Phase Diagram

The figure shows an example of a phase diagram, which summarizes the effect of temperature and pressure on a substance in a closed container. In this diagram, every point represents possible combination of temperature and pressure for the system. You can see the diagram, which partly divided into 3 areas; they are solid, liquid, and gaseous states of the substance.

Always low temperature and high pressures will favor in formation of solid, but high temperature and low pressure will form gases and liquid will lie between these extreme. The best way to remember which area corresponds to each of this state is, remember the conditions of temperature and pressure which are most likely to be linked with a solid, a liquid, and a gas.  Therefore you can test whether you have correctly labeled a phase diagram, how to check it? Just draw a line from left to right across the top of the diagram, which is corresponds to an increase in the temperature of the system at constant pressure. When solid is heated at constant pressure, it melts and forms a liquid. But the solid is ultimately boils to form a gas.

In several ways phase diagram can be used. We can get some idea of the conditions of temperature and pressure which is most likely to produce a gas, a liquid, or a solid by focusing on the regions separated by the lines in the diagram. We can also focus on the lines which is dividing the diagram into states, which is representing the combinations of temperature and pressure at which two states are in equilibrium. In the above phase diagram, the point along the line connecting points A and B show all combination of temperatures and pressures, at which the solid is in equilibrium with the gas. The rate at which the solid sublimes to form a gas is equal to the rate at which the gas condenses to form a solid at these temperatures and pressures.

Along AB Line:

The rate at which solid sublime to form a gas = the rate at which gas condenses to form a solid.

Along BC Line:

The rate at which liquid boils to give the gas = the rate at which gas condenses to form a liquid.

Along BD Line:

The rate at which solid melts to form liquid = the rate at which liquid freezes to form a solid.

Questions:

• Explain phase diagram?
• What are the several ways that phase diagram can be used?