Anything that has mass is made up of matter – an all-encompassing word for atoms and molecules that make up our physical world. We describe this matter as existing in states (sometimes referred to as phases). Most people are familiar with three states of matter – solids, liquids and gases – but there are two more that are less commonly known but just as important – plasmas and Bose-Einstein condensates.

It is important to understand the particle nature of matter. The particles that make up matter are not ‘small bits of solid’ or ‘small drops of liquid’ but atoms and molecules. The physical characteristics of those atoms and molecules decide its state.

Solid

Something is usually described as a solid if it can hold its own shape and is hard to compress (squash). The molecules in a solid are closely packed together – they have a high density.

Right now, you are probably sitting on a chair, using a mouse or a keyboard that is resting on a desk – all those things are solids.

Liquid

In liquids, the molecules have the ability to move around and slide past each other. A liquid will take on the shape of the container it is being held in. While a liquid is easier to compress than a solid, it is still quite difficult – imagine trying to compress water in a confined container!

Water is an example of a liquid, and so is milk, juice and the petrol you put in the car.

Gas

In gases, the atoms are much more spread out than in solids or liquids, and the atoms collide randomly with one another. A gas will fill any container, but if the container is not sealed, the gas will escape. Gas can be compressed much more easily than a liquid or solid.

Right now, you are breathing in air – a mixture of gases containing many elements such as oxygen, nitrogen and carbon.

Plasma

Plasma is very similar to gas, In fact, the easiest way to describe plasma is as a gas that can carry an electrical charge. Plasma is a form of matter that exists when atoms are in an excited state. They are so excited that they jump an energy level and, in doing so, give off light. Plasma particles are spread out and move around randomly, but unlike gas, they contain some free ions and electrons, which gives plasma its ability to conduct electricity.

On Earth, plasmas are commonly found in some kinds of fluorescent lights and neon signs. Another form of plasma on Earth happens during storms as lightning.

Auroras are another form of plasma, where atoms in the upper atmosphere are affected by particles coming in from outer space. The most common form of plasma is in the stars – our Sun exists in the plasma state. Overall, plasmas are the most common state of matter – they make up 99% of the visible universe.

Find out more about gases and plasmas in this article.

Bose-Einstein condensate (BEC)

To understand a Bose-Einstein condensate (BEC), you must first know a bit about temperature.

There is a temperature at which molecular motion (therefore everything) stops, this is called absolute zero (0K or around -273°C). Just a fraction above this temperature – and only for some elements – a BEC occurs. The atoms start behaving like little waves and start overlapping one another until they eventually act like one wave and essentially become a superatom. They are not bonded or mixed – they have become indistinguishable from one another, having the same qualities and existing in the same place.

Daniel Kleppner from the Massachusetts Institute of Technology has a great description. He says the “particles have lost their identity – they all think they are everywhere”. One atom can’t tell itself from another.

Changing states

It is important to understand that matter exists in all states and that matter can also change states. It does this by either using or releasing energy, and it is usually associated with changes in temperature and pressure.

A simple example is water. If you have a block of ice, you have solid water. Add heat (a form of energy) and the ice melts into liquid water that you could drink (it has reached its melting point). Continue to apply heat, and the water will evaporate and turn into steam, which is water in a gaseous state (it has reached boiling point). This works backwards, too. Gas can cool down (by losing energy) and condense back into liquid water and cool down further into a solid. There is even a process called sublimation where a solid can turn straight into a gas when heat is applied.

Gas can also change state to a plasma or BEC:

  • In plasma TVs, little pockets of gas are excited with electricity disrupting the normal balance of atoms so there are lots of free ions and electrons, turning them into plasma, which creates a light.
  • If you super cool gas, you get a BEC superatom wave. Warm it up, and the wave will return to a gas.

Useful links

Watch this YouTube clip of Daniel Kleppner from MIT explaining BECs.

    Published 12 April 2010