States of Matter

David Young
Cytoclonal Pharmaceutics Inc.

At the beginning of their first chemistry class, students are taught that there are three 'states' of matter; solid, liquid and gas. From that point on, additional 'states' of matter are introduced at regular intervals. Many of these states are well defined and have interesting properties, while others seem to be intermediate between commonly found substances and difficult to define or recognize. The following is a listing of the various states of matter with a brief description for each.

There are several different types of states of matter. Gasses, liquids and solids are examples of physical states which can be treated separately from electrical states, magnetic states and optical states. A given substance will have a physical state, a magnetic state, electrical and optical properties.


A gas is a substance which takes the shape of its container and expands to completely fill it's container. There are several types of gases with slightly different behaviors. These are ideal gasses, real gasses, super critical fluids, plasmas and critical opalescent materials.

Ideal Gas
Ideal gasses (sometimes called perfect gases) refer to the behavior which gasses approach as the pressure nears zero. This behavior is described mathematically by the ideal gas law. Although no gas behaves exactly as an ideal gas, many substances come very close to ideal behavior 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 categorized 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 which give non-ideal behavior 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.

Critical Opalescence
The critical point is the temperature and pressure where the boundary between liquids and gasses ceases to exist and the substance becomes a supercritical fluid. Under this one set of physical conditions, the substance is neither gas or liquid. The substance will have liquid like regions of every size from the size of the container down to single molecules. As such there are regions of the size of every wave length of visible light and all wave lengths of light are refracted. This results in a milky, silvery appearing state called critical opalescence.

A plasma is a material which has been heated to a temperature where molecules are not stable. In a plasma state, a substance is a mixture of neutral molecules, ions, atoms, clusters of atoms and free electrons. A spark is an example of a plasma.

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

At very low temperatures certain compounds such as 3He will show a superfluid state. In this state quantum mechanical effects will be visible on a macroscopic scale. For example, spinning a sample of superfluid will give two or four counter rotating vortices in order to conserve angular momentum in the fluid as a whole rather than just at the atomic level.

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

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 which 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.

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 visceoelastic materials.

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

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.

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.

An elastomer is an amorphous solid which can be deformed with out breaking. A rubber band is an elastomer.

Many metals and alloys can be stretched by about 100% before breaking. Some alloys can be stretched by a few thousand percent before breaking. This is referred to as superplasticity.

Bose-Einstein Condensate
Atoms which are bosons behave according to Bose-Einstein statistics. Unlike fermions, many bosons can occupy the same quantum state. A laser beam is a collection of photons, which are bosons, all in the same quantum state, thus giving perfectly coherent light. At very low temperatures, atoms can all occupy the ground state of the system thus giving a coherent matter analogous to the laser. This process is called Bose-Einstein condensation.

Refractory materials viewed on an atomic scale will have small domains of various crystal, liquid and amorphous states. Although this may sound like a very unstable material, refractory materials can be very stable. Refractory bricks are used for lining high temperature blast furnaces. Very little is known about the atomic structure of refractories and why they are so stable.


A diamagnetic compound has all of it's electron spins paired giving a net spin of zero. Diamagnetic compounds are weakly repelled by a magnet.

A paramagnetic compound will have some electrons with unpaired spins. Paramagnetic compounds are weakly attracted by a magnet.

In a ferromagnetic substance there are unpaired electron spins, which are held in alignment by a process known as ferromagnetic coupling. Ferromagnetic compounds, such as iron, are strongly attracted to magnets.

Ferrimagnetic compounds have unpaired electron spins, which are held in an pattern with some up and some down. This is known as ferrimagnetic coupling. In a ferrimagnetic compound, there are more spins held in one direction, so the compound is attracted to a magnet.

When unpaired electrons are held in an alignment with an equal number of spins in each direction, the substance is strongly repelled by a magnet. This is referred to as an antiferromagnet.

Materials which will be repelled by magnetic fields because the magnetic field is excluded from passing through them. This property of superconductors is used to test for the presence of a superconducting state.


A polarizable nonconducting substance in which dipoles will orient randomly until an electric field is present.

A ferroelectric material will be strongly attracted by an electric field. At the molecular level, ferroelectric compounds will have dipole moments all in alignment.

An antiferroelectic material will be strongly repelled by an electric field. The dipole moments in an antiferroelectric are arranged with an equal number pointing in each direction.


An insulator is a compound which does not conduct electricity for all practical purposes. At extremely high voltages, a small amount of conduction will still occur.

A conductor is a material which an electrical current will readily flow through. Conduction occurs either by the movement of electrons or the movement of ionic atoms or molecules. Conductors always have a very small amount of resistance to electrical conduction.

A material that will conduct a small amperage of electricity when the voltage exceeds some critical value. This minimum necessary voltage corresponds to the energy necessary to excite electrons from a filled energy level called the valence band to an unoccupied energy level called the conduction band. This energy difference is called the band gap. Intrinsic semiconductors are materials which have a small band gap in their pure form. Extrinsic semiconducts are insulators which have been made into semiconductors by the addition of a small amount of an element with an excess or a deficiency of electrons to form n-type or p-type semiconductors respectively.

Certain materials at very low temperatures will have a state with no resistance to electrical conduction. Currently, the highest temperature at which this phenomenon has been observed is 160 K. The exact means by which this occurs is not yet completely understood.


Opacity is the measure of how much light will pass through a substance. The opacity will depend upon the wavelength of light also.

Refraction is the bending of light rays. The amount of bending is different for different substances and different wave lengths of light.

Optical Activity
Optically active materials will rotate the plane of polarization of plane polarized light. This is due to the presence of chiral centers in the molecules.

Circular Dichromism
This is the phenomenon of a substances absorbing a slightly different amount of left-circularly polarized light compared to its absorption of right-circularly polarized light.

Non-Linear Optical Properties
The most common optical properties, such as refraction, can be related to the polarizability of the molecule. Polarizability is the change in the dipole moment when an electric field is applied (a first derivative). Non-linear optical properties depend upon the hyperpolarizability of the molecule. Hyperpolarizability is the change in the change in dipole moment due to an electric field (a second derivative). An example of a non-linear optical property is freqency doubling, where the light emitted from a material is twice the frequency of the light being sent into the material.

Further Information

For an introductory chemistry text see
L. Pauling "General Chemistry" Dover (1970)

A physical chemistry text for non-chemists is
P. W. Atkins "The Elements of Physical Chemistry" Oxford University Press (1993)

A physical chemistry text for undergraduate chemistry majors is
I. N. Levine "Physical Chemistry" McGraw-Hill (1995)

An introductory article about superfluids is
O. V. Lounasmaa, G. Pickett Scientific American, p. 104, June (1990)

A mathematical treatment can be found in
D. L. Goodstein "States of Matter" Dover (1985)

Properties of high molecular weight solids (most commonly polymers) are discussed in
H. R. Allcock, F. W. Lampe "Contemporary Polymer Chemistry" Prentice-Hall (1990)

Solid state properties are covered in
A. R. West "Solid State Chemistry and its Applications" John Wiley & Sons (1992)

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