The physical properties of ionic substances
4 marks · NormalTwo ionic compounds, sodium chloride (NaCl) and magnesium oxide (MgO), have the following melting points:
| Compound | Melting Point (°C) |
|---|---|
| NaCl | 801 |
| MgO | 2852 |
Analyse the data. Explain why magnesium oxide has a much higher melting point than sodium chloride. Your answer should consider the type of particles and the forces between them.
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Common mistakes: Referring to molecules; Omitting the role of charge
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Marking points
M1 · 1 mark · Identifies both compounds have a giant ionic lattice.
M2 · 1 mark · States that forces of attraction are between oppositely charged ions (electrostatic attraction).
A1 · 1 mark · Describes that MgO has ions with higher charges (Mg^2+ and O^2-) compared to Na^+ and Cl^- in NaCl.
A2 · 1 mark · Explains that higher charges mean stronger electrostatic forces, so more energy (higher temperature) is required to overcome them in MgO.
Worked solution
Both sodium chloride and magnesium oxide have giant ionic lattices.
In both, strong electrostatic forces of attraction hold the oppositely charged ions together.
In NaCl, ions are Na^+ and Cl^- (charges +1 and -1), whereas in MgO, ions are Mg^2+ and O^2- (charges +2 and -2).
The greater the charges, the stronger the forces. Therefore, MgO needs much more energy to break its lattice, resulting in a much higher melting point than NaCl.