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People who live in Denver or have gone hiking in the mountains are probably aware that water does not boil at 100 degrees C there (its a bit lower because of the pressure). Likewise, people who have made jams and jellies and used a pressure cooker can make it so that water doesn't boil until much higher than 100 degrees C.

In the world of chemistry, things happen at different rates if the variables, are different. Two of these variables are the temperature and pressure. STP - the abbreviation for Standard Temperature and Pressure refers to 0 degrees C (273.15 K) at the pressure of 1 atmosphere (101325 pascals). This is the temperature and pressure at which water freezes at sea level or at which one mole of any gas has the volume of 22.4 liters. Values for density, viscosity, and boiling point are given with respect to the standard temperature and pressure.

'Standard State' is closely related to Standard Temperature and pressure but does not imply a specific temperature, though 25 degrees C is often used. This is because while mercury is a solid at the standard temperature and pressure, most of us see it in its pure form as a liquid, thus its standard state.

Within aviation, the standard temperature is 15 degrees C (closer to the normal temperature that planes operate in), and sea level pressure (because aviators are not chemists they tend to use whatever units they want rather than the scientific units - thus pressure may be seen as 29.92 inches of mercury, 1013.2 milibars, or 101325 pascals). For aviators, this is used to compute performance related figures such as range, airspeed, and fuel consumption.

http://www.nationmaster.com/encyclopedia/Standard-temperature-and-pressure
http://scienceworld.wolfram.com/physics/StandardTemperatureandPressure.html
http://www.ilpi.com/msds/ref/stp.html
http://whatis.techtarget.com/definition/0,,sid9_gci539342,00.html

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