# How does addition of impurities in a solid decrease the melting point? [duplicate]

In my textbook, it is written: The addition of impurities in a solid decreases the melting point of the solid.

How does it do so? Why can't it increase the melting point?

• A solid is a lattice held together by intermolecular forces rigidly (solids are rigid relative to a liquid). As you increase temperature, you add thermal energy to the lattice. At a certain temperature, the thermal energy overpowers the intermolecular forces (lattice energy), and the lattice breaks down. This is the melting point. The lattice energy stems from ordered structures. Impurities interrupt the crystalline structure, thereby weakening the intermolecular forces, which lowers the lattice energy and therefore the amount of thermal energy needed to melt, ie. the melting point decreases. Oct 13 '18 at 11:55
• A liquid btw isnt a rigidly ordered structure. This flexibility allows it to incorporate impurities more efficiently into a network of maximum intermolecular forces, and so impurities can increase the boiling point. The inability to efficiently bring inpurities within an existing ordered crystal is why the mp wont increase. Oct 13 '18 at 12:04
• Agreed. At first, I thought there's no way impurities lower mp. Like, when I add an impurity in any solvent, I'm kind of not allowing it to provide more 'space' for it to melt effectively because of it's addition. But, The kind of 'interuption' that impurities add lowers it's lattice energy and so the mp decreases. Ain't it? Oct 13 '18 at 12:11

Difference between melting points for pure and impure compounds can be quantified with following equation: $$\Delta T = i K_f b$$ where $$\Delta T$$ is melting point difference, $$i$$ is Van't Hoff factor which tells you on how many species does the impurity dissociate, $$K_f$$ is cryoscopic constant which tells you how great the effect is, and $$b$$ is molality of the impurity.