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States of Matter

Physical states

The three common states of matter are solid, liquid and gas. However, there is also a fourth state, plasma, which is actually the most common state of matter in the universe, and a fifth state, a Bose-Einstein condensate (BEC), which is "colder" than a solid.

Plasma lamp.

Plasma is a completely ionised gas. All electrons have been stripped off the nuclei. Instellar examples include nebulae and the centre of stars, while examples on earth include lightning, Jacob's ladders, and plasma lamps.

Gases expand to occupy the space available. They have their molecules moving freely and independently of each other, in random directions and at high speeds. Gas molecules are not normally in contact with each other, although sometimes collisions occur.

Liquids (normally) flow to take up the shape of the container holding them. Molecules in a liquid are free to move in relation to each other and are in contact with one another. Liquids have a certain volume at a given temperature.

Solids are resistant to deformation (ie, they don't want to be changed in shape) and volume (they don't want to be changed in size).

The lightest known solid is man-made and is called aerogel. It is nicknamed frozen smoke, solid smoke or blue smoke due to its semi-transparent nature and the way light scatters in the material; however, it feels like expanded polystyrene (Styrofoam) to the touch. The lightest aerogel produced has a density of 1.9 mg/cm3 or 1.9 kg/m3 (1/530 as dense as water). Air is just 1.2 kg/m3.

Bose-Einstein condensates are achieved at temperatures very very close to absolute zero, when a large fraction of the atoms collapse into the lowest quantum state possible – it is not possible for them to lose any more energy. Liquid helium is an example, and has strange properties such as being superfluid (no surface tension at all) which means it will spontaneously overflow any uncapped container, and two streams of liquid helium can flow through each other without interferring with the other stream.

Changes of state

Each process of changing from one state to another has a name.




boiling or evaporation


freezing or solidification







Solid carbon dioxide is called dry ice because it sublimates. It doesn't have a liquid form at all at atmospheric pressure, but at 5.1 atm (atmospheres) it does form a liquid, and stays a liquid for a wider temperature range the higher the pressure.

Water can also sublimate in sub-zero conditions, and this is why ice cubes get smaller in frost-free freezers and why washing can dry on a clothesline in a freezing environment – even if it gets icicles on it.


The triple point is where all three states of matter (solid, liquid, gas) can exist at the same time. Below the triple point (by pressure, temperature, or both) a substance will change directly from a solid to a gas and back. Water's triple point is at 0.01 °C and at a near vacuum.

A typical phase diagram.

The green lines show how the freezing point can vary with pressure.
The green dotted line gives the anomalous behaviour of water.
The blue line shows how the boiling point can vary with pressure.
The red line shows the boundary where sublimation or deposition can occur.

Wikipedia points out even at a temperature below the boiling point, a liquid will evaporate [on its surface] until the concentration of the vapors belonging to the liquid reach an equilibrium partial pressure in the gas. Therefore no liquid can exist permanently in a vacuum.

A supercritical fluid isn't a fluid which goes about complaining, but rather a fluid that is so hot and so pressured that the liquid and gas states are the same. Wikipedia again:

In 1822, Baron Charles Cagniard de la Tour discovered the critical point of a substance in his famous cannon barrel experiments. Listening to discontinuities in the sound of a rolling flint ball in a sealed cannon filled with fluids at various temperatures, he observed the critical temperature. Above this temperature, the densities of the liquid and gas phases become equal and the distinction between them disappears, resulting in a single supercritical fluid phase.

Water (H2O) has its critical point at 374.15 °C and 218.3 atm.
Ethanol (C2H5OH) is at 240.75 °C and 60.6 atm.
Carbon dioxide (CO2) is at a low 30.95 °C and 72.8 atm.

States of Matter Experiment

Ice was forcibly melted then the resulting water evaporated to observe changes in states of matter, including volume. See the States of Matter Experiment.