Why does increasing the solute concentration decrease the melting point?

Answer 1

Compounds exist as solids when the intermolecular forces keep the molecules locked in place. The addition of a solute interrupts the intermolecular forces between solvent molecules, making the intermolecular forces less strong and easier to disrupt. As a result, the compound will melt at a lower temperature.

Answer 2

Your question really should ask about the lowered freezing point rather than the melting point; a solution is already melted, and what you will be concerned about is the freezing point. But it is an equivalent question since the melting point is also the freezing point, depending upon whether the material is getting hotter or colder. The freezing point gets lower when there is more solute in the solution because the solute has its own bond (a coordinate covalent bond) with the solvent; that's why it dissolved. In order to freeze, the solvent has to first break the bond with the solute and then form a new bond with other solvent molecules, so it is harder to do.

Answer 3

Adding a solute generally depresses the melting point and elevates the boiling point.

Water with salt in it freezes at a lower temperature than 0 oC and boils at a higher temperature than 100 oC