Atoms and Molecules

Physical states of matter

Gas    A gas is a substance which takes the shape of its container and expands to completely fill it's container.

Ideal Gas    Ideal gasses (sometimes called perfect gases) refer to the behaviour which gasses approach as the pressure nears zero. This behaviour is described mathematically by the ideal gas law. Although no gas behaves exactly as an ideal gas, many substances come very close to ideal behaviour at atmospheric pressure and most behave ideally at very low pressures.

Real Gas    Most molecules attract one another until they come very close together, when they become repulsive. This attraction is due to the electrostatic interactions between the two molecules. These interactions are often categorised into dispersion forces, van der Waals forces, hydrogen bonding and dipole-dipole interactions. The repulsion between molecules at very close distances is due to the repulsion between the nuclei of the two molecules. These forces give rise to relationships between the pressure, temperature, volume and quantity of a substance, which do not exactly obey the ideal gas law. Gasses under physical conditions that give non-ideal behaviour are called real gasses.

Supercritical Fluids    At a given temperature, a gas can be compressed until it starts to condense into a liquid displaying a clear boundary between the liquid at the bottom of the container and the gas. Above a certain temperature, called the critical temperature, a gas can be compressed without ever observing a clear liquid - gas boundary. Gasses in this state are called super-critical fluids.

Liquid    A liquid is a substance which takes the shape of it's container and has a fixed volume at a given temperature and pressure. Suspensions, colloids, liquid crystals and viscoelastic materials have properties intermediate between those of a liquid and a solid.

Suspension    A material in which small solid particles are mixed uniformly with a liquid. A suspension behaves as a liquid.

Solution    A material in which other materials are dissolved and mixed uniformly on the molecular scale with a liquid. A solution behaves as a liquid.

Colloid    A colloid is a material which appears to be liquid but actually is a suspension of particles too small to observe with a microscope but bigger than normal molecules.

Liquid Crystal    In crystals, the atoms are arranged in an ordered repeating pattern. In liquids there is no ordered pattern. In liquid crystals there is order in one or two directions while there is no order in the other directions. This gives a number of unique properties such as optical properties that can be turned off and on to make liquid crystal displays for watches and computers. There will also be changes in the viscosity of a substance when it reaches a liquid crystal phase.

Viscoelastic    Some compounds such as natural rubber appear to be solid when they are stretched, bent or set on a table top. However, over a period of time these materials will slowly deform to take the shape of the container. Substances, which act as solid on short time scales and act as liquids on long time scales are, called viscoelastic materials.

Solid    Solid state materials are characterised by having a fixed volume and shape. Crystals, glasses and elastomers are all types of solids.

Crystal    Crystals are solid state materials in which the atoms are arranged in an ordered repeating pattern. Many molecules will form crystals in which the original molecules are still distinguishable only stacked neatly. Organic compounds often form these molecular crystals. In other crystals, such as metal alloys, there is a repeating pattern but no distinguishable molecular units.

Glass    Glasses are amorphous solids, meaning that the atoms are not arranged in any repeating pattern. When a liquid is cooled very slowly it tends to form a crystal, while cooling quickly usually results in amorphous phases. Glasses are distinguished from elastomers by being brittle.

Elastomer    An elastomer is an amorphous solid which can be deformed with out breaking.

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This page was last updated by Martin Chaplin
on 10 February, 2005