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Boiling Point Elevation - 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.

Boiling point elevation is a colligative property of solutions. Solutions boiling points are higher than that of the solute or solvent because the vapor pressure of solutions is lower. A boiling point is when the vapor pressure of the solution becomes equal to the external pressure, so when the initial vapor pressure is lower, it takes more heat to elevate the vapor pressure to the same point.

Alright so one of the properties that you're going to see in class, is we're going to talk about boiling point elevation and that is the temperature difference between the boiling points of a solution and that of a pure solvent. Now as you can probably guess from the title it's actually going to increase from the solvent to the solution, solution is going to have a higher point than the solvent. And why is that? Let's define point first. Boiling point is when vapor pressure of whatever substance you're dealing with is going to equal the atmosphere pressure of the pressure around it. Okay great let's say we have a pure solvent, let's say we have a beaker of water. We know that the surface of water, there's particles that are going to escape into vapor pressure into vapor back and forth back and forth and there're going to have some sort of pressure up here.

And as we heat the substance that pressure is going to raise due to kinetic energy increasing and it's going to eventually reach the atmosphere pressure and then boil okay. So let's over to a solution that you'll put something in it salt, sugar or whatever your solute maybe and to make that solute to actually dissolve these water molecules are going to surround it. So these water molecules are actually really attracted to the solute that's in the solution. So what's going to happen is they'd rather be around these dots or these solute particles than escaping into vapor, so it's already going to start off at a lower vapor pressure. So they actually have to go increase kinetic energy a lot more to reach that vapor pressure to equal the atmosphere pressure because they're already starting in a lower place. So that temperature is going to have to be a lot higher or depending on what the solvent is to that atmosphere pressure and then boil.

Okay so let's talk about different solvents and how the solutes are actually affected by that. So let's about water, we know water boils at a 100 degrees Celsius and it's found for every molar of solute that you put in there it's going to increase the boiling point temperature by 0.512 degrees Celsius. Benzene boils at 80.1 degrees Celsius and every molar or every mole per kilogram of Benzene you put in is going to raise the temperature 2.53 degrees Celsius or boiling point temperature that much and so on and so forth. So basically, in a nutshell, we have a formula here called the boiling point elevation formula and this is a change in temperature of the boiling point that delta T is going to the changing temperature times that constant that you see for every molar that's in the amount of degrees Celsius is going to increase.

Times on molar, in the morality which is mole of the solute divided by the kilograms of the solvent and we're going to multiply it by the Ben Hoff Factor or how much it's actually dividing out for a number of electrolyte actually dissolve and separate themselves in water. So if you have 1 molar of let's say NaCl you're actually going to have 2 molars of particles. So that is what we call a Ben Hoff Factor. So let's actually look at that in real life. It says what is the new boiling point of a 0.029 molar of an NaCl solution? Okay so we put salt in water and we're going to know the new boiling point of this water. So we can just look at our formula and say our change in temperature is going to equal the constant and now we're dealing with water because we know it's an aqueous solution, so our solvent is going to be water so when we look at out table and for every molar it's going to raise 0.512 to degrees Celcius that's what I'm going to say 0.512 degrees Celsius per molar.

We know our molarity is 0.29 and out Ben Hoff Factor is going to be 2 because for every molar of this is going to actually break up, for every 1 mole of NaCl is going to break up into 2 moles of particles. So our Ben Hoff Factor in this case is going to be 2, okay so we multiply all these together and we should get 0.030 degrees Celsius. This is not our new boiling point, this is just a change in boiling point, so our new boiling point is, original boiling point was 100 degrees Celsius, it's going to change or increase because or increase because we know it's elevating by 0.030 degrees Celsius. And so our new boiling point is 100.030 degrees Celsius, the new boiling point of this solution. Okay and this is why you can think about when you're cooking and you're staring some salt into a pot of water not only are you for flavor you're actually increasing the boiling point of that water. So when you're dealing with like let's say pasta you actually can cook pasta a little bit faster because your boiling point is actually a little bit higher than 100 degrees Celsius and you can cook it at something a little bit higher if you put more in, the more the higher the higher the temperature is going to be. But don't put so much salt in there because it's going to be over salted and over flavored. But either way that is boiling point elevation.