Objectives: Students will be able to differentiate between suspended particles and colloidal particles in solution.
Impurities in water may be in one of the three phases of matter: solid, liquid or gas. These impurities are dispersed into three states: suspension, colloidal and dissolved. The state in which an impurity is dispersed is especially important in the methods required for its removal.
A. Suspended Particles
Particles in suspension can be seen by the naked eye or by a microscope. The diameter of suspended particles ranges generally from 100 to 200 micrometers.
Suspended particles contribute to the cloudiness or turbidity of water. The
turbidity
of water is its capacity for absorbing or scattering light.
Demonstration: a sample of suspended particles may be made by taking some clay and adding it to a beaker of water. Stir vigorously to suspend clay.
Students should observe that after some time the suspended particles eventually settle to the bottom of the beaker. Ask students how they might remove the sand from the water. Answer: by filtration or allow to settle and decant
B. Colloidal Particles
The fundamental difference between a colloidal dispersion and a suspension is the size of the colloidal particles between 1 to 5 millimicrons. They can be distinguished only by means of an ultramicroscope or an electron microscope.
There are many types of colloidal dispersions, the following table is a list of some colloidal dispersions.
TABLE OF COLLOIDAL DISPERSIONS
|
Type
|
Name
|
Examples
|
|
gas in liquid
|
foam
|
Whipped cream, soaps suds
|
|
gas in solid
|
solid foam
|
Styrofoam, foam rubber
|
|
liquid in gas
|
liquid aerosol
|
fog, clouds
|
|
liquid in liquid
|
emulsion
|
milk, vinegar in oil
|
|
|
|
salad dressing
|
|
liquid in solid
|
solid emulsion
|
cheese, opals, jellies
|
|
solid in gas
|
solid aerosol
|
smoke, dust in air
|
|
solid in liquid
|
sol
|
india ink, gold sol
|
|
solid in solid
|
solid sol
|
certain gems (ex. rubies)
|
Colloidal dispersions that can be made in the classroom
1. india ink in water
2. add iron (III) chloride solution to boiling water
When colloidal solutions are observed in the path of an incident light beam, they appear perfectly clear. However, when at right angles to the beam many colloids appear turbid even to the naked eye. This phenomenon is known as the
Tyndall Effect
.
Examples of the tyndall effect is to a colloidal dispersion like india ink in water and direct a flash light directly into the solution and then at a right angle. Also, ask your students if they have ever observed dust in air.
Although colloidal dispersions may appear to be turbid with the tyndall effect, colloidal particles contribute little to the normal turbidity of water; but they are largely responsible for the color of natural waters.
To describe Brownian movement, ask your students if they have ever observed dust in the air when the sunlight is coming in a window. Also ask your students to describe this effect. Hopefully you should elicit that the dust appears to be moving rapidly and randomly. This can lead you nicely into the discussion of Brownian movement.
Brownian movement, first reported by Robert Brown in 1827, is the random motion of colloidal particles. This motion is due to the continual bombardment of the colloidal particles with the medium in which it is in. The colloidal particles have a very small mass which allows them to have considerable velocity of translation.
If the Kinetic Molecular Theory has been discussed previously you may also want to tie the brownian movement as visual proof that matter at the molecular level is in constant random motion.
The stability of different dispersions varies with the properties of the dispersed and dispersing phases. Good colloidal dispersions will remain in suspension indefinitely for two reasons. One, because the particle are continually bombarded by the molecules of the dispersing phase, which keeps the particles in motion (Brownian movement) so that gravity does not cause them to settle out. Two, since the colloidal particles have the same kind of electrical charge, they repel each other. This mutual repulsion prevents the dispersed particles from coalescing to larger particles, which would settle out of suspension.
There are many applications for colloidal particles, charcoal is used to remove colloidal impurities from air and water. Another important application is dialysis. Thomas Graham found that a parchment membrane would allow the passage of true solutions but would prevent the passage of colloidal particles.
