Learn about Advanced Placement Chemistry, Solutions 3, in this comprehensive video by bannanaiscool.
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Rob Lederer: Pretty spectacular aren't? It's a flash light actually shining through a glass of distilled water and look at the light that's coming through over here. Now if we take this glass of water and replace it with another glass that got some dirt in it, we can see that the amount of light that passes through is cut down markedly and most of that light is being reflected now off of molecules in solution coming out of the sides, more light actually falling on to the backdrop. that's called the Tyndall effect and it deals with colligative properties of molecules or how the size of molecules in solution determines various properties now of that solution. To understand what boiling point elevation and freezing point depression actually means, you got to understand vapor pressure first, vapor pressure of solvents and solutions, okay. Here is what this is all about. We have a solvent inside of a beaker, nice beaker, huh? Okay, and we have got a bunch of, these are just solvent molecules in here, that they are really big. Okay now, the solvent molecules are really big and here is what you know, that if you have water and you put it in a beaker or glass or whatever, leave it over time and its going to evaporate and take off out of there. Put a lid on top of it and you will get water evaporating, but hits the top of lid, recondenses and forms a liquid again. And so, what do we get when we have the rate of evaporation equaling the rate of condensation. You know that's equilibrium. Okay so, what we have here is a vapor pressure that's achieved because this is, these are vapor particles here and let's just say five ml evaporated although that's going to be certain pressure. That pressure in here is called the equilibrium vapor pressure for that solvent and it changes with change in temperature. So if you have add, increase the temperature here, you know what you are going to get, you are going to get a higher vapor pressure appear. Make sense? Hope so. Now look at this. What if we add a bunch of black solute molecules black solute molecules into the solvent to make a solution. What is that actually going to do to the vapor pressure up here? Well, believe it or not, what really happens is that solid molecules just getting in the way of the solvent trying to evaporate? So because of that interference it's a colligative property coming together as solutions and interference there causes there to be a lower vapor pressure now even at the same temperature than the solvent had before. Now we also got say three molecules evaporated into that state. The vapor pressure goes lower because, you added the solute. Now look at this, what if this was actually the boiling point. So to get to the boiling point, you have to have vapor pressure of five molecules appear. Well, all the sudden by adding the solute in here now to get the five appear to achieve that vapor pressure that we need to get to the boiling point. Well, the temperature has to go up and so if this is boiling at a 100 and this is water, by putting the solute in, you increase the boiling point. So anytime you add a solute into water, the boiling point goes up. Now, conversely if you have water and you put like salt into it, you know that the freezing point goes lower and that's how we keep the roads in Alberta wet in between temperatures of zero and above minus 12 degrees during the winter time because, we don't want to drive on ice, the salt will turn it into liquid. Why? Because, the salt molecules interfere with water trying to freeze together to form the solid they get in the way. So, water stays as a liquid longer in winter time. So, what do we get there? The freezing point has to actually get lower before water can then freeze around that solute and so that's freezing point depression. Cool. Well of course, we got to be able to then calculate that new boiling point temperature. So here is that question and the way to do it. the formula is delta T equals imK, where in this case