The air we breathe and move through daily, the helium in our birthday balloons and the methane used for home heating are all common examples of gases. Gas is one of the three main states of matter, along with solids and liquids.

The states of matter differ according to how closely packed the particles are – a consequence of how much kinetic energy they have – which results in distinct characteristics.

In its solid state, matter is the most closely packed. The molecules in a solid are held together by atomic bonds and attractions. As a result, they vibrate in place rather than flow freely around. Solids have definite shapes and volumes, and they aren't easily compressed; that is, they keep their shape fairly well.

In its liquid state, matter is less closely packed than in a solid, thanks to weaker inter-molecular bonds. When in the presence of a gravitational field, a liquid will take the shape of its container; in the absence of gravity, it forms into spherical shapes.

In its gaseous state, matter experiences weak interactions with itself. Particles can move around quite freely. As a result, gases take on the shape and volume of whatever container they're in. Open the oven after baking a cake, and the gas that was inside will spread throughout the house so that the cake can be smelled from every room.

The newest state of matter known to physicists is plasma, a condition in which the atoms making up matter themselves are breaking down. Plasma occurs only at extreme temperatures and pressures, such as those found in the center of the sun. Because electrons are stripped from atoms in these conditions, a plasma ends up being a mixture of free electrons, the leftover positively charged ions and neutral atoms. In terms of behavior, a plasma acts like a gas, but because of the charges involved, it also has electromagnetic properties.

Matter can change from one state to another depending on conditions of pressure and temperature. Such a transformation is known as a phase change. For example, solid water in the form of ice when heated up to its boiling point will melt into liquid water, which in turn will evaporate into water vapor with even more added heat.

The opposite of evaporation is condensation. When a gas condenses, it becomes a liquid.

A solid can transition directly into a gaseous state of matter by undergoing sublimation. Sublimation happens when a solid is at a particular pressure below its triple point in a phase diagram. For example, dry ice (solid carbon dioxide) sublimates when heated at one atmosphere, unlike "regular" ice (water) which simply melts into liquid when heated at one atmosphere.

The formal physics description of a gas is a substance that does not have a definite volume (also called a fixed volume) or a definite shape. Instead, a gas will take the shape of its container because gas molecules can move freely past one another.

A famous hypothetical problem created by the pre-eminent particle physicist Enrico Fermi helps illustrate this. Fermi asked his students to approximate how many molecules of Caesar's dying breath a human today can expect to encounter with each of their own inhales. Assuming the Roman emperor's last breath has distributed evenly around the globe by now (and has not been reabsorbed by the ocean or plants), calculations show that today's living beings breathe in about one molecule of his dying breath with each of theirs.

Although a liquid can also take the shape of its container, a liquid doesn't change its volume without help. But a gas will always spread out to fill its container and, conversely, it can be compressed into a smaller container.

An important measurement to describe a gas is pressure. The pressure of a gas is the force per unit area that the gas exerts on its container. More pressure leads to more force, and vice versa.

For example, a bike tire pumped up to a high pressure feels taught and hard from the outside. A low-pressure tire, on the other hand, exerts less outwards force, and as a result, it feels floppier and softer.

Another key characteristic of a gas is its temperature. The temperature of a gas is defined as a measure of the average kinetic energy per molecule in the gas. Because all molecules vibrate, they all have some amount of kinetic energy.

Both pressure and temperature are needed to determine if the state of matter is gaseous. Some materials are gases only at high temperatures, while others are gases at low temperatures or room temperature. Meanwhile, some materials are only gases at high temperatures and low pressures. A phase diagram shows the state of matter for a given substance at various combinations of temperature and pressure.

Gases abound in the world around us. Carbon dioxide, a common greenhouse gas, is given off when burning fuel to power many of humanity's current activities. When liquid water vaporizes, it becomes steam or water vapor – a process that occurs on stove tops and in puddles outside under the sun.

The mixture of gases known as air – which is typically 78 percent nitrogen, 21 percent oxygen and 1 percent other gases – surrounds all terrestrial creatures and exchanges with their bodies through the respiratory system. When breathing, many animals extract oxygen from the air and eliminate carbon dioxide from their bodies, while many plants do the opposite, taking in carbon dioxide and giving off oxygen.

To help better explain the behavior of gases, physicists like to approximate how the gases would behave if they were made of many point particles moving in straight lines and not experiencing intermolecular forces – in other words, without interacting with one another.

Of course, no gas is actually ideal, but by considering how a gas would act under such a description, physicists are able to combine multiple simple laws about gaseous properties into one: the ideal gas law.

Specifically, the ideal gas law is derived from four simpler gas laws that show pieces of the relationships in the combined gas law. They are:

• Boyle's law: A gas' pressure is inversely proportional to its volume at a constant temperature and amount of gas.
• Charles' law: A gas' volume and temperature are proportional when pressure is held constant.
• Avogadro's law: The volume of a gas is proportional to the amount of gas when pressure and temperature are constant.
• Amonton's law: The pressure and temperature of a gas are proportional so long as the amount and volume of the gas are held constant.