Explain the following differences in terms of the type of bonding present.

a. Aluminium oxide has a melting point of 2980 °C but aluminium chloride changes to a vapour at 178 °C.

b. Magnesium chloride conducts electricity when molten but not when solid.

c. Iron conducts electricity when solid but the ionic solid iron(II) chloride does not conduct when solid.

d. Sodium sulfate dissolves in water but sulfur does not.

e. Propanol, CH₃CH₂CH₂OH, is soluble in water but propane, CH₃CH₂CH₃, is not.

f. A solution of hydrogen chloride in water conducts electricity.