There are 6 states of matter.

1. Solid matter

is relatively cold, since the atoms and molecules, though vibrating, stay in the same positions with respect to each other, which is why solid matter holds the shape it has.

2. Liquid

occurs at higher temperatures, when the atoms and molecules can slide around each other. This is why liquids take the shape of their container (or a puddle).

3. Gas

is matter in a relatively high temperature range, where the molecules bounce off each other, at relatively high speeds, which makes gas take up a lot more space.

4. Plasma

refers to ionized, or charged, particles, which behave somewhat like a gas, since the particles bounce off each other at relatively high speeds. The electrons in a lightning bolt or in a 'plasma ball' are examples. Since groups of electrons are neither molecules or atoms, they can't be solid or liquid or a gas.

5. Fluid

is a state somewhere between gas and liquid, and is considered a distinct state.

In a closed container of water, for example, if some of the container is 'empty', you will see a surface which indicates where the liquid ends and the gas begins.

If you heat it, more of the container will become gas and less liquid, as the water boils, becoming vapor. The gaseous part will increase in pressure, but if there is enough liquid, the container will not become completely gaseous.

Eventually, if the temperature is raised enough , the pressure will increase to about 25 times our atmospheric pressure (25 ATM) and the line between the states will start to blur, literally. Shortly, the 'mixture' of gas and liquid will spread until the entire container is somewhat cloudy-looking. This is a rather unstable (short-lived) phenomenon.

When it clears, there is no line between the states. Is it a liquid or a gas? The answer is neither; it's fluid.

6. Condensedmatter comes in four flavors

Typical atoms are mostly empty space. The electrons and nuclei only take up a tiny percentage of the entire volume that one atom fills, because their interaction is based on the charges of the electrons.

Close atoms can't get too close because their electrons repel each other, keeping the nuclei 1000s of times further away from each other than the sizes of the nuclei. There are two ways to negate the forces which preserve these separations; remove the energy, or overpower them.

6.1 Degenerate electron gas

is what makes up a white dwarf, the condensed core of a star which has run out of fuel.

A star is just the balancing act which occurs as collapsing cloud of gas heats up, until the pressure and temperature become so high that nuclear fusion occurs. When it does, it releases huge amounts of energy, which balance the weight of the collapsing gas. So much radiant energy is created that the pressure increases enough to stop the collapse. Our sun is one example of such a balancing act, with about 1/3 of it's internal pressure being produced by radiation alone.

Eventually, the fuel (hydrogen, and later, helium) will run out, allowing the collapse to continue. A white dwarf is the core of a star which has collapsed to the point that electrons and nuclei have been 'crushed' together. The nuclei no longer have any electrons associated with them, but instead flow freely in an electron 'gas,' with no space between the particles.

Such material, because it does not have the empty space which comprise most of the atoms we are familiar with, is very dense. A thimble-full weighs as much as a mountain.

Don't worry, it won't happen to our sun for several billion years. Long before then, humans will have either become extinct, gone elsewhere, or altered the sun to extend it's life-span.

6.2 Bose-Einstein Condensate

describes gas at extremely low temperatures. Liquid helium, the typical material used in experiments of this nature, has the highest temperature at which this occurs: -271.29 Celsius. In this state, all the electrons collapse into the 'ground state' of their associated atoms.

Electrons 'orbit' nuclei in shells. Because each electron repels the others, having the same electrical charge, but not by the Pauli Exclusion Principle, they sort themselves out into different orbitals around an atom, each with an associated energy level.

At sufficiently low temperatures, the energy which supports this structure is gone, and the electrons collapse into the lowest orbital. It is not nearly as dense as the degenerate electron gas, because the lowest shell still has it's integrity, but it is denser, and has several strange behaviors.

• The viscosity is extremely low, since the atoms are so much smaller than normal.
• Most containers can't hold it at all, instead allowing it to flow through the walls.
• In many ways, it acts as a vacuum, as if it weren't there at all.
• He II, the name given to helium in this state, also has the strange property that at the right temperature and pressure, it must be heated up to turn it from a liquid into a solid

6.3 Neutron Stars

A neutron star occurs when a sufficiently large star collapses at the end of it's fuel-supply, causing a supernova, the most violent interstellar event we know of. Larger supernovae can release more energy than the star emitted during it's entire normal life-span.

After a supernova, the core remains, but exists in a condensed state significantly different than a white dwarf. In a white dwarf, the material is a sea of electrons and nuclei crushed together. In a neutron star, the pressure due to gravity is so high that the electrons and protons are essentially 'combined' via a process known as 'reverse beta-decay,' into neutrons.

In one sense, a neutron star is one giant atomic nucleus. It is so dense that a teaspoon weighs as much as the Earth.

6.4 Black Holes

occur at the end of the life of the largest stars. Basically, the neutron star that remains after the supernova is so small and dense that nothing, including light, can escape from it, hence their name.

No one really knows what a black hole is like. It may be a neutron star, hidden by the fact that light can't escape it, or there may be some other state that occurs when neutrons are crushed enough. General Relativity predicts even more esoteric possibilities.

What we do know is that black holes have a temperature, they can rotate, they very slowly evaporate, via Hawking Radiation, and they can hold an electrical charge.

Interestingly, the four traditional states of matter map very neatly (well, nearly so) to the four ancient and medieval elements.

The last is a bit of a stretch, as "fire" usually refers to the process of oxidation, or a gas whose molecules are all in an excited energy state. However, the ancients also considered the Sun to be a ball of fire, and I bet if you showed a medieval alchemist an example of a plasma, he would probably have considered it "fire". Oh, and ionization is about as excited an energy state as you can get.

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