Gaseous state of matter (gas)

A gas is one of the four fundamental states of matter. In gases, the atoms or molecules are far apart due to they are not bounded at all, meaning, they do not have any attractive forces but only repulsive forces.

The interaction of gas particles in the presence of electric and gravitational fields are considered negligible. Due to that, they can occupy a large volume. They do not have their shape or volume but assume the shape and the volume of the container.

Finally, gas particles spread apart or diffuse in order to homogeneously distribute themselves throughout any container. Some typical examples are oxygen, hydrogen, and helium at room temperature.

The term “gas” attributed in 1620 by the chemist J.B. van Helmont to substances that are in the gaseous state and therefore have no proper volume.

The gaseous state, like any other state of aggregation, depends on the temperature and pressure conditions and is not characteristic of certain substances.

Saying that a substance, for example, air is a gas, it only means that it is such in the ordinary conditions of temperature and pressure, by varying these conditions it can instead manifest itself as a liquid or even as a solid.

Physical characteristics of a gas

The physical properties or macroscopic characteristics of a gas are four:

  1. pressure;
  2. volume;
  3. number of particles (chemists group them by moles);
  4. temperature.

Compared to the other states of matter, gases have low density and low viscosity. Since gas molecules can move freely within a container, their mass is normally characterized by density.

Pressure and temperature influence the particles within a certain volume. This variation in particle separation and speed is referred to as compressibility. This particle separation and size influences optical properties of gases.

In a gas (at standard temperature), the attractive forces existing between the molecules are not such as to keep the molecules bound together; when instead the gas is brought to a very low temperature, it happens that the short-range forces end up prevailing on the tendency of the molecules to remain independent one from the other.

Under normal conditions of pressure (1 atm) and temperature (25 °C), the molecules are practically free from each other, and this is the reason why a gas always tends to occupy the entire volume at its disposal.

To characterize a gas we need different parameters, unlike liquids or solids, for which even a single parameter is often sufficient: for example, if we have 1 liter of water, there is no possibility of confusion, as it is permissible to neglect the phenomenon of “cubic expansion” of liquids (in relation to sudden changes in pressure or temperature), so that a precise volume will always correspond to 1 liter of water. Same thing for solids: to study a substance in the solid state it is generally not necessary to specify under which experimental conditions we conduct our analysis.

Different is the case of gases because the “quantity” for gas is something entirely different from the volume that contains it. Given a certain mass (m) of a gas, or a certain quantity of such gas, it is necessary to use other parameters to conduct further analyzes: the pressure (P), the volume (V) of the vessel containing the gas and the temperature (T) at which the gas is located.

Of these four quantities (P, V, T, m), each can be expressed as a function of the other three: while in solids and liquids the dependence of “V” and “m” from “P” and “T”, it is not possible to neglect this dependency for gases (it is correct to speak of a specific volume only if we specify in what conditions of pressure and temperature we consider it). This is the only way we can get the mass of gas available.

Noble gas

The noble gases (historically called also the inert gases, but this term is not strictly accurate because several of them do take part in chemical reactions) make up a group of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low chemical reactivity and made up of atoms with complete electron shells. They are the most stable due to having the maximum number of valence electrons their outer shell can hold. Therefore, they rarely react with other elements since they are already stable.

Other characteristics of the noble gases are that they all conduct electricity, fluoresce, and are used in many conditions when a stable element is needed to maintain a safe and constant environment.

They constitute the group 18 of the periodic table according to the current IUPAC nomenclature. Noble gases are all monoatomic gases, not easily liquefied, typically non-reactive, present in the atmosphere in various percentages.

The six noble gases that occur naturally are:

  1. helium (He)
  2. neon (Ne)
  3. argon (Ar)
  4. krypton (Kr)
  5. xenon (Xe)
  6. and the radioactive radon (Rn).

Oganesson (Og) is variously predicted to be a noble gas as well or to break the trend due to relativistic effects; its chemistry has not yet been investigated.

Perfect gas (ideal gas)

A perfect gas (or ideal gas) is defined as a gas that follows the laws of Boyle and Gay-Lussac.

The perfect gas model explains the behavior of gases using the kinetic-molecular theory; here are the characteristics of an ideal gas:

  • the average kinetic energy of the gas molecules (thermal agitation motion) is directly proportional to the absolute temperature;
  • the gas molecules do not attract each other reciprocally; therefore the distance forces of interaction and any other type of energy other than kinetic energy are null. In a real gas, the situation is generally more complicated, because there are, even if weak, reciprocal cohesion forces between the gas molecules, and furthermore they also possess a particular potential (gravitational) energy. Moreover, in a real gas subject to compression, the distances between the molecules become too small to be able to neglect the reciprocal cohesion forces, while in gas at very low temperature the collisions between the particles become so sporadic that they are not significant. However, the behavior of a real gas, provided it is sufficiently rarefied, can be assimilated to that of a perfect gas. For this reason gases, unlike solids and liquids, have no shape of their own and tend to expand, occupying the entire volume of their container;
  • the volume occupied by the molecules is negligible, this feature is also valid for real gases since the particles are assumed as point-like;
  • the molecules interact with each other and with the walls of the container through perfectly elastic collisions (i.e., there is no dispersion of kinetic energy during impacts) the impacts against the walls determine the pressure exerted by the gas;
  • the ideal gas molecules are assumed as rigid spheres, having all identical mass and a negligible volume compared to that occupied by the entire gas;
  • the motion of molecules is random and disordered in every direction but subject to deterministic laws.


Biogas is a mixture of different gases produced by anaerobic decomposition (with methanogen or anaerobic organisms), or fermentation, of biomass – organic material (including animal dung, human sewage, food waste, crop residues, and industrial and municipal wastes). Biogas is a renewable energy source.

It is composed primarily of methane (up to 60%), which is the combustible component, carbon dioxide, and hydrogen sulfide.

Biogas is produced in an air-tight container, called an anaerobic digester, biodigester or a bioreactor.

Is used as a fuel to heat stoves, lamps, run small machines, and to generate electricity. The residues of biogas production are used as a low-grade organic fertilizer. Biogas fuels do not usually cause any pollution to the atmosphere, and because they come from renewable energy resources they have great potential for future use.