Giant ionic structures
4 marks · NormalCalcium chloride is an ionic compound with the formula CaCl_2.
Name the ions present in calcium chloride.
Explain why calcium chloride is soluble in water but does not conduct electricity when solid.
What must happen for solid calcium chloride to conduct electricity?
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(a) Ca^{2+} and Cl^- ions · must show Names or formulae of both ions
(b) Ions are attracted to water molecules, which helps them dissolve. In solid, the ions are fixed in the lattice and cannot carry charge, so do not conduct. · must show Ions fixed in solid, Ions separated in water, Conductivity linked to mobility
(c) It must be melted or dissolved in water so the ions can move. · must show Ions must be mobile
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Marking points
B1 (a) · 1 mark · Names or gives correct formula of both ions (Ca^{2+} and Cl^-).
M1 (b) · 1 mark · States that ions are attracted to water molecules and this helps them dissolve (or similar).
A1 (b) · 1 mark · Explains in solid, ions are fixed and cannot carry charge.
B2 (c) · 1 mark · States it must be melted or dissolved so ions can move.
Worked solution
a) The ions present are Ca^{2+} and Cl^-.
b) Calcium chloride is soluble because the water molecules are attracted to the ions, separating them from the lattice. In the solid, the ions are stuck in the lattice and cannot move, so it does not conduct electricity.
c) The solid must be melted or dissolved in water for the ions to be free to move and carry charge.