Skill 15.5 Analyzing vapor pressure curves and phase diagrams

Skill 15.5 Analyzing vapor pressure curves and phase diagrams 

Whether a substance exists as a gas, liquid, or solid depends on the nature of its intermolecular attractive forces and on its temperature and pressure. A phase diagram is a graphical way to summarize the environmental conditions under which the different states of a substance are stable. The diagram is divided into three areas representing the three possible states of the substance (gas, liquid, or solid). Temperature and pressure determine the phase of a substance and are shown on the x-axis and y-axis of the phase diagram, respectively. 

The curves separating each area represent the boundaries of phase changes Below is a typical phase diagram. It consists of three curves that divide the diagram into regions labeled solid, liquid, and gas.  

Curve AB, dividing the solid region from the liquid region, represents the conditions under which the solid and liquid are in equilibrium. Usually, the melting point is only slightly affected by pressure. For this reason, the melting point curve, AB, is nearly vertical. 

Curve AC, which divides the liquid region from the gaseous region, represents the boiling point of the liquid at various pressures. This temperature is much more dependent on atmospheric pressure because of the effect of pressure on gases. 

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