A solute adds particles to a solution, such as:
NaCl-> Na+ + Cl- This puts 2 particles in the solution for every 1 formula unit of table salt. Any particle that gets added to a liquid interferes with the liquids ability to form crystals (ice), it interferes with the liquids ability to break surface tension ( boiling), and since less liquid molecules can escape there is less vapor pressure.
Addition of a solute means:
To freeze the solution you would have to slow the molecules down more than normal to encourage crystal growth, so the temp must be lowered.
To boil the solution you would have to speed the molecules up more than normal to encourage the molecules to leave the liquid, so the temp must be increased.
To increase the vapor pressure of the solvent has the same explanation as boiling.
