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Properties of Substances

In order to predict how systems will behave, we need to understand their properties at different equilibrium states.


Pure Substances

A pure substance is one that is chemically homogenous (it has the same chemical composition everywhere in the substance). Examples include

  • hydrogen

  • pure water

  • a mixture of water and steam

  • air (all the gaseous substances are distributed evenly, everywhere)

Examples do not include

  • Mixtures of oil and water (oil is not soluble in water, so the two will always be separate)

  • Mixtures of liquid and gaseous air (different parts of air condense at different temperatures, so the mixture is not homogeneous)



Phase Changes

While there are three main phases a substance can be in (solid, liquid and gas), there are also a number of sub-phases within these:

As you can see, the liquid phase is broken into two bands:

  • subcooled (compressed) liquids are when the liquid is not about to evaporate. For example, water at 20°C

  • saturated liquids are when the liquid is about to evaporate. For example, water at 100°C

The gas phase is also split into two bands:

  • saturated vapours are vapours that are about to condense. This region overlaps with the saturated liquids phase, so at 100°C, water exists as a mixture of liquid about to vaporise and vapour about to liquify

  • superheated vapours are gases that are not about to condense. For example, steam at 300°C

The boiling point is also referred to as the saturation temperature and pressure. This is the given temperature and/or pressure that the liquid-vapour mixture is seen. It is important to note that the temperature remains constant during a phase change.


There are two other important point between phases: the critical and the triple point.

  • At the triple point, gas, solid and liquid phases can coexist.

  • Above the critical point, liquid and vapour can no longer be distinguished. It is the high-pressure form of the liquid-vapour mixture phase, known as the supercritical fluid phase.