# Boiling point and VP

#### MynameisMike

10+ Year Member
Why does the addition of a nonvolatile solute lower the vapor pressure, but elevate the boiling point.

When a solutions VP = atmospheric pressure, it will boil. Thus, it would have a lower BP since its VP is lower.

These seem to contradict each other.

#### matth87

Why does the addition of a nonvolatile solute lower the vapor pressure, but elevate the boiling point.

When a solutions VP = atmospheric pressure, it will boil. Thus, it would have a lower BP since its VP is lower.

These seem to contradict each other.
You still have to get your solution's vapor pressure greater then atm pressure. If you lower the vapor pressure its going to take more energy to get all those little particles back up to the vapor pressure greater then atmospheric pressure. If its going to take more heat then your solution will boil at a higher temperature.

#### G1SG2

10+ Year Member
5+ Year Member
Why does the addition of a nonvolatile solute lower the vapor pressure, but elevate the boiling point.

When a solutions VP = atmospheric pressure, it will boil.
Thus, it would have a lower BP since its VP is lower.

These seem to contradict each other.
Ding! The answer lies in the bold statements above. In order to boil, the solutions VP has to equal the atmospheric pressure, which is 1 atm (unless otherwise stated). If you LOWER the vapor pressure (say, from 0.7 atm to 0.4 atm), do you think you are closer or farther from 1 atm? You just lowered the vapor pressure and are now further from the 1 atm mark.

I'm going to copy and paste what I wrote in a similar thread a while ago:

What happens is that the molecules exert a pressure above the liquid as they try to escape into the gas phase. This is the vapor pressure. Now, if you add NaCl, a nonvolatile solute, it will block some of the molecules from escaping at the surface since NaCl will also occupy some room. This lowers the vapor pressure, since you don't have that many molecules crashing up at the surface trying to escape. By LOWERING the vapor pressure, you elevated the boiling point because remember, boiling occurs when the vapor pressure=atmospheric pressure. Think about adding salt to a pot of hot water and pasta. You want to raise the boiling point so your pasta can cook longer/cook well. Here's a silly way to make you understand:

liquid molecules attempting to escape into the gas phase: muahaha, let's become gas molecules! We're creating more vapor pressure by hanging out at the surface and trying to escape into the gas phase. That pasta won't be in there for long!

nonvolatile solute: not so fast! Let me block your way for a while.

liquid molecules attempting to escape into the gas phase: dammit! You just lowered the vapor pressure. We worked so hard! Now this pasta will have to stay in here longer because we have to create more pressure to equal to the atmospheric pressure. We almost had it! Dang.

Another example: think of 20 runners that have to run 5 miles. They can stop running as soon as they hit the 5 mile mark. Now, suppose you drive by some runners who are at the 3 mile mark (they have 2 more miles to go). You stop, stuff them in your car, drive them back to the 1 mile mark and drop the off. Now they have to run 4 more miles instead of 2 more miles to get to the 5 mile mark. Same goes for vapor pressure. When you add a nonvolatile solute, you lower the vapor pressure, but the 1 atmosphere requirement doesn't change. They solution still has to equal its vapor pressure to 1 atm. It just has to work for longer now because you lowered its vapor pressure. So, lowering the vapor pressure does not affect the atmospheric pressure. Hope this helps.

OP
M

#### MynameisMike

10+ Year Member
lol wow...it all seems so simple now. Thanks guys!

#### Hemichordate

##### Peds
10+ Year Member
So if you have two pure volatile solutes, A and B, and you mix them together, is the solution going to have a vapor pressure that is between the vapor pressures of the two compounds? What about boiling point?

#### dragon529

##### MS-III
10+ Year Member
So if you have two pure volatile solutes, A and B, and you mix them together, is the solution going to have a vapor pressure that is between the vapor pressures of the two compounds? What about boiling point?
The vapor pressure for the mixture will depends on the mole ratio of the two solutes, and it should be always in between the two solute's individual vapor pressure.

As for BP, I can only say the BP will be higher than any single solute's BP.. due to BP elevation.

#### thebillsfan

##### Unseasoned Veteran
10+ Year Member
5+ Year Member
how does the addition of a volatile solute to a solution relate to deviations in raoult's law? also what about henry's law? sorry, I realize that is sort of a broad question, but it is also a broad concept

#### G1SG2

10+ Year Member
5+ Year Member
how does the addition of a volatile solute to a solution relate to deviations in raoult's law? also what about henry's law? sorry, I realize that is sort of a broad question, but it is also a broad concept
I think that the addition of a volatile solute would cause a positive deviation and raise the vapor pressure even more because volatile substances bring their own vapor pressures. Volatility in general refers to the tendency of a substance to vaporize. Since they would vaporize more easily, they would contribute to the increasing vapor pressure.

Henry's law tells you how much of a gas can dissolve in a certain volume of a certain type of liquid. It's given by the equation p=K*c, where p is the partial pressure of the solute, c is the concentration of the solute, and k is Henry's law constant specific for each substance. When the temperature increases, the partial pressure increases, but the amount of gas dissolved DECREASES because remember, the solubility of gases decreases with increasing temperature.