Logic puzzle to figure out what molecules can make foam
Beforehand, make up dropper bottles of six liquids: water, distilled water, drink mix, soap, skim milk and whole milk (make sure the milks are cold).
Students drip each liquid separately into a tube to fill it half way.
Cap and shake hard.
Look for foam above the liquid i.e. bubbles that remain for more than a couple of seconds. (Make sure the milks are really cold, when they will make foam. The soap will also make foam. The waters and the drink mix should not make foam.)
Figure out which molecules make foam from the liquids that do and don't make foam. This puzzle is most easily done by figuring out which molecules do not make foam, and then deducing which ones must be making foam in the liquids that do foam. Use the attached foam test worksheet to help.
Testing molecules to see which ones make the foam in a recipe or environment
Beforehand, make up dropper bottles of the molecules that are in the recipe or environment:
To find out what makes the foam in meringue make bottles of water, protein (in the egg white), sugar and salt (the cream of tartar).
To find out what makes the foam in milkshake make bottles of water, protein, sugar, salt and fatty acid (all components of milk).
To find out what makes the foam in ocean waves breaking on a beach, make bottles of water, salt, protein (from living things in the ocean) and fatty acids (from living things in the ocean).
In these activities, protein and fatty acids will make foam, but water, salt and sugar will not.
How is the foam made?
Before the tube was shaken, the protein/fatty acid molecules were spread out. During shaking, air bubbles are mixed in. The protein or fatty acid molecules cluster around these air bubbles, holding them in place. The foam you see is hundreds of tiny air bubbles held in place.
More detail: the protein/fatty acid molecules have two different parts - one end of the molecule likes to be in water ("hydrophilic") and the other end does not like to be in water ("hydrophobic"). The hydrophobic parts stick into the air bubbles (so only touch air) and the hydrophilic parts project into the water surrounding the bubbles. The protein/fatty acid molecules surrounding each air bubble stabilizes them so that they remain suspended in the mixture.
Salt/sugar molecules don't cluster around air bubbles, so they don't make foam.
A foam is a kind of mixture called a colloid, with a gas suspended in a liquid. (See the attached mixtures summary for more information on mixtures.)
The attached ocean foam activity booklet is a self-guided activity investigating the molecules that make the foam in the ocean.