Phase Diagrams

Phase Diagrams of water and carbon dioxide

What does LPG stand for? How can a gas be liquefied? What conditions are needed to convert a gas into a liquid?

What does LPG stand for? How can a gas be liquefied? What conditions are needed to convert a gas into a liquid?

What is phase diagram?

• A phase diagram is a graphical representation of the physical states of a substance under different conditions of temperature and pressure.
• It gives the possible combinations of pressure and temperature at which certain physical state or states a substance would be observed.
• Each substance has its own phase diagram.

What are the features of a phase diagram?

What are the features of a phase diagram?

• Phase diagrams are plots of pressure (usually in atmospheres) versus temperature (usually in degrees Celsius or Kelvin).
• The diagram is divided into three areas: solid, liquid and gaseous states.
• The boundary between the liquid and gaseous regions stop at point C, the critical temperature for the substance.

A. The Three Areas

A. The Three Areas

The three areas are marked solid, liquid, and vapor. Under a set of conditions in the diagram, a substance can exist in a solid, liquid, or vapor (gas) phase. The labels on the graph represent the stable states of a system in equilibrium.

B. Three Lines (Curves)

B. Three Lines (Curves)

The lines that serve as boundaries between physical states represent the combinations of pressures and temperatures at which two phases can exist in equilibrium. In other words, these lines define phase change points.

B. Three Lines (Curves)

B. Three Lines (Curves)

1. The green line divides the solid and liquid phases, and represents melting (solid to liquid) and freezing (liquid to solid) points.