# Q & A: Evaporating at Low Temperatures

Q:
how much vacume dose it take to turn liquid mercury into a vapor?
- Duane Jones (age 39)
conway sc usa
A:
Here's something that seams a little odd. You can cause a liquid to boil at a temperature below its boiling point! To understand this, you need to know the real reason liquids will boil. There is a special number called the "vapor pressure" of a liquid or solid. The vapor pressure changes with temperature. When the vapor pressure of the liquid is the same as the pressure around it, the liquid will begin to boil. You may have heard about how water boils faster up in the mountains. That's because the pressure is lower high above gound. As the water heats up, the vapor pressure equals the outside pressure sooner.

So the amount of vacuum needed to vaporize mercury is the same as the vapor pressure of mercury at whatever temperature you're looking at.

Here's a few numbers for you about mercury's vapor pressure:
22C --> 0.0014 torr
38C --> 0.0050 torr
100C --> 0.26 torr

Notice how the vapor pressure gets higher as the temperature goes up.