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Q & A: vapor pressure and solutes

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Most recent answer: 10/22/2007
Q:
Sir i’m doing a project that involves boiling water at room temperature. The vacuum pump i’m using is an ordinary household vacuum cleaner, which could create only minimum vacuum. So i need information about chemicals/additives that could increase vapour pressure of water and evaporation rate. can addition of surfactants to water, increase its vapour pressure, hence cause a boiling point depression. As boiling point is a function of inter molecular forces(IMF), can i expect a surfactant ,that lowers IMF, to reduce boiling point of water. whether there are anyother additives that increase vapour pressure of water ,when added to it.
- subbu (age 12)
INDIA
A:
Interesting question. It certainly is not easy to find solutes to raise the vapor pressure of water. I think it can't be done, and here's my reasoning.


I hate to introduce a new term, but doing so makes this argument a lot easier. There's something called the chemical potential, which is a measure of how easy it is for molecules of some type to leave some material. There's a spontaneous net flow of molecules from regions of high chemical potential to regions of low chemical potential, until the chemical potentials become equal. That's just like the spontaneous flow of heat from regions of high temperature to ones of low temperature, until the temperature becomes uniform. Let's call the chemical potential of water mu.

Now think of four regions. One is the solution. One is some vapor in equilibrium with the solution. Another is pure water, and the last is the vapor it equilibrates with. In equilibrium mu(above solution) = mu(solution) and mu(water)= mu(above water).

Let's imagine we were wrong about how solutes affect vapor pressure, and the vapor pressure above the solution were more than the vapor presure above the water. Higher pressure gas has higher mu. That would mean the mu(above solution) > mu(above water). So then mu(solution) > mu(water). That would mean that water would spontaneously flow from the solution to any region of pure water. So the solution would spontaneously separate into a pure water region and a denser region. So it wasn't in equilibrium to begin with. So our assumption that you could have, in equilibrium, higher vapor pressure above the solution is self-contradictory.

The equilibrium vapor pressure above the solution is always lower.

Mike W.

(published on 10/22/2007)

Follow-Up #1: water vapor pressure

Q:
i want, how to pressurize the water and increasing the boiling temperature at 150 degree centigrade?.which method is suitable for increase the presure?
- balaji (age 25)
coimbatore ,tamil nadu,india
A:
There is a relationship between equilibrium pressure and temperature of water.  For example at 150 degrees C the pressure is about 5 atmospheres.  You would need a very strong vessel in order to contain that pressure.   For other temperatures you can consult the tables available at:
 

Lee H

p.s. In case that link should expire, the key phrases to search for are "vapor pressure" and "Clausius-Clapeyron". The latter is the name of the thermodynamic relation between vapor pressure and temperature.
Mike W.

(published on 10/22/2007)

Follow-up on this answer.