In a phase diagram, a triple point is the place where a substance exists simultaneously as a solid, a liquid, and a gas. Based on atmospheric pressure and temperature, triple points have very specific values and exist in equilibrium. Though they have relatively few practical applications, triple points do tell scientists information about the behavior of a substance and are used to precisely calibrate thermometers.
A phase diagram is most often used in thermodynamics to denote the conditions necessary for a transition of a substance between its three states: solid, liquid, and gas. These diagrams look similar to a graph, with temperature listed along the x-axis and, usually, pressure listed along the y-axis. For unary, or simple, one-component substances, like water, the lines on the diagram form a Y shape. Each of the Y's three lines will denote one state, and where the three connect is the triple point.
A substance may exist easily in one state or even two at a range of temperatures. For example, water under 32°F (0°C;) exists as ice, a solid. Warm the ice above its freezing point, and it begins to melt, existing as both a solid and a liquid. Conversely, water above 32°F (0°C;) is in a liquid state. Heat it and steam is released, allowing it to exist as both a gas and a liquid.
Although existing in one or two states may occur over a range of temperatures, existing in all three states at the same time requires very confining conditions and so is only seen in specific circumstances. For example, water's triple point occurs at a temperature of 32.018°F (0.01°C) and a pressure of 0.006 atmosphere (atm). Since triple points are so confining, they are usually only seen in closed systems.
Water's triple point, however, actually has a practical application outside of a closed system. It is its triple point that allows ice skaters to slide across the ice. The pressure of a skater's body weight on the single blade of his or her skates raises the temperature of the ice just enough, while exerting just enough pressure, to reach water's triple point and allow the skater to move across the surface of the liquid while vapor is also being released.
Another practical application of triple points is in the calibration of thermometers. Using a cell which may contain water or liquid nitrogen and maintaining a constant temperature on that cell over a fixed period of time, a scientist can determine the exact temperature reading necessary for an accurate thermometer. Though there are many methods to calibrate thermometers, calibration using triple points is usually thought to be the most accurate.