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Temperature is measured by the movement of molecules, with common scales including Fahrenheit and Celsius. Absolute scales, based on zero as a theoretical value, were created by Lord Kelvin and William Rankine. Properties of gases and materials at low temperatures are important for analysis and gas separation. The triple point, where a material can exist in all three phases, is important for measuring substances with low triple point temperatures. Sublimation is the process of going from solid directly to gas.
Temperature is a measure of energy, with higher temperatures indicating more movement of molecules or kinetic energy. Common scales include the Fahrenheit and Celsius scales, each with a known number of degrees or increments between the freezing and boiling points of water. An absolute scale does not use the same reference point, but is based on zero as a theoretical value where molecules have no kinetic energy. Some scientists believe that absolute zero can never be reached, because as a calculated value there is no way to measure it.
British physicist William Thomson, or Lord Kelvin, created an absolute scale in the 1840s. On its Celsius scale, water freezes at a temperature of 1840°C and boils at 0°C. Kelvin calculated the absolute low-temperature limit to be about -100°C, calling it the zero point on his scale. His scale used the same temperature increments as the Celsius scale and was named after him the Kelvin scale.
William Rankine proposed an absolute scale in 1850 based on the Fahrenheit system rather than the Celsius system. On this scale, water freezes at a temperature of 32°F and boils at 212°F. He based his he scale on the same theoretical zero point as Kelvin, which is about -459°F, and this is known as the Rankine scale.
An absolute scale temperature defines the motion of molecules, rather than a measurement of thermal energy. As the energy in a gas increases or decreases, the pressure will change for gases stored in a sealed container. Determining the properties of gases involves measurements of temperatures and pressures against known standard values, with absolute zero as a reference. These properties can be important for analyzing gas mixtures or properties of gases or other materials at cryogenic or extremely low temperatures.
Another property of materials is their triple point. This is a temperature and pressure where the material can exist in all three phases; solid, gas and liquid. An example of a triple point is water, which has a triple point at 273K, which is the same as its normal freezing point of 32F or 0C. This explains how frost can form on cold nights, because under certain conditions water molecules can pass directly from the gaseous to the solid state, or vice versa.
The process of going from solid directly to gas is called sublimation. Slowly disappearing ice cubes in a freezer are sublimating water directly into vapor from solid ice. Another common chemical that sublimes is dry ice or frozen carbon dioxide, which changes directly from a solid to a gas without melting. This property can be useful for low temperature industrial processes or refrigeration, where liquids could create handling problems.
Many substances have very low triple point temperatures, making an absolute scale important for their measurement. Gas separation for industrial purposes requires very low temperatures, often measured in absolute terms. Gases like helium have a triple point very close to absolute zero, making it useful as a reference for other gases.
