Use this activity to show what molecules make foams, in the context of another activity.
e.g. foam in a meringue, milkshake or the foam on the ocean (see attached booklet).
Make dropper bottles of each of the components to test for whether they make a foam:
For foam of a meringue you need to test protein, sugar and salt (cream of tartar).
For the foam in ocean water you need to test salt, protein and fat.
Students guess which molecules make foam, then drip some into a small tube to half fill it, then shake vigorously.
They can shake water alone in a second tube as a control - this should make no foam.
If the air bubbles stick around as a foam on top of the liquid, the molecule makes foam.
Students can figure out which of the components are responsible for making the foam.
Results should show that protein and fat make foam, whereas salt and sugar do not.
How is the foam made?
Before the tube was shaken, the protein/fat molecules were spread out. During shaking, air bubbles are mixed in. The protein or fat 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/fat molecules have different parts, some which like water ("hydrophilic") and some which do not ("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/fat 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 attachment for more information on colloids and mixtures.)
The attached foam booklet is a self-guided activity investigating the molecules that make the foam in the ocean.