[wallaby]

On a graph of a equilibrium vapor pressure curve (pressure vs. temperature) why is it that the substance is a liquid if it is above and to the left of the curve, and a gas if it is below and to the right of the curve?

For example if substance X has a vapor pressure of 170 mm Hg at 50 degrees C, then how come the substance is a gas if the pressure is 170 mm Hg, and a liquid when the pressure is greater than 170 mm Hg at 50 degrees C.

[lancenti]

You could think of it based on the kinetic model of matter. Since the temperature is the same, the molecules have equal amount of kinetic energy to push out the walls of the container.

But at a higher pressure, or smaller volume, you'll notice that the molecules come closer together. If the molecules are sufficiently close, it becomes a liquid. There is, of course, much more to this explanation but it might be better if you read up on it.