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Q.
Which of the following substances has the highest melting point?
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Solution:
Melting points depend upon lattice energy. $ \text{NaCl} $ and $ \text{KCl} $ have unit charge on their ions while $ \text{MgO} $ and $ \text{BaO} $ have two units of charge, therefore lattice energies of $ \text{MgO} $ and $ \text{BaO} $ are expected to be larger than those of $ \text{NaCl} $ and $ \text{KCl}\text{.} $ Since, $ \text{M}{{\text{g}}^{2+}} $ is smaller than $ \text{B}{{\text{a}}^{\text{2+}}}\text{,} $ therefore, $ \text{MgO} $ has the higher lattice energy and hence, has the highest melting point.