All matter exists in one of three phases or states. These phases are solid, liquid, or gas. A fourth state also exists, which is plasma, but in this article we will focus only on the first three, since they are far more common.

Most of the matter in the Universe exists in a gas state. For example, hydrogen, which is the most abundant element in the Universe exists only as a gas. However, in some places, such as on planets, liquids and solids can be found in abundance. Solids make up most of the Earth’s surface and over 70% of all known chemical compounds on the Earth which all exist in solid state. Gases are also abundant, making up about 22% of the Earth’s matter. The rest of the Earth’s mass is made up of liquids.

Solids form when particles are pushed so close together that they cannot move past one another. Particles in solids are very tightly packed, but can vibrate. Heat energy separates particles when it is applied to a solid, causing the particles to become less dense and slow down vibration. As heat energy dissipates, particles will be drawn closer together again because of their mutual attraction.

Liquid states occur when particles aren’t as tightly packed together as in solids, although they still have enough contact with each other that all the molecules remain in contact at some point. There’s room between individual atoms for them to slide past one another easily; liquids are able to take on any shape because there’s no fixed structure holding them together.

Gas states occur when particles are as far apart as possible and move very fast, colliding constantly with one another. Since there is such space between particles, gas states will take on the shape of their container. Gas states also have no fixed volume; they can be compressed or expanded to fill whatever space they occupy.

Thus, at low temperatures substances tend to be mostly solid. However, as the temperature rises, they gradually turn into liquids and then into gasses. A familiar example of this is water. Which forms a solid (ice) at low temperatures, then becomes a liquid (water) at moderate temperatures, and finally becomes a gas (steam) at high temperatures.

All matter is found in one of these states or phases.