為甚麼冰晶石能降低melting point？

The melting point of any substance would be lowered if it contains impurity.
This is because the existence of impurity particles in the lattice of the substance would disturb the regularity of the lattice, and this would make the lattice easier to break. Therefore, the lattice would be broken at a temperature lower the normal melting point of the pure substance. In other words, the melting point of the substance is lower.

The cryolite (冰晶石) is an impurity of aluminium oxide, and would lower the melting point of aluminium oxide.
Such technique is usually employed in industry. For example, in electrolysis of molten sodium chloride, a small amount of calcium chloride is added as impurity to lower the melting point of sodium chloride.