A gas is one of the four states of matter. In a gas, the molecules move freely and are not attached to each other. This makes it different from a liquid where the molecules are loosely attached to or touching each other. It is also different from a solid where the molecular bonds are strong and hold the molecules together in one shape.
A gas does not have only one volume like a liquid or solid does. Instead, gas can expand until it fills whatever container it is in.
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.
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 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 (gives more pressure) to 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.
The temperature of a gas is how hot or cold it is. In physics it is usually measured in kelvins although degrees Celsius are used more elsewhere. In a gas, the average velocity (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.
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.