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Freezing Point Depression - Concept

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

Freezing point depression is a colligative property of solutions. Solutions freezing points are lower than that of the pure solvent or solute because freezing, or becoming solid, creates order and decreases entropy. Solutions have high entropy because of the mix of solvent and solute, so it takes more energy to decrease their entropy to the same point.

Alright when dealing with solutions, you're going to be coming across a colligative properties and one of the colligative properties that you're going to see is freezing point depression and that says in a solution solute particles interfere with attractive forces among the solvent particles. And this prevents solution into entering a solid state. So essentially what they're saying is, because a liquid has like all these extra particles in it to make a solution, and it's not a pure solvent those get in the way of the intermolecular forces that make it a solid, a solid. Like the hydrogen bonding, dipole-dipole interaction and the London dispersion forces, those extra particles that are there kind of get in the way and they actually help to like lower the freezing point to get, to push those particles out so it'll be a pure solvent when it's actually frozen.

Freezing point states that the particles are no longer have sufficient kinetic energy to overcome intermolecular attractive forces so when those particles are there those attractive forces are not necessary. So like they're going to have to just push those particles out so they can actually have the inner particle attractive forces present. So let's actually talk about different substances and their freezing points. So we're talking about water which is a universal solvent and we know that water freezes typically at 0 degrees Celsius at normal freezing point. Now for every molal of substance I'm going to put within that water in a pure substance is actually going to drop the temperature the freezing point temperature by 1.86 degrees Celsius.

Benzene freezes at 5.5 degrees Celsius well higher than water and for every mol of substance that you have for a kilogram of solution the boiling point is going to drop even more 5.12 degrees Celsius and so on and so forth. So if you were to look at this, this is very similar to boiling point elevation but this set formula is exactly the same but there's some slight differences. So the change in temperature of the freezing point is equal to the constant that we had discussed, time similarity of this solution that we're dealing with, time is a Ben Hoff Factor and a Ben Hoff Factor is how much the particle actually separates in solution. So we're talking about ionic compounds, they separate into solution depending on how many particles they have or how many ions they have in that but molecular compounds don't at all. So let's actually put that in action.

Alright so what a freezing point of a 0.029 molal of NaCl aqueous solution so we know it's aqueous and the aqueous tells us that our solvent is water. So we're going to say our delta T are changed in temperature to freezing point is going to equal to the constant of water which is 1.86 degrees Celsius for every molar. And the molar solution is 0.029 and because it's NaCl I know it's ionic for every one molal it's going to actually separate into 2 substances Na plus and Cl minus. So we're actually multiplying this by 2, we have substances when it's in solution. So when you multiply all of these together you get 0.11 degrees Celsius and we're going to say alright our original freezing point is 0 it's going to lower by 0.11 and so our new freezing point is 0.11 degrees Celsius negative because it dropped that much. So we can actually like talk about, when you think about when it snows outside and the reason that you put salt on the roads there isn't even salt on the roads that's actually going to lower the freezing point so there's not going to be sheets of ice on your drive way or on the roads or on the side walks. So that's why they use salt and they actually use calcium chloride typically which is actually better than sodium chloride because this actually breaks up into 3 particles so it'll drop the freezing point 3 times much as another solute would. So this is an example of frizzing point depression.