In school, you were probably taught that there were three states of matter: solid, liquid and gas. Then, as you learned more science, you might have come across a fourth: plasma.
However, there are also many less familiar states of matter, with names like 'Bose-Einstein condensate' and 'time crystals'. So how many states of matter are there, really?
The answer is that there are four fundamental states of matter – solid, liquid, gas and plasma. These are the ones that occur naturally in the Universe. On top of these, there are exotic states of matter. These are states of matter that you certainly won't encounter in your day-to-day life, but are allowed by the laws of physics.
What is a state of matter?
The same material can exist in many different forms, depending on factors like the temperature and pressure. Any one of these forms is called a 'state of matter'.
The state of matter dictates how the molecules that make it up are arranged, how much they move about, and the strength of the forces between them – called intermolecular forces.
What are the states of matter?
In a solid, the molecules are densely packed together, and exert strong intermolecular forces on each other. The molecules do not have room to move freely, and vibrate in place. A solid will keep its shape unless a force acts on it.
In most materials, a solid is the most dense natural state of matter. One exception to this rule is ice, which floats on water.
A typical solid is crystalline, meaning the molecules are arranged in a repeating pattern. However, there is another type, called amorphous solids, which don't have this long-range pattern. Glass is an example of an amorphous solid.
When you heat up a solid to its melting point, its molecules gain too much energy to stay in the densely packed structure. The material will melt, leaving you with a liquid.
In a liquid, the intermolecular forces are still strong, but now, the molecules can move around each other. As a result, the liquid can flow and change to the shape of its container. The molecules of a liquid are slightly less densely packed than a solid.
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Heat up a liquid past its boiling point and you have a gas. The molecules have completely escaped from each other, and the intermolecular forces between them are very small.
A gas expands to fill the space it's in, which also means that a gas can be easily compressed. The molecules have a lot of energy, and they zip around in random directions until they collide with something.
Even when a liquid is well below its boiling point, it can also turn into a gas by evaporation. As the molecules move around randomly within the liquid, every now and again, one near the surface will have enough energy to escape. Over time, all the molecules make the leap and the entire liquid will evaporate.
A plasma is formed when a hot gas is electrically charged, though it behaves very differently from a gaseous material.
The molecules of a gas are made up of individual atoms, and an atom has a positively charged nucleus surrounded by negatively charged electrons. When the temperature of the gas is high enough – or when it's subjected to a strong electromagnetic field – these electrons are ripped away from the nuclei. The resulting mixture of positively and negatively charged particles is called a plasma.
Since it contains charged particles, a plasma can conduct electricity.
Exotic states of matter
There are many other states of matter, some of which are listed below.
Superconductivity is when matter is in a state with no electrical resistance – that is, its electrical conductivity is greatly increased. A superconducting material has a critical temperature below which this change happens; this point is usually close to absolute zero.
Bosons are a type of particle that include photons, gluons and the Higgs boson. When bosons are cooled to incredibly low temperatures at low density, they start to show quantum mechanical effects at large scales.
An ordinary crystalline solid has its molecules arranged in repeating patterns in space. The molecules of a time crystal, however, follow a repeating pattern in time. The particles are in constant motion, following the same repetitive movements without losing any energy.