When describing the properties of a gas you can use the values, characterizing the molecular world (microcosm), for example the energy of the molecules, the velocity, mass and t. p. The numerical values of these quantities we can define only by calculating. All such quantities are called microscopic (from the Greek "mikros" small).

However, to describe the properties of gases can be used and such values, numerical values are found by a simple measurement with instruments, for example pressure, the temperature and volume of gas. The values of such quantities are determined by the joint action of a vast number of molecules, therefore they are called macroscopic (from the Greek "macro" — large).

 The ratio: p=(2/3)n₀E_post: establishes a link between microscopic and macroscopic values for gases. That's why the formula is called the basic equation of molecular-kinetic theory of gases. Macroscopic quantities, uniquely characterizes the gas state, called thermodynamic parameters of gas. The most important thermodynamic parameters of gas are its volume V, the pressure p and temperature T.

If you take a certain mass of gas m, when permanent p, V and t of gas will be in equilibrium. When there is change of these parameters, the gas flows in a particular process. If this process consists of a number of continuously following each other equilibrium States of gas, he is called the equilibrium process. The equilibrium process must take place slowly enough, since the rapid change of the parameters of pressure n temperature can have respectively the same values throughout the volume of the gas. This Chapter describes only the equilibrium processes in gases, in which the mass of gas remains constant.

When the process ends in Gaza, the gas passes into a new state, and his options are becoming the new permanent numeric values, generally speaking, different from their values at the beginning of the process. If the constant mass of gas the values of all its parameters at the beginning and at the end of the process will be the same, the process is called circular or closed.

The ratio between the values of certain parameters at the beginning and end of the process is called gas z a K o n o m. Gas law, expressing the relationship between all three gas parameters, called combined gas law.

It, what such a process in Gaza, which would change only one parameter of the gas, does not exist, since the values of these parameters are interrelated. An example of this is the law of Charles, expressing the relationship between p and T.