 Collegative Properties 1. Boiling Point, Elevation and Freezing Point, Depression What is a Collegative Property? A Collegative Property is a property that is dependent on the number of solute particles dissolved. It is an intrinsic property and it does not depend on the identity of the solute. It simply depends on the quantity of the solute that dissolves. Let's have a look at the first Collegative Property. Boiling Point, Elevation The boiling point of pure water is 100 degrees Celsius. But when you add solute to it, the boiling point will increase. The more you add, the more it will increase. The boiling point elevation is a Collegative Property. But how does solute make the boiling point go up? Boiling point is the temperature at which the vapor pressure of a liquid being equal to the pressure surrounding the liquid, in which case the liquid is allowed to freely escape the liquid phase to go into the gas phase. The presence of the solute gets in a way and prevents the water from being able to escape from the surface. So you would need to add more heat in order to get the water to escape into the gas phase. Hence, higher boiling point. Let's look at the equation for boiling point elevation. So, elevation temperature, which is the temperature difference between the boiling point of the solution and the boiling point of the solvent, is equal to kb, kb is more of a boiling point constant, times molality, which as you recall from the alternative concentration, is equal to the number of moles of solute divided by kilograms of solvent, times van Haar factor i. Now the van Haar factor is an interesting one. Van Haar factor depends on the number of molecules that a solute produces when dissolved in the solvent. For example, sugar molecules. When dissolved in water remains sugar molecules. They don't get split into anything. So i equal 1. Sodium chloride however, when dissolved in water, gets split into sodium cation and chlorine anion. So i equal 2. Magnesium chloride, MgCl2, when dissolved in water, gets split into 3. One magnesium cation and two chlorine anions. So i equal 3. We talk about colligated properties not concerning the identity of the solute and that remains true. But you gotta look at what actually happens, because if your solute split in say 2, once dissolved, then adding one portion of the solute effectively means adding two portions. Ok, let's look at the second colligated property, freezing point depression. As we discussed in the pure water videos, freezing point is the temperature at which a liquid becomes solid. And when you ask solute to water, the freezing point decreases. This is because the solutes mess with the water's crystal structures, getting in the way of the water from forming hexagonal lactides, as we talk about in the pure water video. So the freezing point depression, or the difference between this freezing point of the solution and the freezing point of the solvent, is equal to minus kf times molality times i, the Van Hopf factor. So this equation is similar to the boiling point elevation equation of both. The only difference is kf, the freezing point constant or as we would call it, the triocuspic constant. And it has a negative sign to indicate that the freezing point will decrease. For water, kf is equal to 1.86 which is bigger than kb, for water which is 0.51. So freezing point changes much more rapidly than the boiling point of water, and the unit for kb or kf is degrees per molal. It doesn't matter whether you have Celsius or Kelvin here, everything is scaled up at the same rate.