Question

How kcl and kf is added to sodium chloride to decrease the melting point of nacl

Answer 1

Adding an impurity like calcium chloride into molten sodium chloride lowers the melting point (likewise, the freezing point) because, as Kristina Kučanda stated, the entropy is increased.  Why?  Because with the presence of calcium ions, the sodium ions are farther from each other and moving more randomly in respect to one another in the molten solution.  Thus the sodium ions need to be cooler (lower temperature, lower energy) to solidify (freeze) or, conversely, melt.  [Melting and freezing points are the same for a specific substance or solution.]

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