Expansion and Compression of a Gas

This classic state of matter can be difficult to see but it has some amazing properties.

Along with liquids and solids, gases are one of the three major states of matter. Typically they result when a substance is heated in its liquid state to its boiling point, or when evaporation occurs from the surface of a liquid. There are numerous types and classifications of gases, including elements that naturally exist in a gaseous form, compound gases comprising more than one element, and mixtures of individual pure gases.

Gas particles are much more loosely connected than those found in liquid or solid states, which results in lower density-and this is ultimately what sets a gas apart from the other two phases. Without changes in pressure or temperature, gas particles move around freely and randomly.

They have no set shape and only change direction and momentum when bouncing off one another or off the inside of a container. Negatively charged areas of particles are attracted to positively charged areas – how these interact varies depending on the gas and are part of what makes each one unique. Because most gases are colorless, they are measured by four different properties: volume, temperature, pressure and number of particles; the latter property is more commonly known as moles. When put into a container (and notĀ pressurizedĀ  gas molecules will evenly distribute themselves.


Expansion and Compression of a GasGases typically expand, or increase in volume, when they are heated. Specifically, gas molecules move apart when heated, making the gas as a whole take up more space. All gases expand at the same rate if they are under the same amount of pressure. Gases can also expand without heat if the pressure lowers. Hot-air balloons float by heating propane in its liquid state until it becomes a gas. The propane gas is ignited and expands, causing the air around it to heat as well. Hot air is less dense than cooler air, so the balloon rises.


Compressing a gas means putting it under greater pressure, and this also results in an increase in temperature. This is because the molecules are closer together and they generate more energy as they collide with one another. Many gases are kept under specific pressures – usually higher pressure than that of the atmosphere – for use in various industries. Basic bicycle pumps function in the same way as pistons. Drawing up the handle pulls air into the valve; then the downstroke compresses the air and forces it into the bicycle tyre.

gas under pressureUnder pressure

When a gas is under pressure in a container, it’s exerting force against the inside of that container. Gas particles travel around in straight lines until they collide with one another and the walls of the object holding them; this changes their momentum and creates pressure. If you shake a can of soda before opening it, you’ll have quite a mess; this is a result of the carbon dioxide gas sitting on top of the soda, along with the gas that’s dissolved in the liquid itself. Shaking it forces the extra gas into the liquid, creating large bubbles that rapidly push their way out as you open the can.


Vapour is a gaseous phase in which a substance is at a slightly lower temperature than its critical point – the conditions, like temperature and pressure, under which it changes to a different state of matter. When it reaches its critical point, it can be condensed. A vapour is a sort of halfway state. Substances can also exist in both states at the same time. One example occurs when boiling water. Once water reaches its boiling point, it begins to turn to a gas; in the case of boiling water, we better know the vapour as steam.