Vapor Pressure/Colligative Properties/Boiling Point

[2PacClone23]

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So vapor pressure is the pressure at equilibrium between the liquid and gas phase in a closed container.

I get that.



So when you add solutes to it, that will LOWER the vapor pressure? What does this mean? Does this mean, it will not want to escape to the gas phase as much?


Thus, if that's true, then that means more energy is needed to go to the gas phase, and thus, an elevation in boiling point.


This all makes sense....so far.


Then, this solute added will also lower the melting point, meaning it will melt more often because LESS energy is needed?

But I thought if you're lowering the melting point, then you're also lowering the freezing point....which means more things will want to become liquid...which is why we pour salt on ice right?


Am I correct here?

 

[gigawatt]

If you add a non-volatile solute (something that contributes low-to-zero vapor pressure), then that lowers the solution vapor pressure and so the boiling point (point where vapor pressure equals and then exceeds atmospheric pressure) will be raised.

The reason that adding solutes to a liquid depresses the freezing point is that it interferes with the solvent molecules forming the lattice structure needed for freezing. For example, water forms a hexagonal lattice as it freezes but addition of solutes molecule make it harder for the water molecules to cleanly organize in this way.

 

[loveoforganic]

Vapor pressure is decreased because the solvent forms strong cohesive forces with the solute, making it harder for those solvent particles to leave.

 

[r8 dds]

Vapor pressure is decreased because the solvent forms strong cohesive forces with the solute, making it harder for those solvent particles to leave.

Thus, increasing BP.

 

[loveoforganic]

yep!

 

[2PacClone23]

Still kinda confusing 🙁

 

[duodenum]

just remember that BP and VP are inverses. So adding a solute will increase the BP (higher temp to boil), lower the FP (cause water to freeze at a lower temperature) and lowers the VP. If u keep these facts in mind, u can tackle any problem...

best of luck

 

[2PacClone23]

just remember that BP and VP are inverses. So adding a solute will increase the BP (higher temp to boil), lower the FP (cause water to freeze at a lower temperature) and lowers the VP. If u keep these facts in mind, u can tackle any problem...

best of luck

That works. I get really confused with the MP/FP comparisons. If you pour salt on ice, you're adding a solute, meaning it should decrease VP, elevate BP, and most importantly of all, lower the FP (meaning it takes less energy to melt it)