C), instead of quartz. Explain why sodium chloride would not be a suitable choice as a permanent electrical insulator if there were any risk of it melting or being exposed to water, contrasting this with quartz's behaviour under the same conditions.

   [6 marks]

   Total for question 4: 12 marks

End of questions