[ygao85]

Is it always true that adding a solute to a solvent will always dec. the vapour pressure of the solution or does the solute have to be nonvolatile for this to be true?


Thx

[jdurg]

Well, just think about what vapor pressure really is.  Vapor pressure is a measure of how much of the substance is able to escape from the liquid into the space around it.  It's really just a measurement of how strong the intermolecular forces are between the molecules/atoms of the substance.  If a substance has very weak intermolecular attractions, it will have a high vapor pressure as the atoms/molecules are able to escape from each other out into the open space.  If there is a very large intermolecular attraction, then the atoms/molecules are unable to get away from each other and therefore their vapor pressure is very low.

Take a look at the element bromine.  Bromine is held together via simple Van der Waals intermolecular forces.  There is no real force keeping the bromine molecules together as a liquid.  Therefore, the molecules are able to move away from each other fairly easily and bromine has a very high vapor pressure.  Mercury, although it is a liquid, is still a metal and therefore exhibits intermetallic bonding with itself.  Metallic bonding is quite strong, so the mercury atoms have a hard time escaping from the surface of the liquid.  Because of this, mercury has a low vapor pressure.  (However, for a metal mercury has a high vapor pressure.  It's just that in comparison to most liquids that are known, the vapor pressure is miniscule).

As you can see, a strong intermolecular attraction results in a low vapor pressure.  Water, H2O, has fairly strong hydrogen bonding going on between its molecules.  Therefore, the water molecules are less likely to escape the surface of the liquid and water's vapor pressure isn't all that high.  In order for something to dissolve in water, the water has to exhibit some type of attraction to the substance being dissolved.  The water molecules move in and overcome the intermolecular attraction of the solute and thus dissolve the substance.  Now the water molecules are attracted more to the solute than they are to each other.  Because of this, the vapor pressure of the water decreases and its boiling point increases.  It is not possible to lower the boiling point of a liquid by adding another substance to it, therefore that means that you cannot elevate the vapor pressure of a liquid by adding something to it.  (Now you may have heard of something called an azeotrope, the most famous being that of ethanol and water.  The azeotrope has a boiling point which is less than that of either of the components of the mixture.  However, when the stuff does boil, it's not a pure substance boiling out of solution.  It's the mixture that is boiling together.  Therefore, you techincally cannot say that it's the water that is boiling at a lower temperature or that it's the ethanol that's boiling at a lower temperature.  In reality, it's neither of them that are boiling at a lower temperature).

[GCT]

the interaction has to be ideal, try reading up on it in your text under colligative properties.