Why does ice have a low melting point while magnesium has a high melting point?
Answers
As water freezes the kinetic energy of the molecules decreases, and intermolec forces increase and a lattice of water molecules form which we call ice. During the phase change from liquid to solid, there is an equilibrium between the two phases. In the presence of salt (ions) this equilibrium is distorted and fewer water molcules enter the solid phase and more remain in the liquid phase. This is ultimately seen as a lowering of the freezing point/melting point.
I think you have it backwards when you suggesr that adding ions increases the melting point of solids. The presence of ions or impurities DECREASES the melting point because they interfere with the normal intermolecular forces holding the crystalline lattice together. Thus, less heat is needed to break the lattice and melt the solid and hence melting point is decreased.
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