Hydrocarbon compounds, either individually or in mixtures, will change their state or phase - from liquid to gas or vice versa - by changing temperature and pressure. This change is called phase behavior.
For each hydrocarbon component such as methane, ethane, propane, etc., there is a given pressure for every temperature at which the hydrocarbon can exist as a liquid and a gas. The vapor pressure curve shown in Figure connects those points. The critical point is the temperature above which the material will exist as a gas regardless of pressure.
When two or more components are mixed, instead of a single line an envelope represents a broad region in which two phases (liquid and gas) co-exist, Figure 1. This envelope is bounded by a bubble point line and a dew point line which meet at the critical point. The bubble point is the pressure, for a given temperature, where the first bubble of gas evolves from solution. The dew point is the pressure, for a given temperature, where the first drop of liquid condenses.