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Boiling Point
Definition
- Boiling point is the temperature at which a liquid changes to a gas (vapor) at normal atmospheric pressure.
A more specific definition of boiling point is the temperature at which the vapor pressure of a liquid is equal to the external pressure. - The normal boiling point is the temperature at which the liquid boils when the external pressure is one atmosphere (760 torr = 760 mm Hg = 1 atm = 101.3 kPa = 14.7 psi).
- Pure substances have a discrete boiling point. Mixtures consisting of two or more chemicals can have a single boiling point, but depending how the components interact, boiling may instead occur over a range of temperatures and the term boiling point range is a better descriptor. For example, petroleum ether (a mixture of various aliphatic hydrocarbons) typically has a boiling point range of 42 to 62 °C (108 to 144 °F) .
Additional Info
How can one determine the temperature at which water boils? After all, bubbles start to appear in water well below the known boiling point of 100 degrees C (212 °F).
The answer lies in monitoring the temperature of the material with time. When the boiling point is reached, the temperature will not rise again until all of the liquid has evaporated. This is due to the high heat capacity of water (it takes much more energy to convert water from liquid to gas than it does to raise the temperature of liquid water).
Of course, if water is heated under pressure this may raise the boiling point above its normal boiling point of 100 degrees C. Likewise, the addition of a solute may also raise the boiling point, a phenomenon called boiling point elevation (see Further Reading below for more information).
Not all substances have a boiling point. Some substances may decompose into other materials when heated instead of boiling. Wood does not boil, and neither does calcium carbonate, which decomposes to calcium oxide and carbon dioxide when heated. Other substances, such as solid carbon dioxide, may sublime to give gases without ever forming a liquid under normal conditions. However, under higher pressure, carbon dioxide will turn to liquid and then boil as the temperature is raised. Therefore, it is a best scientific practice to always report the external pressure when reporting a boiling point.
SDS Relevance
If applicable and known, the boiling point of a material is required to appear in Section 9 (physical and chemical properties) of the material's Safety Data Sheet.
Knowing the boiling point of a substance is an important consideration for storage. For example, storing a chemical with a boiling point of 50 oC (122 oF) in direct sunlight or next to a boiler could cause the material to completely vaporize and/or result in a fire or explosion.
Items with a low boiling point generally have a high vapor pressure. Containers of such material can build up signicant pressure even when they are below their boiling point. Likewise, low-boiling materials easily produce large amounts of vapor which can be flammable or even explosive.
Reactions run more easily with laboratory heating mantles and temperature controllers from Safety Emporium.
Further Reading
- Melting Point, Freezing Point, Boiling Point at Purdue University.
- Boiling point and related concepts at Carl Nave's HyperPhysics site.
- Wikipedia's entry for boiling point.
- ChemSpider has information on over 63 million structures and includes all kinds of data such as boiling points (search and then click on the Properties tab in the result).
- The International Boiling Point Project - a great K-12 experiment. Available thanks to the Internet Archive.
- Safety Emporium sells a variety of Thermometers and Thermocouples for accurate temperature measurement in the laboratory.
See also: evaporation rate, flash point, freezing point, vapor pressure.