All matter exists in one of four states: solid, liquid, gas, or plasma. Solid matter has densely packed molecules and does not change volume or shape. Liquids have more loosely packed molecules and can change shape but not volume. Gas molecules are widely dispersed, and gases can change both shape and volume. Plasma is similar to a gas but contains free-moving charged particles (although its overall charge is neutral).
Particles in gases, liquids, and solids all vibrate. Those in gases vibrate and move at high speeds; those in liquids vibrate and move slightly; those in solids vibrate yet stay packed in place in their rigid structure.
Changes in temperature and pressure can cause matter to change states. Generally, adding energy (in the form of heat) changes a substance to a higher energy state (e.g., solid to liquid). Transitions from a high to lower energy state (e.g., liquid to solid) release energy. Each of these changes has a specific name:
The occurrence of these processes depends on the amount of energy in individual molecules, rather than the collective energy of the system. For example, in a pool of water outside on a hot day, the whole pool does not evaporate at once; evaporation occurs incrementally in molecules with a high enough energy. Evaporation is also more likely to occur in conjunction with a decrease in the gas pressure around a liquid, since molecules tend to move from areas of high pressure to areas of low pressure.
Phase diagrams are used to indicate the phase in which a substance is found at a given pressure and temperature. Phase diagrams are constructed on an x, y-coordinate system where temperature is plotted along the x-axis and pressure is plotted along the y-axis. Phase regions are areas on a phase diagram (corresponding to specific temperature and pressure combinations) at which the substance will exist in a particular physical phase. Lines called phase boundaries separate these phase regions, representing pressure and temperature combinations at which the substance undergoes phase transitions.
Every phase diagram includes two important points. The triple point is the point at which the lines of equilibrium intersect and all three phases (solid, liquid, and gas) exist in equilibrium. The second special point on a phase diagram is the critical point. This point is found along the phase boundary between liquid and gas, and is the point at which the phase boundary terminates. This represents the fact that at very high temperature and pressure, liquid and gas phases become indistinguishable. This is known as a supercritical fluid.