In principle, all substances composed of atoms and molecules can exist in three states of matter: solid (s), liquid (l), and gaseous (g). In thermodynamics, the term phase is often used synonymously.
The solid state has already been introduced in the context of idealized rigid bodies and real elastic materials. In this state, the particles are tightly bound and arranged in specific structures. They can only perform periodic vibrations around fixed positions relative to one another.
There are essentially two types of structures in solids: a crystalline form, like in table salt or granulated sugar, where particles form regular patterns; and an amorphous form, where the particles are randomly arranged and lack long-range order—glass is a good example. Intermediate forms exist as well, called polycrystalline materials. These are composed of crystalline regions that are themselves arranged in a non-uniform or random fashion.
Liquids represent the second state of matter. In this state, the particles are still connected by intermolecular forces but can move freely relative to each other. Liquids can easily change shape and adapt to the container they are in, offering little to no resistance to volume changes. For instance, water can easily be poured from one glass to another. More viscous liquids like honey flow more slowly, but are still classified as liquids by definition.
The gaseous state differs fundamentally from the two previous states—collectively referred to as condensed matter—in that the interactions between particles are negligible. Gas particles move freely and can occupy any volume.
A well-known substance that naturally occurs on Earth in all three states is water:
- As solid (ice or snow) in cold regions like the Arctic.
- As liquid in oceans and rivers.
- As gas (water vapor) in the air, often produced by solar heating and involved in Earth's water cycle.
In all three states of matter, the motion of particles is a measure of temperature: the faster the particles move, the higher the temperature.