A phase diagram shows the conditions under which a substance exists in different phases. It typically includes solid, liquid, and gaseous phases, with pressure on the vertical axis and temperature on the horizontal axis. The curves on the diagram represent points of equilibrium between phases, and the open spaces represent distinct phases. The triple point is where all three phases are in equilibrium. The curve dividing liquid and gas represents the boiling point.
A phase diagram is a graph commonly used in the sciences, especially chemistry, to visualize the conditions under which a substance exists in one phase and changes in another phase. The phases addressed by a phase diagram are typically the solid, liquid and gaseous phases. The diagrams are set up as graphs with pressure on the vertical axis and temperature on the horizontal axis. The curves on the diagram represent points where the different phases are in a state of equilibrium and the open spaces represent distinct phases. A lot of valuable information can be gleaned from such a diagram, such as the boiling point and freezing point of a substance at a given pressure.
The typical phase diagram begins at very low temperature and pressure and begins with only one curve. Above the curve is the solid phase and below is the gaseous phase. The curve itself represents the equilibrium between solid and gas; under conditions of very low pressure, liquid rarely exists. This balance is also called the sublimation point. Eventually, at a given temperature and pressure, the curve branches into two distinct curves, and the space between the two represents the liquid phase.
The point where the curve branches into two curves is called the triple point. It is important because it represents the specific conditions of temperature and pressure under which the solid, liquid and gaseous phases are all in a state of equilibrium. This means that the substance represented by the graph is constantly shifting between phases: all three are present at any given time.
At a certain temperature and pressure, the lower curve ends at a point called the critical point. This usually occurs at relatively high pressure and temperature. Under conditions of pressure and temperature above the critical point, there is no perceptible distinction between the liquid phase and the gaseous phase.
Phase diagrams are useful and can be very important to chemists. The curve on the phase diagram dividing liquid and gas, for example, represents the conditions under which the two will be at equilibrium. In other words, the curve represents the boiling point of the substance.
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