A triple point is where a substance exists as a solid, liquid, and gas at the same time, with specific values based on atmospheric pressure and temperature. They provide insight into a substance’s behavior and are used to calibrate thermometers. The triple point of water has a practical application in ice skating and thermometer calibration.
In a phase diagram, a triple point is where a substance exists as a solid, liquid, and gas at the same time. Based on atmospheric pressure and temperature, triple points have very specific values and exist in equilibrium. While they have relatively few practical applications, triple dots provide scientists with insight into a substance’s behavior and are used to precisely calibrate thermometers.
A phase diagram is most often used in thermodynamics to indicate the conditions necessary for a substance to transition 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, such as water, the lines on the diagram form a Y shape. Each of the three lines of the Y will denote a state, and where the three connect is the triple point.
A substance can easily exist in one state or even two at a range of temperatures. For example, water below 32°F (0°C;) exists as ice, a solid. It heats 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 up and the vapor is released, allowing it to exist as both a gas and a liquid.
While existence in one or two states can occur over a range of temperatures, existence in all three states simultaneously requires very confined conditions and thus only occurs under specific circumstances. For example, the triple point of water occurs at a temperature of 32.018°F (0.01°C) and a pressure of 0.006 atmospheres (atm). Because the triple points are so contiguous, they are usually only seen in closed systems.
The triple point of water, however, does indeed have a practical application outside of a closed system. It is its triple point that allows skaters to glide across the ice. The pressure of a skater’s body weight on the single blade of his skates raises the temperature of the ice just enough, exerting just enough pressure, to reach the triple point of water and allow the skater to move across the surface of the liquid as the vapor is also released.
Another practical application of triple points is in the calibration of thermometers. By using a cell that may contain water or liquid nitrogen and maintaining a constant temperature on that cell for a specified amount of time, a scientist can determine the exact temperature reading needed for an accurate thermometer. While there are many methods for calibrating thermometers, triple point calibration is generally considered to be the most accurate.
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