The physical properties of ionic substances
3 marks · EasyWhat does it mean if a substance is described as 'crystalline'?
Explain how the structure of sodium chloride leads to its crystalline appearance.
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(a) It means the substance has a regular repeating structure or shape.
(b) The ions in sodium chloride are arranged in a regular, repeating lattice. This repeating arrangement is seen as a crystalline shape.
Common mistakes: crystalline means colour; molecules
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Marking points
B1 (a) · 1 mark · Defines crystalline as a regular repeating structure or shape
M1 (b) · 1 mark · States ions are arranged in a regular repeating lattice
A1 (b) · 1 mark · Links the regular structure to the appearance of crystal shapes
Worked solution
a) 'Crystalline' means that the substance has a regular repeating structure or definite geometric shape. b) The structure of sodium chloride is a regular, repeating giant ionic lattice, so its physical crystals are also regular and geometric in shape.