Do you know Raoult's law? You can use it to derive most of what is being taught as "colligative properties" (hint: at temperatures involved salts are virtually non volatile).
Of course. Raoult's law states that P solution = P solvent + P solute = X solvent * P° solvent + X solute * P° solute.
If the solute is non-volatile, P solute is negligible, so Psolution = P solvent.
Given the presence of the solute, X solvent <1, therefore P solution < P° solvent. This is the vapor pressure drop, one of the colligative properties. Now, if the vapor pressure of the solution is reduced, in the phase diagram of the solvent the liquid-vapour curve is "lowered", so that, consequently, the boiling point rises and the freezing point lowers (other colligative properties).
My doubt arises right here: it seems to me that cryoscopic lowering and ebullioscopic raising don't ALWAYS go hand in hand. Here is an example: considering a solution of ethanol in water (therefore where ethanol acts as a solute and IS VOLATILE), an increase in boiling point is not observed, on the contrary, the boiling temperature is lower than that of pure water, this because even ethanol participates to vapor pressure.
However, this does not exclude the occurrence of cryoscopic lowering: in fact, a solution of water in ethanol or ethanol in water reaches an eutectic.
Therefore, to obtain the freezing point lowering it is not necessary for the solute to be non-volatile, but it is necessary for it to be insoluble in the solid.
Conversely, boiling point elevation should necessarily require a non-volatile solute. If the solute is volatile, then the boiling temperature of the solution will be between that of the two compounds, so in the phase diagram the liquid-vapor curve does not "go down", but the solid-liquid curve can still shift to the left.
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