A pure substance is one that contains only one type of atom or chemical molecule (the opposite of a mixture).

Pure substances such as water (H2O) can be found in 3 forms

• solid
• liquid
• gaseous

Water has the advantage of being naturally present on Earth in these 3 forms, because its properties allow it to be at ambient temperature. This is not the case for other substances, which require extreme conditions to change state, such as :

• copper: solid
• oxygen : gaseous
• ethanol: liquid

For example, for oxygen to become solid, it requires a temperature lower than -218.79°C!

Thermodynamics is concerned with the conditions required to change from one form to another and the way this transition takes place, which requires a significant exchange of heat.

An important tool for determining the stable form of a pure substance is the equilibrium phase diagram.

To name the homogeneous state of a pure substance, we use the concept of phase. We will thus have the liquid, solid and gaseous phase.

• Solid: the particles are in contact with each other around a defined position. They make small oscillations and the solid keeps a well defined shape.
• Liquid: no fixed shape, the particles are in contact, but can move.
• Gaseous: the particles are not in direct contact and are distant from each other.

When the particles are in permanent contact (which is the case of the liquid and gaseous phase), we speak of a condensed phase.

Those whose compound can flow and which do not have their own form are called fluid phases (this is the case of the liquid and gaseous phase).

The phase diagram

It allows to predict the state of a given pure substance at equilibrium thanks to the temperature and pressure.

Logically, at high temperature and low pressure, the stable state is the gaseous form. At very low temperatures, the solid appears as the stable state. The liquid is stable only in an intermediate zone between the two other zones, there is a minimum pressure and a minimum temperature for the existence of the liquid state.

The shape of a phase diagram is almost the same for each pure substance.

Several phases can coexist if the pressure and temperature conditions are on the phase separation curve. The most common example is an ice cube in a glass, if the temperature between the liquid and the solid is precisely 0°C and the air is also at 0°C, then their state will remain frozen ad vitam æternam since there is no more heat transfer.

Thus, we realize that to store a gas in liquid form, we often have the choice between increasing the pressure or lowering the temperature.

The triple point is the coexistence of these three phases (here, it is θ)

The critical point (here, the point C), when we exceed this point, the notion of liquid or gas loses its meaning, there is no more boiling or temperature plateau. This area of the phase diagram is called the "zone of continuity of the fluid state".

Latent heat of change of state

Enthalpy of change of state

When water is boiled, its temperature increases continuously until it boils. Then, the temperature remains constant as the heating continues. Why does this happen?

According to the phase diagram, the temperature of the water cannot increase any more, because after this temperature it is no longer in a stable state. Thus, the amount of heat supplied is used for the transformation of the water into steam. In order for the temperature to continue to rise, all the water must have become steam.

This is how we define :

The latent heat of change of state is the amount of heat required to move a pure substance from one state to another reversibly, with the pressure held constant.

To melt an ice cube, it takes as much heat as to raise the temperature of the same amount of liquid by 80°C. As for the transformation of this same quantity of liquid into gas, it requires no less than 5.4 times the amount of heat necessary to make the liquid go from 0°C to 100°C! For example, if it took 5 minutes to boil, it will take more than 25 minutes to turn all the water into gas. Thus, there is no point in keeping the heating at maximum for cooking pasta, the temperature remains the same, it is enough to keep the boiling time to a minimum to save energy.

Here is a diagram to name the passage from one state to another, for example to pass from a liquid state to a solid state, it is the solidification:

The latent heats of change of state are always positive values and correspond to the passage of lower enthalpy to that of higher enthalpy. That is to say from solid to liquid or from liquid to gas.

We will see soon: the chemical potential and the mixtures.