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## Chemistry library

### Course: Chemistry library>Unit 11

Lesson 3: Mixtures and solutions

# Boiling point elevation and freezing point depression

Boiling point elevation is the raising of a solvent's boiling point due to the addition of a solute. Similarly, freezing point depression is the lowering of a solvent's freezing point due to the addition of a solute. In fact, as the boiling point of a solvent increases, its freezing point decreases. An example of this would be the addition of salt to an icy sidewalk. The solute (salt) reduces the freezing point of the ice, which allows the ice to melt at a lower temperature.​. Created by Sal Khan.

## Want to join the conversation?

• Hello, I looked this up because I'm getting ready for a test that will most likely have this kind of stuff on it... It's a great video but I have one question.

My current book says that the formula for freezing point depression is ΔT=(-i)(Kf)(m), but I don't see where the (i) comes in for this video. My book isn't clear on the definition of (i) and I can't really find anything else on it.

I came to this video to figure out how to calculate (i), but how do you do without it?

Jaydo.
• The value "i" is called the van't Hoff factor (named after the first person to win the Nobel Prize in Chemistry). It is a little weird at first, so I will show you some examples and then explain in a moment:

For sodium chloride, NaCl, i = 2
For magnesium chloride, MgCl2, i = 3
For glucose, C6H12O6, i = 1
For sodium sulfate, Na2SO4, i = 3

Essentially, the van't Hoff factor tells you how many "units" the compound in question will separate into when dissolved in water.

Now, for table salt, sodium chloride, it dissociates into a sodium cation and a chloride anion. The van't Hoff factor is two because there are two species.

For magnesium chloride, there are three species: one magnesium cation and two chloride anions. Therefore, i = 3.

For glucose, i = 1 because glucose is not an ionic compound like the others. When it dissolves in water, the covalent bonds holding a glucose molecule together are not broken. Sure, it will dissolve, but it just dissolves into glucose sub-units.

For sodium sulfate, i = 3 because it would break down into two sodium cations and one sulfate anion. Notice that the sulfate anion does not break up any further. The sulfur-oxygen bonds are covalent, whereas the bonds holding together the sodiums and the sulfate were ionic.

The beauty of "i" is that it doesn't depend on the identity of the substance. If you were to create a 1m solution of each of the four compounds mentioned above, the freezing point depression and boiling point elevation for Na2SO4 and MgCl2 would be the same. Sodium chloride would be your second most effective, and glucose would be the worst.
• Was that a mistake that he wrote solute lowers the boiling point at , it should be melting point right?
• It actually should have been the freezing point I believe, because at he says freezing point, and throughout the beginning of the video he talks about freezing the water.
• I don't understand why NaCl when dissociated in water is suddenly 2 moles rather than 1... Regardless of whether its a whole molecule or in anion and cation form, wouldn't we still have the same mass?
• Moles are not mass. Moles are counts of particles. Thus, when one particle breaks into two particles you double the number of particles. Likewise, when you have 1 mole of NaCl and the particles break apart, you now have 2 moles in total -- 1 mole of Na⁺ ions and 1 mole of Cl ⁻ ions.
• At , Sal, says that with NaCl breaking down, you get twice the moles of the solute that you have originally. His example being that two moles of NaCl being dissolved in water gives you four moles of the solute. SO my question is, does it always just double by two, or does it depend on the elements amu and/or how many elements are in the molecule?
• It depends on the molecules involved. For example, when is dissolve 1 mol of MgCl2 in water, I get three moles of solute, as I have 1 mole of Mg and 2 moles of Cl, making 3 moles overall. With AlCl3, I'd have 4 moles of solute for every 1 mole I dissolve.
• so does that mean adding solute will lower surface tension too?
• Surface tension is about strength of interactions between molecules AND mixing, while boiling point and freezing point changes are ONLY about mixing. So surface tension will be more complicated.
(1 vote)
• Why do the Carbon-dioxide particles have some difficulty escaping through the water molecules when the is low pressure or low temperature and vice-versa is there is a high pressure or temperature??
• Because at low pressure/low temperature the CO2 particles have much less kinetic energy than at high pressure/high temperature. CO2 moving from being dissolved in water to free in the atmosphere is endothermic - it requires energy (heat!) input.

CO2 (aq) + heat ---> CO2 (g)
• delta t(v) = 2 degrees what?
is it Celsius or Fahrenheit?
• In science, temperature is measured in Kelvin or Celcius.
For this example, since an increase in temperature of 2 degree Kelvin or 2 degree Celcius is the same, it doesn't matter you use. Note that delta T is the change in temperature.
• I'm confused a solute makes the boiling point lower? I thought it made the boiling point higher.
• So what is clear to me is that since it is even harder for the molecules of a liquid like water to bond when they are mixed with some solute molecules, it requires less kinetic energy for the molecules to bond and create a solid. (Lower temperature decreases the kinetic energy.).
But what happens if we put a solute into an amorphus liquid? Would decrease its solidification temperature too, or because the amorphus compound doesn't want to create a geometric and clear structure it doesn't affect its freezing point?