We will define gases, describe how they are categorized, and discuss some characteristics. What are its features and examples?

Gases are characterized by little attraction between their particles.

What are gases?

By gas, we mean one of the three main states of aggregation of matter (along with liquids and solids). It is characterized by dispersion, fluidity, and little attraction between its constituent particles.

Gases are the most volatile forms of matter in nature and they are extremely common in everyday life. Thus, when a substance is in a gaseous state, we usually call it gas.

Liquid or solid substances can be transformed into gas using different processes. This transformation implies a change in the physical properties of the substances, such as their state of aggregation. However, their chemical properties do not change since the substances continue to have the same chemical structure. That is, chemical bonds do not break, and new substances are not generated.
Gases are found everywhere: from the heterogeneous mass of gases that we call the atmosphere and that we breathe as air, to the gases that are generated within the intestine as a product of digestion and decomposition, to the flammable gases with which we feed our kitchens and ovens.

See also: States of aggregation

History of gases

The word “gas” was invented in the 17th century by the Flemish scientist Jan Baptista van Helmont from the Latin term “chaos”.
He chose the name because of the apparent degree of disorder exhibited by the molecules of a gas. Also, this state was known as the “aerial state,” but this term fell into disuse.
The first laws governing the behavior of gases were developed as a result of intensive research at the end of the same century, particularly their relationships between pressure, temperature, and volume.
This led Émile Clapeyron to formulate the ideal law for all gases (“Ideal Gas Law”) in 1834.

The ideal gas and real gas

An ideal gas is a model gas created by humans, and it has no interactions between the particles that form it; that is, they have neither attraction nor repulsion between them. On the other hand, real gas does exhibit these interactions.
The simpler the chemical formula of a real gas and the lower its reactivity, the more closely it can resemble an ideal gas. Thus, monatomic gases, for example, helium (He), are the ones that behave most similarly to ideal gases.

More in: Ideal Gases

Gas laws

The volume of a gas varies inversely proportional to the pressure at a constant temperature.

One of the most commonly used laws to describe the behavior of gases is the Ideal Gas Law, which, in turn, can be understood as a combination of other laws:

  • Boyle-Mariotte law. It is determined that the volume of a gas varies inversely proportional to the absolute pressure of the vessel where it is contained if the temperature remains constant. It is expressed according to the equation:
  • Gay-Lussac Law. He explains that the pressure of a mass of gas whose volume remains constant is directly proportional to the temperature (expressed in degrees Kelvin) it possesses. This is represented as follows:
  • Charles’ Law. It means that the temperature and volume of a gas are directly proportional when the pressure is constant. This law is represented by the following equation:
    In all previous cases V1P1, and T1 are the volume, pressure, and initial temperature. Whereas V2P2, and T2 are the volume, pressure, and final temperature.
  • Avogadro’s Law. It states that under the same conditions of pressure and temperature, different gaseous compounds contain the same number of particles.
  • Law of Ideal Gases. From the combination of the above laws, the Law of ideal gases is obtained, whose equation is represented as follows:
    Where PV, and T are the pressure, volume, and temperature, while n is the number of moles of the gas, and R is the constant of the ideal gas whose value is 8.31451 J/molK.

Types of gases

Gases are classified chemically into the following groups:

  • Combustible or flammable. Those that can burn, that is, generate explosive or exothermic reactions in the presence of oxygen or other oxidants.
  • Corrosive. Those that, when in contact with other substances, subject them to intense reduction or oxidation processes, generating damage to their surface or wounds in the case of being organic matter.
  • Oxidizers. Those that allow you to keep alive a flame or a flammable reaction since they induce combustion in other substances.
  • Toxic. Those that pose a health hazard by the reactions they introduce into the bodies of living things, such as radioactive gases.
  • Inert or noble. Those that present little or no reactivity, except in certain situations and conditions.

Gas properties

gases - aerosol
Gases can be compressed by applying pressure to them.

Gases have the following properties:

  • They have no volume of their own. They occupy the volume of the container in which they are located.
  • They have no shape of their own. They also assume that of their container.
  • They can expand and contract. Like solids and liquids, gases expand when their temperature is increased and contract when cooled.
  • They have great fluidity. Gases flow much more than liquids because their particles have fewer interactions. They can easily move through a hole from one container to another.
  • They have high diffusion. Gases can easily mix due to the great movement of their particles.
  • Solubility. Gases can be soluble in water or other liquids.
  • Can be compressed. By applying pressure to a gas, its particles can be brought closer together; that is, the gas is compressed.

Changes in states of gases

dry ice sublimation gases
Dry ice is an example of sublimation.
  • Sublimation. It is a physical process of phase change, which allows converting a solid into a gas directly, without first going through a liquid stage. This process is rare and usually involves specific conditions of pressure and temperature. We can observe it in dry ice (or ice) at room temperature: the solid block releases a slight vapor that is the substance recovering its original gaseous state.
  • Boiling. It is the process by which a liquid turns into a gas. It occurs when the entire mass of the liquid is heated to a temperature equal to its boiling point.
  • Evaporation. It is a very common phase change process that leads a liquid to become a gas when the temperature of the liquid is increased. It happens slowly and gradually. We put it into practice, for example, in the shower when very hot water turns into observable steam as a whitish cloud.
  • Condensation. It is the opposite process to evaporation; that is, a phase change process that leads from the gaseous to the liquid state due to the loss of heat energy. This lost energy causes the gas particles to vibrate more slowly, allowing them to approach and interact more closely, such as on cold glass on a rainy day, or plants and other surfaces with dew.
condensation - gases
Gases can become liquid due to the loss of heat energy.
  • Reverse sublimation. It is the opposite path of sublimation, that is, the passage from the gaseous state to the solid state without first passing through a moment of liquidity. This process requires very specific pressure and temperature conditions.


Lava lamps use cold plasma.

The plasmatic state of matter is considered the fourth state of aggregation, but it has enormous similarities with the gaseous state since it is an ionized gas—that is, a gas whose particles have lost electrons and have acquired a certain electromagnetic charge. There are cold plasmas, like the ones used in lava lamps, or hot plasmas, like the fire that surrounds the Sun.

Examples of Gas

methane gas
Methane is one of the components of the gas used in homes.

Some examples of gases are:

  • Hydrogen (H2). It is the most common diatomic gas in the entire universe.
  • Helium (He). It is tasteless, colorless, and inert. It is the least soluble in water of all gases.
  • Methane (CH4). It is a gaseous hydrocarbon with an unpleasant odor that is obtained as a product of the decomposition of organic matter.
  • Air. It is the heterogeneous mixture of hydrogen, nitrogen, oxygen, argon, and other gases that living beings breathe.

Continue with: Fluids

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