The three phases are solid, liquid and gas. A change of phase occurs when a substance passes from one phase state to another. The freezing point is the point at which the substance passes from a liquid to a solid. The melting point occurs where the substance passes from a solid to a liquid. (These two names actually represent the same point.) The boiling point occurs where the substance passes between its liquid and gaseous phases. Evaporation is the process of changing from a liquid to a gas; condensation is the process of changing from a gas to a liquid. Fusion or melting is the process of changing from a solid to a liquid; solidification or crystallization is the process of changing from a liquid to a solid. Most substances pass from a solid to a liquid to a gas and vice versa. Under certain circumstances, however, some substances may change directly from a solid to a gas. This process is known as sublimation. A common example is dry ice. Dry ice is frozen carbon dioxide. When exposed to room temperature air (at standard pressure), dry ice changes directly to a gas.
Associated with each substance is a phase diagram, which represents the phase of the element at various temperatures and pressures. Temperature is plotted on the x-axis; pressure is plotted on the y-axis. The graph is then divided into sections representing the phases of the substance. Any point on the line dividing the liquid and gaseous phases represents the boiling point for those conditions. This dividing line ends at a specific temperature known as the critical temperature. At any temperature higher than the critical temperature, the substance can only exist as a gas. Any
