1. 1
   A technician places crystals of solid iodine, I₂, in a test tube and gently warms them. The iodine melts at 114°C and boils at only 184°C, both relatively low temperatures. The technician then sets up a circuit with a battery and ammeter and places electrodes into a sample of liquid iodine to test its electrical conductivity.

   (a)

   Which statement explains why iodine has such low melting and boiling points?

   [1 mark]

   • AIodine molecules are held together by weak intermolecular forces, which require little energy to overcome
   • BThe covalent bonds inside each iodine molecule are weak and easily broken on heating
   • CIodine is a giant covalent structure with few covalent bonds
   • DIodine atoms are not chemically bonded to each other at all

   (b)

   When the technician tests liquid iodine in the circuit, the ammeter reads zero, showing no current flows. Which statement best explains this?

   [1 mark]

   • AThe battery used did not have enough voltage to move the iodine molecules
   • BLiquid iodine is too hot for a current to pass through it
   • CIodine molecules are positively charged and repel the electrons in the circuit
   • DIodine molecules have no overall electrical charge and there are no free ions or delocalised electrons to carry current

   (c)

   Explain, in terms of forces, why only a small amount of energy is needed to melt and boil iodine, compared with an ionic compound such as sodium chloride.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Methane, CH₄, is used as a fuel and is stored as a gas at room temperature because it boils at −162°C, an extremely low temperature. An engineer checks whether a pipeline carrying methane gas conducts electricity, to rule this out as a fire risk from stray currents.

   (a)

   Which statement explains why methane has such a low boiling point?

   [1 mark]

   • AThe covalent bonds between carbon and hydrogen atoms in methane are very weak
   • BMethane consists of simple molecules with only weak intermolecular forces between them, which are easily overcome
   • CMethane is a giant covalent structure with a small number of atoms
   • DMethane molecules repel each other strongly, forcing them apart at low temperatures

   (b)

   The engineer finds that methane gas does not conduct electricity at all. Which statement correctly explains this observation?

   [1 mark]

   • AMethane conducts electricity only when it is burned
   • BMethane is a gas, and all gases are unable to conduct electricity
   • CMethane molecules are neutral overall, with no free ions or delocalised electrons available to carry a current
   • DMethane molecules are too small to carry an electric current

   (c)

   Explain why methane exists as a gas at room temperature, while a substance such as sodium chloride exists as a solid with a much higher melting and boiling point.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student compares two substances: wax (a simple molecular substance) and quartz (a giant covalent structure). She heats each substance separately and records the temperature at which each melts. The wax melts at around 60°C, while the quartz does not melt even at over 1700°C.

   (a)

   Explain, in terms of structure and bonding, why wax melts at such a low temperature compared with quartz.

   [3 marks]

   (b)

   The student also tests both substances for electrical conductivity in the solid state and finds that neither wax nor quartz conducts electricity. Explain why wax does not conduct electricity, and explain what this shows about the similarity between wax and quartz in this respect, even though their bonding is very different.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist bubbles hydrogen chloride gas, HCl, a simple molecular substance, through water. HCl gas has a boiling point of −85°C and does not conduct electricity as a gas, but the resulting solution formed in water does conduct electricity.

   (a)

   Describe, in terms of structure and bonding, why HCl has such a low boiling point, and explain why HCl gas itself does not conduct electricity even though the aqueous solution it forms does.

   [6 marks]

   (b)

   The chemist then compares the properties of HCl with those of a giant covalent structure, silicon dioxide (SiO₂), which has a melting point of over 1700°C and never conducts electricity, even when molten. Explain why silicon dioxide has such a much higher melting point than HCl, and explain why, unlike an ionic compound, molten silicon dioxide still does not conduct electricity.

   [6 marks]

   Total for question 4: 12 marks

End of questions