The three laws of thermodynamics describe systems that exchange energy with each other or with some environment. This exchange may be the system doing work or heat being absorbed. In principle, the number of particles could also be changed. These processes may be quasi-static, so that the condition of equilibrium is maintained during the process and the process is reversible, but it may also be that the system goes from one equilibrium state to another via a non-equilibrium process (for example, the free expansion of a gas). The ‘executive summary’ consists of the following statements:

• 1 The first law states that heat is a form of energy and that energy is conserved.

• 2 The second law tells us that a system cannot convert all absorbed heat into work. Machines that are one hundred percent efficient do not exist. In the second law a new important state variable, the entropy S, is introduced.

• 3 There is a lowest temperature, at which a system is maximally ordered, and where the entropy trends to zero. This is a consequence of the quantum-mechanical nature of any physical system, which becomes relevant at very low temperatures.

It is striking that states of macroscopic systems consisting of very many particles which are in equilibrium can be described effectively in terms of a very small number of variables and parameters. This is quite a general phenomenon, holding for large classes of liquids, gases and solids, and mixtures thereof.

In thermodynamics we are interested in describing processes of macroscopic (sub)systems in which some kind of energy exchange takes place. That may be because we bring them into thermal contact, or we exert forces on them, etc. Now we will be concerned with changes in the variables that characterize the state of the system. The number of independent variables is equal to the number of ways that energy can be supplied or extracted from the system. Each of these variables is paired with a dependent variable. Half of these state variables are extensive (i.e. proportional to the amount of material or the size of the system), like for example the volume V, and the other half are intensive, like P and T.