250 ml beaker
Table salt (NaCl)
Dirt from your garden
Stir 5 grams (1 teaspoon) of table salt (NaCl) into a beaker with 250 ml (about 8 ounces) of water. Stir until all the salt (solute) dissolves in the water (solvent). Shine the laser pointer thru the beaker containing the saline solution…and…nothing interesting happens. When the NaCl dissolves in water it separates into sodium (Na+) cations and chloride (Cl-) anions too small to be seen with the naked eye and will not scatter the light from the laser beam. Solutions are homogeneous mixtures, that is, the water molecules, sodium cations, and chloride anions are uniform throughout the mixture. The mixture is stable (the salt, once dissolved, won’t settle to the bottom of the beaker) and the salt cannot be filtered from the water.
You’ll need an eyedropper with a small amount of milk, a teaspoon, and a beaker with 250 ml of water. Squeeze a few drops of milk from the eyedropper into the beaker and stir. Shine the laser through the beaker and you should now be able to observe the Tyndall effect:
You’ll notice that you can’t see the laser beam piercing through the air, but you can see the beam in the diluted milk and water mixture. A glass of milk is an example of a colloid and the Tyndall effect is what gives it its translucent appearance. Milk is mostly an emulsion of milk fat and water. An emulsion is “a suspension of small globules of one liquid in a second liquid with which the first will not mix” (source). Oil and vinegar do not mix, but vinaigrette is an emulsion of oil and vinegar. “Emulsion” is used to describe a colloid of two or more liquids (in the case of milk, milk fat droplets dispersed in water) as opposed to, say, an aerosol colloid like fog (water droplets dispersed in air). The milk fat globules are too small to be seen with the naked eye or even through an optical microscope, but (unlike a solution) are large enough to scatter light and create the Tyndall effect. Colloids are visually homogenous (uniform throughout), but microscopically heterogeneous (lumpy/grainy--in this case, the globules of milk fat remain separate from the water). Generally, colloids cannot easily be filtered nor settle at the bottom of the beaker.
Stir 5 grams (1 teaspoon) of dirt from your garden into a beaker with 250 ml (about 8 ounces) of water. Before the dirt settles, shine the laser pointer thru the beaker. You should be able to observe the Tyndall effect before the particulate (the particles of dirt suspended in the water) settles to the bottom of the beaker. Suspensions are heterogeneous (lumpy/grainy—the grains of dirt suspended in the water). Particles in a suspension are usually large enough to see with the naked eye or be viewed through an optical microscope. They are often large enough to be filtered from the water, and, of course, will eventually settle to the bottom of the beaker.--------------------
For more information about the Science Play and Research Kit (SPARK):
If you are designing your own SPARK kit you may wish to include an inexpensive laser pointer in your kit for the Tyndall effect demonstration. Also take a moment to look at the Celtic Mad Scientist video in my DIY Laser Interferometer article. You may find it useful if you want to design your own laser interferometer to demonstrate the wave/particle duality of light.