[hiswifetheirmom]

Can anyone explain in simple terms why a gas that is subjected to decreasing temperature and increasing pressure would undergo condensation?

Thanks!

[BetaAmyloid]

Well, just think, if a gas is decreasing in temperature, the molecules are slowing down. If you then add pressure, the molecules slow down even more.

Molecules slow down more and more from gas to liquid to solid. So...a low temperature and high pressure equals condensation and eventually a hard substance (such as you have water vapor...then liquid water...then ice water).

[hiswifetheirmom]

Thank you.  I knew it had to do with the molecules slowing down, but this really helped.  Thanks!

[cth]

Well, just think, if a gas is decreasing in temperature, the molecules are slowing down. If you then add pressure, the molecules slow down even more.

Indeed, decreasing the temperature results in slower motion for molecules.
But, pressure and molecular motion are not directly linked. Pressure comes from the number of molecules that hit a surface during a period of time. For example, it is possible to increase the pressure while keeping the temperature constant (so the molecular motion constant) by decreasing the volume.

Can anyone explain in simple terms why a gas that is subjected to decreasing temperature and increasing pressure would undergo condensation?

Thanks!

In a gas, you have the thermal agitation that tend to keep the molecules moving, and you have the interactions between molecules when the bounce into each other. Those supramolecular interactions are attractive: Van der Waals forces http://en.wikipedia.org/wiki/Van_der_Waals_force, hydrogen bonds, pi-pi interactions, ionic forces, halogen bonds http://en.wikipedia.org/wiki/Halogen_bond,...
* When you decrease the temperature, molecules are moving more slowly. They have less kinetic energy. Then, as the temperature goes down, comes a point where interactions forces become stronger than the kinetic energy that keep the molecules moving  you get condensation. The molecules aggregate with each others to form a liquid or a solid.
* When you increase the pressure, the gas becomes more dense (you have more molecules by unit of volume, so the molecules are bouncing with each other more often). As the density increases, the attractive forces become more and more important. Then comes a point where they get predominant over the thermal energy  you get condensation.

BTW, that illustrates why some compounds have low boiling points while other boil at high temperature:
* If a molecule is heavy, you need more energy (so higher temperature) to break the attractive interactions within the solid and to make it move away.
* If attractive forces are strong (like ionic forces for salt compounds), you need a lot more energy to break them and let the molecules fly freely  salts usually have very high boiling points.
* If the attractive forces are weak, no need of much energy to break them, so a low temperature is enough. For example, butane is a gas at room temperature and atmospheric pressure. It is because butane in its solid form is held together only by Van der Waals forces which are weak. In comparison, water (which is lighter weight than butane) boils at much higher temperature because of the hydrogen interactions that keep the liquid together more efficiently.

[BetaAmyloid]

Ah, sorry cth, almost had it right.