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.
Also consider the definition of the 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.
Now consider this intuitive 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.
Also 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.
I know that this argument is not true, as experiments proves the colligative properties are true.
Where and why my "intuitive" argument is failing?
For some of you who failing to understand my question, this is another form of it . A logical explanation for depression in freezing point , where inevitability of atmospheric pressure regardless of solution's vapor pressure is considered? If not considered , appropriate reason for the same.