Question

how vapor pressure and boiling point relate to colligative properties

Answer 1

Colligative properties depends on ONLY solute particles present in solution. (Remember when we speak word colligative properties means we are talking about only solute particles).

When a solute is added to to a pure solvent then that solution boiling point is increased (Boiling point elevation) than pure solvent.

Example: when you add salt to water its boiling point is increased. (As shown in below image)

When a solute is added to to a pure solvent then that solution freezing point is increased (Depression in freezing point) than pure solvent. According to Raoult's law the vapor pressure of a solution is equal to the mole fraction of the solvent times the vapor pressure of the pure liquid.

Example: when you add salt (NaCl) to ice the temperature is further lowered (Vapor pressure is also lowered due to condensation). (As shown in below image)

For a detailed lengthy explanation can be found at

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch15/colligative.php

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