Gas is aeriform whose temperature is higher than the critical temperature; as a result, gases cannot be liquefied without first being cooled, unlike vapors. Gas is a fluid that has no volume of its own (tends to occupy all the volume at its disposal) and that is easily compressible. A gas is one of the four fundamental states of matter, in which, 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 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 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 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.

Elemental gases

The only chemical elements that are stable diatomic homonuclear molecules at standard conditions for temperature and pressure are hydrogen (H2), nitrogen (N2), oxygen (O2), and two halogens: fluorine (F2) and chlorine (Cl2). When the pressure is changed and is higher or lower, or when the temperature is changed and is higher or lower, then the element may exist in a different form such as in liquid form or solid form. When grouped together with the monatomic noble gases – helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn) – these gases are called “elemental gases”.

Physical characteristics of a gas

The physical properties or macroscopic characteristics of a gas are:

  • pressure;
  • volume;
  • number of particles (chemists group them by moles);
  • temperature.

From the microscopic point of view, however, the properties of gases are:

  • kinetic theory of particles: the model of the kinetic theory of gases describes a gas as a large number of identical submicroscopic particles (atoms or molecules) with the same mass, all of which are in constant, rapid, random motion. Their size is assumed to be much smaller than the average distance between the particles. The particles undergo random elastic collisions between themselves and with the enclosing walls of the container. The basic version of the model describes the ideal gas and considers no other interactions between the particles and, thus, the nature of kinetic energy transfers during collisions is strictly thermal.
  • brownian motion: is the mathematical model used to describe the random motion of particles suspended in a fluid (a liquid or a gas) resulting from their collision with the fast-moving molecules in the fluid.
  • intermolecular forces: momentary attractions (or repulsions) between particles affect gas dynamics; the name given to these intermolecular forces is van der Waals force. These forces play a key role in determining physical properties of a gas such as viscosity and flow rate. Ignoring these forces in certain conditions allows a real gas to be treated like an ideal gas. This assumption allows the use of ideal gas laws which greatly simplifies calculations.

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.

Inert gas

An inert gas is generally non-reactive with other substances, used to avoid unwanted chemical reactions degrading a sample. The term ’inert’ means non-reactive. We refer to gases as being chemically inert if their atoms don’t combine with other atoms in chemical reactions under a set of given conditions. The noble gases often do not react with many substances and were historically referred to as the inert gases. These undesirable chemical reactions are often oxidation and hydrolysis reactions with the oxygen and moisture in the air.

Unlike noble gases, an inert gas is not necessarily elemental and is often a compound gas. Like the noble gases, the tendency for non-reactivity is due to the valence, the outermost electron shell, being complete in all the inert gases. This is a tendency, not a rule, as noble gases and other “inert“ gases can react to form compounds. Inert gases, in particular helium, are used as coolants in gas-cooled reactors. In laboratory experiments and accident simulation tests, inert gases, mostly argon, are applied as a cover gas or a filling gas for the cladding tubes to detect cladding damage.

The presence of inert gases has the following consequences. The inert gases decrease the partial pressure of the reactive gases like oxygen, steam or hydrogen, and under special conditions, they can isolate the reactive gases from the reaction site. The decrease in the partial pressure of hydrogen by inert gases was discussed earlier. The vapor pressure of oxygen is so low that oxygen impurities of the order of ppm in noble gases result in oxidation reactions. The second effect, the formation of a boundary layer between the reactive gas and reaction site, plays a more important role. In the case of lamellar gas flows, the reactive gases flowing at the metal/gas interface are consumed and the inert gas remains.

New reactive molecules have to diffuse through the inert gas layer at the interface. However, diffusion in gases is fast and gas diffusion does not influence or only slightly influences the reaction rate. The situation changes when the concentration of the reactive gases is so low that starvation conditions are reached, at least locally. Inert gases play a determining role under conditions of limited diffusion. Such conditions occur, for instance, in breakaway oxide layers during oxidation. The gases penetrate into the cracks and are transported to the oxide/metal interface. The oxygen or steam is consumed there and the noble gases remain in the cracks. New oxygen or steam then has to diffuse through the cracks filled with the inert gases. The result is an, at least partly, suppressed breakaway effect.

Noble gas

The noble gases (historically called also inert gas, 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:

  • helium (He)
  • neon (Ne)
  • argon (Ar)
  • krypton (Kr)
  • xenon (Xe)
  • 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)

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.

These approximations lead to formulate the law known as the ideal gas law, which describes, in thermodynamic equilibrium conditions, the relationship between pressure, volume and temperature of the gas:

PV = nRT

where P is the pressure, V is the volume occupied by the gas, n the amount of substance of the gas, R = 8.314462618… [J/mol·K] is the universal constant of perfect gases and T is the temperature.

Real gas

Real gases are non-hypothetical gases whose molecules occupy space and have interactions; consequently, they adhere to gas laws. An attempt to produce an equation that describes the behavior of gases more realistically is represented by the equation of real gases.

The corrections made to the equation of perfect gases are two: the proper volume of the molecules is taken into account, which is therefore no longer considered point-like, and the interactions between molecules that were neglected in the case of perfect gases are considered.

The first correction has the effect of making the gas not indefinitely compressible; its empirical finding is the liquefaction to which real gases are subjected if compressed (and cooled) sufficiently. The second correction ensures that the real gases do not expand infinitely but reach a point where they cannot occupy more volume (this is because a very small force is established between the atoms, due to the random variation of the electrostatic charges in the individual molecules, called Strength of van der Waals).

For this reason, the ideal gas law does not provide accurate results in the case of real gases, especially in low temperature and/or high-pressure conditions, while it becomes more accurate in case of rarefied gases, at high temperature and low pressure, that is when intermolecular forces and molecular volume become negligible. So, to understand the behavior of real gases, the following must be taken into account:

  • compressibility effects;
  • variable specific heat capacity;
  • van der Waals forces;
  • non-equilibrium thermodynamic effects;
  • issues with molecular dissociation and elementary reactions with variable composition.

Industrial gas

Industrial gases are the gaseous materials (at ambient temperature and pressure) that are specifically manufactured for use in industry (which include oil and gas, petrochemicals, chemicals, power, mining, steelmaking, metals, environmental protection, medicine, pharmaceuticals, biotechnology, food, water, fertilizers, nuclear power, electronics, and aerospace).

They are chemicals which can be an elemental gas or a chemical compound that is either organic or inorganic and tend to be low molecular weight molecules. They could also be a mixture of individual gases.

Related keywords

  • Biogas

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