The physical properties of ionic substances
3 marks · NormalA student looks at two samples of different ionic compounds under a microscope. Sample A forms visible cube-shaped grains, but Sample B appears as a fine white powder and individual grains are not visible to the naked eye.
Using ideas about the arrangement of ions, calculate and explain how the crystalline property of ionic compounds is demonstrated in both samples.
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Common mistakes: Describing ionic compounds as molecules
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Marking points
M1 · 1 mark · Makes reference to the regular arrangement of ions in an ionic lattice.
M2 · 1 mark · States that this regular lattice leads to crystalline properties.
A1 · 1 mark · Explains that large visible crystals or many small crystals both show crystallinity from the lattice.
Worked solution
Ionic compounds are crystalline because their ions are arranged in a regular repeating lattice.
In sample A, the cube-shaped grains are visible crystals, each made from this repeating ionic lattice.
In sample B, the fine powder also consists of tiny crystals, but they are too small to see without a microscope. Both samples demonstrate crystallinity because both are made of a regular lattice of ions.