1. 1
   A student heats a sample of solid sodium chloride, NaCl, in a crucible and measures its melting point. NaCl melts at 801°C. The student then tests whether solid NaCl, molten NaCl, and NaCl dissolved in water conduct electricity, using a circuit with a battery, ammeter and two electrodes.

   (a)

   Which statement explains why NaCl has such a high melting point?

   [1 mark]

   • AWeak intermolecular forces exist between NaCl molecules
   • BStrong electrostatic forces of attraction act between oppositely charged ions in all directions
   • CCovalent bonds between sodium and chlorine atoms require a lot of energy to break
   • DDelocalised electrons hold the sodium ions together in a lattice

   (b)

   When the student tests the solid NaCl in the circuit, no current flows and the ammeter reads zero. Which statement explains this observation?

   [1 mark]

   • AThe ions in solid NaCl are fixed in place in the lattice and cannot move to carry charge
   • BSolid NaCl has no ions present at all
   • CThe electrodes used were not connected to a power source
   • DSolid NaCl contains delocalised electrons that repel the current

   (c)

   When the NaCl is melted, or dissolved in water, the ammeter shows a current flowing. Explain why molten NaCl and dissolved NaCl conduct electricity.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Magnesium oxide, MgO, is used to line the inside of furnaces because it withstands extremely high temperatures without melting (melting point over 2800°C), much higher than NaCl. A technician tests a sample of furnace lining material and finds it does not conduct electricity while solid.

   (a)

   Which statement best explains why MgO has a higher melting point than NaCl?

   [1 mark]

   • AMgO ions are larger than NaCl ions
   • BMgO is a gas at room temperature so it needs less energy to melt
   • CMgO contains covalent bonds while NaCl does not
   • DMg²⁺ and O²⁻ ions have greater charges than Na⁺ and Cl⁻ ions, so the electrostatic forces of attraction between them are stronger

   (b)

   The technician finds that solid MgO does not conduct electricity, but this property is not a problem for its use as a furnace lining. Which statement correctly describes the electrical conductivity of MgO?

   [1 mark]

   • AMgO conducts electricity in the solid state because it contains ions
   • BMgO never conducts electricity under any conditions
   • CMgO does not conduct electricity as a solid, but would conduct if melted, because the ions would then be free to move
   • DMgO conducts electricity only because it is used in a furnace

   (c)

   Explain, in terms of structure and bonding, why magnesium oxide is a suitable material for lining furnaces.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A chemist dissolves potassium bromide, KBr, an ionic compound, in water to form a solution. She places two carbon electrodes connected to a battery and ammeter into the solution and observes that the ammeter shows a steady reading. She compares this with a separate sample of solid KBr, which does not conduct.

   (a)

   Explain what happens to the ions in the KBr solution when the current is switched on, and why the ammeter shows a reading.

   [3 marks]

   (b)

   The chemist repeats the experiment with solid KBr instead of the solution and finds the ammeter reads zero. Using ideas about ionic structure and bonding, explain the difference in conductivity between solid KBr and dissolved KBr, and predict what would happen if the solid KBr were melted instead of dissolved.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A materials scientist is comparing calcium fluoride, CaF₂ (an ionic compound with melting point 1418°C), with a simple molecular substance, iodine, I₂ (melting point 114°C), for use as a high-temperature electrical component.

   (a)

   Describe, in terms of structure and bonding, why calcium fluoride has such a high melting point, and explain what would need to happen to the structure for it to conduct electricity as a solid, referring to why it currently does not.

   [6 marks]

   (b)

   The scientist also has a sample of iodine, I₂, a simple molecular substance with a much lower melting point (114°C) than calcium fluoride. Explain, in terms of structure and bonding, why iodine has a much lower melting point than calcium fluoride, and why neither solid iodine nor molten iodine conducts electricity, unlike calcium fluoride which conducts when molten.

   [6 marks]

   Total for question 4: 12 marks

End of questions