The atoms or molecules that comprise a solid are packed close together and are not compressible.

Because all solids have some thermal energy, its atoms do vibrate. However, this movement is very small and very rapid, and cannot be observed under ordinary conditions.

What are the different types of solids?

There are four types of crystalline solids --

	Molecular solids are represented as repeating units made up of molecules. An example would be ice. View the 3-D structure of ice.
	Metallic solids are repeating units made up of metal atoms. The valence electrons in metals are able to jump from atom to atom.

Amorphous solids

Amorphous solids do not have a definite melting point or regular repeating units. An amorphous solid is a solid in which there is no long-range order of the positions of the atoms unlike those in crystalline solids. An example of an amorphous solid is window glass. In addition many polymers such as polystyrene are amorphous.Amorphous solids can exist in two distinct states, the 'rubbery' state and the 'glassy' state. The temperature at which they transition between the glassy and rubbery states is called their glass transition temperature or Tg.

Interesting Note on Solids: While no mass at all is an impossibility, aerogels seem to come pretty close. Aerogels are the lightest solids and have a density of 1.9 mg per cm3 or 1.9 kg/m3 (526.3 times lighter than water). Sometimes called frozen smoke, aerogels are open-cell polymers with pores less than 50 nanometers in diameter.

What are the properties of a Liquid?

Liquids have a definite volume, but are able to change their shape by flowing.

Liquids are similar to solids in that the particles touch. However the particles are able to move around.

Since particles are able to touch the densities of liquid will be close to that of a solid.

Since the liquid molecules can move they will take the shape of their container.

What are the specific properties of liquids?

Viscosity --The resistance of a liquid to flow is called its viscosity

Surface Tension -- The result of attraction between molecules of a liquid which causes the surface of the liquid to act as a thin elastic film under tension. Surface tension causes water to form spherical drops.

Vapor Pressure -- The pressure that a solid or liquid exerts when it is in equilibrium with its vapor at a given temperature.

Boiling Point -- when vapor pressure = atmospheric pressure.

What are the properties of a Gas?

Gases have no definite volume or shape. If unconstrained gases will spread out indefinitely. If confined they will take the shape of their container. This is because gas particle have enough energy to overcome attractive forces. Each of the particles are well separated resulting in a very low density.

Gas Laws -- There are several excellent interactive java applets which we recommend.

Molecular Model for an Ideal Gas
2 dimensional gas of hard spheres
Gas Simulation
Brownian Motion for Gass

What is the fourth state of matter?

The fourth state of matter is plasma. Plasma is an ionized gas, a gas into which sufficient energy is provided to free electrons from atoms or molecules and to allow both species, ions and electrons, to coexist. In effect a plasma is a cloud of protons, neutrons and electrons where all the electrons have come loose from their respective molecules and atoms, giving the plasma the ability to act as a whole rather than as a bunch of atoms. Plasmas are the most common state of matter in the universe comprising more than 99% of our visible universe and most of that not visible. Plasma occurs naturally and makes up the stuff of our sun, the core of stars and occurs in quasars, x-ray beam emitting pulsars, and supernovas. On earth, plasma is naturally occurring in flames, lightning and the auroras. Most space plasmas have a very low density, for example the Solar Wind which averages only 10 particles per cubic-cm. Inter-particle collisions are unlikely - hence these plasmas are termed collisionless.

And now a fifth state -- Bose Einstein?

The collapse of the atoms into a single quantum state is known as Bose condensation or Bose-Einstein condensation is now considered a 5th state of matter.

Recently, scientists have discovered the Bose-Einstein condensate, which can be thought of as the opposite of a plasma. It occurs at ultra-low temperature, close to the point that the atoms are not moving at all. A Bose-Einstein condensate is a gaseous superfluid phase formed by atoms cooled to temperatures very near to absolute zero. The first such condensate was produced by Eric Cornell and Carl Wieman in 1995 at the University of Colorado at Boulder, using a gas of rubidium atoms cooled to 170 nanokelvins (nK). --Under such conditions, a large fraction of the atoms collapse into the lowest quantum state, producing a superfluid. This phenomenon was predicted in the 1920s by Satyendra Nath Bose and Albert Einstein, based on Bose's work on the statistical mechanics of photons, which was then formalized and generalized by Einstein.