The three fundamental phases of matter are solid, liquid and gas. A phase change occurs when a substance transitions from one phase to another. In everyday life, phase changes -- such as liquid water boiling into steam -- are caused by increasing or decreasing temperature, but pressure is equally capable of inducing a phase change. The combined effects of temperature and pressure lead to a condition in which the three phases of matter can coexist.

A phase diagram is a valuable tool for analyzing the influence of temperature and pressure on phase changes. With pressure on the vertical axis and temperature on the horizontal axis, a phase diagram uses curved lines to depict the temperature and pressure conditions that result in phase changes. A phase diagram has three lines that depict the temperature-pressure combinations resulting in a transition from solid to liquid, liquid to gas, and solid to gas. The point at which these three lines intersect is called the triple point -- at this exact combination of temperature and pressure, a substance can assume any of the three phases. The triple point for water is a temperature of 0.01 degrees Celsius (32.018 degrees Fahrenheit) and a pressure of 611.7 Pascals (.006 atmospheres). With this combination of temperature and pressure, water can exist as liquid water, ice or steam.