In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium. For example, the triple point of mercury occurs at a temperature of −38.8344 °C and a pressure of 0.2 mPa.
In addition to the triple point between solid, liquid, and gas, there can be triple points involving more than one solid phase, for substances with multiple polymorphs. Helium-4 is a special case that presents a triple point involving two different fluid phases (see lambda point). In general, for a system with p possible phases, there are triple points.
The triple point of water is used to define the kelvin, the SI base unit of thermodynamic temperature. The number given for the temperature of the triple point of water is an exact definition rather than a measured quantity. The triple points of several substances are used to define points in the ITS-90 international temperature scale, ranging from the triple point of hydrogen (13.8033 K) to the triple point of water (273.16 K).
Read more about Triple Point: Triple Point Cells, Table of Triple Points
Famous quotes containing the words triple and/or point:
“Their martyred blood and ashes sow
O’er all the Italian fields where still doth sway
The triple tyrant; that from these may grow
A hundredfold, who, having learnt thy way,
Early may fly the Babylonian woe.”
—John Milton (1608–1674)
“You should go to picture-galleries and museums of sculpture to be acted upon, and not to express or try to form your own perfectly futile opinion. It makes no difference to you or the world what you may think of any work of art. That is not the question; the point is how it affects you. The picture is the judge of your capacity, not you of its excellence; the world has long ago passed its judgment upon it, and now it is for the work to estimate you.”
—Anna C. Brackett (1836–1911)