The presence of a nonvolatile solute causes the melting point of the solution to be higher than that of the pure solvent. True or false?
The correct answer and explanation is:
False.
The presence of a nonvolatile solute in a solvent actually lowers the melting point of the solution compared to the pure solvent. This phenomenon is known as freezing point depression.
When a nonvolatile solute dissolves in a solvent, it disrupts the orderly arrangement of solvent molecules necessary to form a solid crystal structure. The solute particles interfere with the solvent’s ability to solidify, meaning the solution requires a lower temperature to reach the solid phase. As a result, the melting point of the solution decreases relative to the pure solvent.
This effect occurs because the solution has a lower chemical potential of the solvent compared to the pure solvent. In simpler terms, the presence of solute particles reduces the escaping tendency of solvent molecules, making it harder for the solvent to organize into a solid state at the original melting point. Therefore, the solution must be cooled further to freeze.
Freezing point depression is a colligative property, which means it depends only on the number of solute particles in the solution, not their identity. The magnitude of the decrease in melting point is proportional to the molal concentration of the solute particles.
Common examples include adding salt to ice to melt it faster or antifreeze in car radiators, which lowers the freezing point of water, preventing it from freezing in cold weather.
In summary, the presence of a nonvolatile solute lowers the melting point of a solvent, so the statement that it causes the melting point to be higher is false.