Gases are one of the four fundamental states of matter alongside solids, liquids and plasma. They are made up of atoms and molecules like other matter but don’t have a fixed shape or volume. Gases take the shape of whatever container they are confined to and even expand to fill that container.
Though gases are often thought to be invisible, many actually have distinctive colours. For example, nitrogen dioxide is a browny-orange sort of colour, while chlorine is a greenish yellow. Even common gases like oxygen and water vapour absorb light, just not at ultraviolet and infrared wavelengths that we can see.
“There’s a sweet spot between the absorption spectra of oxygen and water where not much light gets absorbed. Lo and behold, that’s exactly the range of light that we’ve evolved to see!” chemist and science communicator Professor Mark Lorch, from the University of Hull, previously told Science Focus.
“So it’s not that gases are invisible, as such, it’s just that we can’t see atmospheric gases as they don’t have a colour in the visible range.”
What is the structure of gas?
The particles in a gas are able to move freely and separate from each other without a fixed arrangement. This makes the structure of a gas very different to that of a solid or liquid.
In a solid, particles are densely packed together in a fixed arrangement. In a liquid, molecules can slide past each other without a fixed arrangement but are still packed together. (Plasma is a superheated matter, such as lighting, formed when the normally neutral particles in a gas become electrically charged.)
Properties of gases
Some of the main properties of gases are as follows:
- Volume: Gases don’t have a set shape or volume. They assume the shape of whatever containers to which they are confined and expand to fill that container.
- Density: The densities of gases are low as the particles inside separate out. Solids and liquids both have much higher densities.
- Compressibility: Because the particles of a gas are separated from each other, and there’s plenty of space between the particles, Gases can be easily compressed. This distinguishes them from liquids and solids, in which the particles are too densely compact to be easily compressed.
- Particle movement: Particles within gas move rapidly in all directions.
- Pressure: Gas particles exert pressure by colliding with each other and the walls of a container.
What are some key laws of gases?
Boyle's law
Boyle's law states that the pressure of an ideal gas will be inversely proportional to its volume at a constant temperature. In other words, increasing volume will decrease pressure, and decreasing volume will increase pressure.
The law is named after Anglo-Irish philosopher and scientist Robert Boyle, who discovered it in 1662. It is also known as Mariotte's law because French physicist Edme Mariotte came up with the law independently in 1676.
Charles's law
Charles's law states that the volume of an ideal gas is directly proportional to the temperature at a constant pressure. This means that as the temperature of gas rises and drops, so does volume, and explains why gas expands when heated.
The law is named after French inventor and scientist Jacques Charles and his experiments in around 1787. However, fellow French scientist Joseph Gay-Lussac more formally established the law with experiments in 1802.
Avogadro's law
Avogadro's law states that equal volumes of different gases - at the same temperature and pressure - will have an equal number of molecules. Therefore, the amount of gas is directly proportional to the volume of gas. Italian scientist Amedeo Avogadro first proposed this law in 1811.
Ideal gas law
The ideal gas law combines the other gas laws to create an equation (PV = nRT) for a hypothetical ideal gas, which can be used to determine how gas behaves under different conditions.
What are some examples of gas?
- Nitrogen (N2) - Most of the air we breathe (78 per cent) is nitrogen gas.
- Oxygen (O2) - This is the vital 21 per cent of the air that keeps us alive.
- Carbon dioxide (CO2) - The stuff we exhale. CO2 is a natural part of our world, but it’s also one of the greenhouse gases emitted by human activity that’s causing climate change.
- Nitrous oxide (N2O) - The “laughing gas” commonly used as an anaesthetic.
- Krypton (Kr) - One of the rarest gases in our atmosphere. Also used in fluorescent lights.
What are the elemental gases?
The elements that are gases at room temperature (22°C) are as follows:
- Hydrogen (H2)
- Nitrogen (N2)
- Oxygen (O2)
- Fluorine (F2)
- Chlorine (Cl2)
- Helium (He)
- Neon (Ne)
- Argon (Ar)
- Krypton (Kr)
- Xenon (Xe)
- Radon (Rn)
What are the noble gases?
The noble gases, or inert gases, are colourless and odourless gases made up of elements that generally don’t react with other elements or compounds. There are seven in total.
- Helium (He)
- Neon (Ne)
- Argon (Ar)
- Krypton (Kr)
- Xenon (Xe)
- Radon (Rn)
- Oganesson (Og)
This list includes the synthetic element Oganesson, which is still being researched but not predicted to be a gas under normal conditions.
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