Can anyone explain vapor pressure to me?

Question

If you could, please apply it to this question:

b. Explain in a paragraph why (and how) liquid mercury could be vaporized.
c. The freezing point of Hg is -38.87 degrees C; does crystalline Hg exert a vapor pressure? Is the vapor pressure of Hg(l) larger or smaller than that of Hg(g)? Hg(s)?

For b, it is my understanding that when a liquid has great pressure, it vaporizes. Is this correct?

Answer 1

Vapor pressure is a measure of the tendency of a chemical to want to spontaneously leave the bulk liquid/solid state and enter the vapor state. The higher the vapor pressure, the more the chemical wants to escape from the bulk phase (liquid or solid) and move to the vaporous (gaseous) phase.

The process of vaporization requires energy, usually in the form of heat (the latent heat of vaporization). As you heat a bulk chemical, its temperature rises, and the vapor pressure rises along with it. When the vapor pressure of the chemical reaches atmospheric pressure, the chemical boils.

Mercury has a vapor pressure of 0.002 Torr at 25°C. The heat of vaporization is 14.652 kcal/mole at 25°C. If you wish to increase the vapor pressure (and rate of vaporization) of the mercury, turn up the heat.

Even frozen Mercury has a vapor pressure, although it is very low. Gaseous Mercury does NOT have a vapor pressure, because it is already in the vaporous state.

I hope that helps.

Answer 2

Picture the liquid mercury sitting in a wide flat dish inside a sealed plexiglass container. Some of the atoms of mercury, especially those on the surface, have enough energy to change to the gas (vapor) phase. These gas atoms will hit the walls of the container and bounce off it. The more atoms do this and the harder they hit, the higher the vapor pressure. Normally this happens more when the temperature inside the box goes up.
This always happens, but, maybe the sun is shining on the box, or it is on top of a bunsen burner. If the sealed box gets hot enough, you can imagine how the vapor pressure inside might even explode the box.

If the temperature is very cold, below-38.87 degrees C, mercury is in the solid, crystalline phase. Even then, a few atoms on the surface will have enough energy to sublime (change directly from a solid to a gas). So there will be a small vapor pressure even at that low temperature.

So which phase has the highest vapor pressure - solid, liquid or gas?
Since gas is the hottest phase of any substance, the molecules are all vapor and moving very fast, so gas has the highest vapor pressure, next comes liquid then solid.
If someone broke open a liquid mercury filled instrument and didn't clean it all up, the mercury would slowly vaporize and be breathed in by people over time causing mercury poisoning, even if they didn't touch it.