Gas

A gas is one of the main forms, or states, of matter, like solids, liquids, and plasmas. What makes gases special is that they do not have a fixed shape or a set volume. Instead, gas particles, tiny atoms or molecules, are spread out, move freely, and have a lot of energy. Because the particles are so far apart and do not stick to each other much, gases can spread out to fill any space or container they are in. Gases can also be squeezed into smaller spaces, which is called compression. Gases are really important in science. They help us understand how heat, pressure, and motion work. Gases are all around us, from the air we breathe to the fuel in engines, and they play big roles in nature and technology.

Gases have special physical properties that make them different from solids and liquids. For example, gases can be easily compressed (squeezed into a smaller space), they have low density (they are much lighter than the same volume of solid or liquid), and they can spread out and mix with other gases easily (this is called diffusion). Gases also push against the walls of whatever container they are in. This push is called pressure and comes from gas particles bouncing around and hitting the container’s walls. Scientists use a formula called the ideal gas law to describe how gases behave. It is written as PV = nRT, where:
 P is pressure,
 V is volume,
 n is the amount of gas (in moles),
 R is a constant,
 T is temperature.

This formula helps predict how a gas will act if you change its temperature, pressure, or volume. But it is not perfect. Real gases do not always follow this rule, especially when they are under very high pressure or low temperature. In those cases, the gas particles get closer together and start to interact, which changes how they behave. Scientists use more detailed equations, like the van der Waals equation, to describe real gas behavior more accurately.

Gases can change into other forms of matter, like liquids or solids, when the temperature or pressure changes. For example, when a gas cools down or gets squeezed tightly (compressed), it can turn into a liquid in a process called condensation. If the conditions are right, a gas can even turn directly into a solid without becoming a liquid first. This is called deposition. The opposite can also happen. If you heat a liquid, it can become a gas through vaporization (like boiling water turning into steam). And sometimes, if you heat a solid, it can turn straight into a gas without melting first. This process is called sublimation (like dry ice turning into carbon dioxide gas). These changes, called phase transitions, are very important. They happen all around us, in cloud formation, breathing, and even during volcanic eruptions. They are also used in many industries, such as refrigeration, distilling liquids, and making chemicals.

Gases can be made of just one kind of element or of different elements combined. Some gases are elemental, meaning they are made of only one type of atom. Examples include hydrogen (H₂), oxygen (O₂), and the noble gases like helium and neon. Other gases are compounds, which means they are made of two or more different atoms joined together. Examples of these include carbon dioxide (CO₂), methane (CH₄), and ammonia (NH₃). Gases can also be mixtures, where many gases are blended together. A good example is air, which we breathe every day. Air is mostly made of nitrogen and oxygen, but it also contains small amounts of other gases like argon and carbon dioxide. These gas mixtures are super important. The mix of gases in Earth’s atmosphere helps regulate the planet’s temperature and makes breathing possible. In factories and hospitals, special gas mixtures are used for welding, surgery, and even to keep food fresh.

Some gases belong to special groups because of how they behave or affect the world around us. Noble gases are a group of gases like helium, neon, and argon that do nott easily react with other substances. This is because their atoms have a “full” outer shell of electrons, which makes them chemically stable or inert. That means they usually do not join with other atoms to form compounds. Greenhouse gases, like carbon dioxide (CO₂) and methane (CH₄), are gases that trap heat in Earth’s atmosphere. They let sunlight in but stop some of the heat from escaping, like a blanket. This helps keep the planet warm, but too much of these gases leads to global warming and climate change. There are also toxic gases, which can make people sick or be dangerous to breathe, and flammable gases, which can catch fire easily. These types of gases must be handled carefully to avoid accidents. To study and work with gases, scientists use special tools like gas chromatography and mass spectrometry. These tools help them identify what gases are present, measure how much there is, and even separate or analyze them in great detail.

Introduction
In a pure gas, each molecule may be made of an individual atom. It may be elemental, where each molecule is made of more than one of the same atom bound together. It may be compounds where molecules are made of many types of atoms together. An example of a monoatomic gas is neon, an example of an elemental gas is hydrogen and an example of a compound gas is carbon dioxide.
Examples
Composition
A gas mixture contains a mix of any of the above types. air comprises 78% nitrogen, 21% oxygen, less than 1% argon, around 0.03% carbon dioxide and more other in very small quantities.
Applications
War
Poison gases were used as chemical weapons in WWI.

Characteristics
All gases can flow, like liquids. This means the molecules move about independently of each other. Most gases are colourless, like hydrogen. Gas particles will spread about, or diffuse, in order to fill all the space in any container such as a bottle or a room. Compared to liquids and solids, gases have a very low density and viscosity. We cannot directly see most gases since they aren't coloured. However it is possible to measure their density, volume, temperature and pressure.
Pressure

Pressure is the measure of how much pushing force something is putting on another object. In a gas, this is usually the gas pushing on the container of the object or, if the gas is heavy, something inside the gas. Pressure is measured in pascals. Because of Newton's third law, we can change the pressure of a gas by putting force on the object containing it. For example, squeezing a bottle with air inside pressurises the air inside.

When talking about gas, pressure is often related to the container. A lot of gas in a small container would have very high pressure. A small amount of a gas in a big container would have low pressure.
Gas can create pressure itself when there is a lot of it. The weight of the gas creates pressure on anything underneath it, including other gas. On a planet, this is called atmospheric pressure.
Temperature

The temperature of a gas is how hot or cold it is. In physics it is usually measured in Kelvin although degrees Celsius are used more elsewhere. In a gas, the average velocity, i.e. how fast they move, of the molecules is related to the temperature. The faster the gas molecules are moving, the more they collide, or smash into each other. These collisions release energy, which in a gas comes in the form of heat. Conversely if the temperature around the gas becomes hotter then the gas particles will convert the thermal energy to kinetic energy, making them move faster and making the gas hotter.

State changes
A gas can go through two different state changes. If the temperature is low enough the gas can condense and turn into a liquid. Sometimes, if the temperature is low enough it can go through deposition, where it changes straight to a solid. Normally a gas must first condense to a liquid, and then freeze to become a solid, but if the temperature is very low it can skip the liquid stage and instantly become solid. Frost on the ground in winter is caused by this. Water vapour – a gas – goes into the air which is very cold, and instantly becomes ice due to deposition.

 Related pages 
 Ideal gas

 References