Phase Diagrams

Phase diagrams are graphs that show the relationship between pressure, temperature, and the state of matter for a substance. The y-axis represents pressure (increasing upwards) and the x-axis represents temperature (increasing from left to right).

Using water as an example, at standard atmospheric pressure (1 atm) and 0°C, water is at its freezing/melting point. At 1 atm and 100°C, water boils. These points define the boundaries between solid, liquid, and gas phases.

The triple point is where all three states (solid, liquid, and gas) coexist in equilibrium. For water, this occurs at very low pressure and temperature. The critical point is where the distinction between liquid and gas ceases to exist, forming a 'supercritical fluid' beyond this point.

Substances can change state by altering either temperature or pressure. Heating water at constant pressure can melt and boil it. Conversely, reducing pressure using a bell jar can cause water to boil at room temperature without changing its internal temperature.

Each substance has a unique phase diagram. At standard atmospheric pressure, water melts from solid to liquid, then boils to gas. However, for carbon dioxide (dry ice), at standard pressure, it sublimes directly from a solid to a gas, which is why it appears to give off 'smoke'.