Recently, I learnt that by adding a non-volatile solute to a solvent, the vapour pressure of the solution is decreased and consequently its boiling point.

Facts and definitions

freezing point: "The temperature at which the vapour pressure of the substance in its liquid phase is equal to its vapour pressure in the solid phase."

So as the solution cools down, the vapour pressure decreases and eventually equals the vapour pressure of the solid.

My argument as to why depression in freezing point shouldn’t take place

consider this argument:the atmospheric pressure is constant regardless of the temperature of the solution.

As we cool down a solution, we are basically removing the kinetic energy of the solution's particles so due to less kinetic energy and due to atmospheric pressure the Van der Waal's forces come to play, hence turning the solution into a solid state.

the pressure exerted on the liquid by the atmosphere is the same whether you add a solute or not, so no depression of freezing point should take place.

Reality check

I know that this argument is not true, as experiments proves the colligative properties are true.

Where and why my "intuitive" argument is failing?

  • $\begingroup$ Kindly reopen the question , I have tried my best to make it understandable and aesthetic. $\endgroup$ – Chemist Apr 13 '19 at 14:38
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    $\begingroup$ I still don't get it, but there is one obvious error in your reasoning: The atmospheric pressure is totally irrelevant, only the partial pressure of the substance in question plays a role. $\endgroup$ – Karl Apr 14 '19 at 19:44

You argument is largely based on enthalpy, whereas freezing point depression and boiling point elevation is (in the situation when the solute only weakly interacts with the solvent) mainly an entropic phenomenon.

Because of the increased entropy by adding a solute (i.e. there are more configurations by having two types of molecules instead of one), the solution will liquify at a lower temperature as compared to the pure solvent and with a similar reasoning, the boiling point will increase with respect to the pure solvent.

  • $\begingroup$ but the reason given by my teacher and and textbooks is that , add solute -vapour pressure decreases-bp and mp decreases-as they are dependent on the pressure above the solution $\endgroup$ – Chemist Nov 11 '18 at 18:46
  • $\begingroup$ Perhaps I am overlooking something, but to the best of my knowledge the boiling point increases when you add a weak-interacting solute to a solvent. For instance, have a look at this page: en.wikipedia.org/wiki/Boiling-point_elevation Did your teacher (or textbook) by any chance mentioned some additional details? $\endgroup$ – Ivo Filot Nov 11 '18 at 18:48
  • $\begingroup$ yeah boiling of liquids takes place when vapour pressure is equal or greater than atmospheric pressure $\endgroup$ – Chemist Nov 11 '18 at 18:49
  • $\begingroup$ but adding of solute decreases the vapor pressure. now the elevation of boiling point completely makes sense to me whereas depression in freezing point does not $\endgroup$ – Chemist Nov 11 '18 at 18:51
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    $\begingroup$ Let us continue this discussion in chat. $\endgroup$ – Chemist Nov 11 '18 at 19:11

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