In nature, matter exists in three phases: solid, liquid, and gas. The transition between phases requires phase equilibrium between two phases and occurs at a specific temperature for a given pressure. These conditions can be represented graphically with pressure and temperature as the axes, known as a phase diagram. A phase diagram represents the phases of a substance that coexist in thermodynamic equilibrium and undergo phase transition under different conditions, such as temperature, pressure, or volume. Water exists in the liquid phase at room temperature and atmospheric pressure. At temperature 0oC or below, the phase changes from liquid to solid, whereas when the temperature is above 100oC, it changes to a gas phase. The transition between solid and gas is called sublimation, while the transitions from solid to liquid and from liquid to gas are known as fusion and vaporization, respectively. The point when all three phases coexist is known as the triple point, and the point when the properties of two phases become indistinguishable is known as a critical point.