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If we were to place a substance in an evacuated, closed container, some of it would vaporize. The pressure in the space above the liquid would increase from zero and eventually stabilize at a constant value, the vapor pressure.
Finally, recognize that liquids that aren't in a closed container still have a vapor pressure. However, the material will eventually evaporate or vaporize (turn into a gas) completely.
Vapor pressure and boiling point have an intimate relationship. The boiling point is the temperature at which the vapor pressure of the liquid equals the external pressure. For example, because the air pressure is lower in a city far above sea level such as Denver, the boiling point of water is lower than in a sea level city such as New York.
Most materials have very low vapor pressures. For example, water has a vapor pressure of approximately 20 torr at room temperature (22 °C = 72 °F). But remember that vapor pressures increase with temperature; water will have a vapor pressure of 760 torr = 1 atm at its boiling point of 100 oC (212 oF).
In general, the higher the vapor pressure of a material at a given temperature, the lower the boiling point. In other words, compounds with high vapor pressures are volatile, forming a high concentration of vapor above the liquid; this can sometimes pose a fire hazard.
Other materials may emit enough vapor to exceed the permissible exposure limit (PEL) for inhalation. This vapor pressure does not need to be large. For example, while the vapor pressure of mercury is quite low (0.00185 torr), this can easily exceed the PEL for mercury vapor. Always minimize your exposure to volatile chemicals and use proper protection such as fume hoods or respirators if their use can not be avoided or minimized.
Flammable liquid handling charts and more are available at Safety Emporium.
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