Solutions are homogeneous. Dissolved molecules as large as 1000 pm never separate as a result of gravitational forces, even in an ultracentrifuge. When suspended particles reach µm size (106 pm), they separate readily under gravity, and we classify the mixtureA combination of two or more substances in which the substances retain their chemical identity. as definitely heterogeneous. Surprisingly, suspensions of particles between these sizes, in the range 5 000 - 200 000 pm, never settle under gravity or centrifugation, yet the mixtures are definitely heterogeneous because beams of light passing though them are visible. If a laser pointer beam passes through a solution, it is invisible, but if it passes through a colloidal suspension it is easily seen. Laser shows would be invisible if it weren't for the colloidal smoke or fog that renders the beams visible.
Colloids are stable suspensions of a dispersed medium, like the solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions. particles in smoke or fat particles in milk, in a continuous medium, like air or water. The particles and medium may be any combination of solid, liquidA state of matter in which the atomic-scale particles remain close together but are able to change their positions so that the matter takes the shape of its container, and gasA state of matter in which a substance occupies the full volume of its container and changes shape to match the shape of the container. In a gas the distance between particles is much greater than the diameters of the particles themselves; hence the distances between particles can change as necessary so that the matter uniformly occupies its container. (except that they both can't be gases, because gasses are completely miscibleWith respect to two liquids, able to dissolve in each other in any proportion.), as the following table  shows:
(All gases are mutually miscible
| Liquid Aerosol|
Examples: fog1, mist, hair spray
| Solid Aerosol|
Examples: smoke, cloud1
Example: Whipped cream
Examples: milk, mayonnaise, hand cream
Examples: pigmented ink, blood
|Solid|| Solid Foam|
Examples: aerogel, styrofoam, pumice
Examples: agar, gelatin, jelly, silica gel
| Solid Sol|
Example: red stained glass
1 Average water droplets in fogs and clouds are around 0.01 mm in diameter, so they are ten times as big as typical colloids. Clouds "float" mostly because of air updrafts, and the hardly noticeable (~0.3 cm/s) terminal falling velocity of small droplets.
Colloids are stable, that is, their particles don't aggregate and precipitate, because the particles all tend to adsorb ions of one charge from solution. The colloidal particles then repel one another electrostatically. The individual particles are kept in suspension by Brownian motion.
When white light passes through a colloidal suspension, light scattering is highly wavelengthThe distance between the crests of adjacent waves (or between any adjacent corresponding points in waves); used in the context of electromagnetic radiation. dependent, so colloids may appear to be brightly colored. This is the case with the gold colloid that is responsible for the deep red color in most stained glass. Colloidal gold is easily made in the laboratory by reducing a gold saltAn ionic compound that can be formed by replacing the hydrogen ion of an acid with a different cation. dissolved in water to give elemental gold clusters. As the clusters change in size, the color of the colloid may change through virtually every color of the rainbow.
HCl + Na2S2O3 → 2 S + 2 NaCl + SO2 + H2O
H3O+ + S2O32- → 2 S + Cl- + SO2 + 2 H2O
As the colloidal particles grow, the suspension appears blue because blue light is scattered, but the transmitted light hitting the screen behind the beaker is red, like the light we see transmitted through the atmosphereA unit of pressure equal to 101.325 kPa or 760 mmHg; abbreviated atm. Also, the mixture of gases surrounding the earth. as the sun sets. You can do this at home by adding milk dropwise to a container of pure water while a beam of white light from a bright flashlight or projector is passed through the suspension and toward a white screen.
- ↑ http://en.wikipedia.org/wiki/Colloid
- ↑ http://www.suite101.com/article.cfm/science_sky/116567
- ↑ http://en.wikipedia.org/wiki/Sunset
- ↑ The colloidal sulfur can also be prepared by dissolving sulfur in methanol and adding the solution dropwise to water: http://www.woodrow.org/teachers/ci/1986/exp14.html