Standard Temperature and Pressure is defined as 0 degrees Celsius and 1 atmosphere of pressure. Do not confuse STP with the STP Products Company, a maker of oil and fuel additives...unless you are reading one of their MSDS's! STP is used in many thermodynamic calculations and tabulations. Certain properties of matter such as density, viscosity, boiling point etc. will vary with changes in temperature or pressure. Having one common set of conditions ("state") for tabulating these values makes comparisons possible and eases calculations. The concept of matter in its standard state (also called "standard conditions") is closely related. "Standard state" does not generally imply a specific temperature, but 25 °C (298 K) is most often used: State of Matter: Gas, Standard State: 1 atm of pressure; State of Matter: Liquid, Standard State: Pure liquid; State of Matter: Solid, Standard State: Pure solid; State of Matter: Solution, Standard State: 1 molar; State of Matter: Elements, Standard State: The most stable allotrope at STP, with Gf0 = 0. Many chemistry calculations are for materials that are in their standard state. One very useful rule for gases that does not necessarily require standard state conditions is the Ideal Gas Law (PV = nRT): Where P: Pressure (atm), V: Volume (liter), n: # of moles of material (mole), R: The Ideal Gas Constant (0.08206 L.atm.mol-1.K-1), T: Temperature (Kelvin). MSDS Relevance: The letters "STP" occur most commonly on an MSDS after a physical property such as the density or flammable limit of a gas or thevapor pressure of a material. Remember: STP is 0 °C, NOT Room Temperature. Usually the properties at STP versus room temperature vary by less than 10% for gases and even less for liquids or solids. For example, in the calculation in the previous section, the volume would be 858 liters at 20°C, a difference of 7%. And the density of water is 0.99987 g/ml at 0 °C versus 0.99823 g/ml at 20 °C, a difference of 0.16%. However, sometimes the differences can be extreme - dimethylamine is a volatile liquid with a specific gravity of 0.680 at STP, but it is a gas above its boiling point of 7 °C! See also: Mass Units, Mole, Pressure Units