More Hydrophobic and Hydrophilic Interactions: Marbling with Shaving Cream
[[Shopping List: Shaving cream; food coloring; squeegees; cookies sheets; wax paper; small pieces of cardstock; toothpicks; paper plates]
- Mound a small amount of shaving cream on a paper plate. Add three-four drops of food coloring to the shaving cream and swirl with a toothpick. Be careful not to overswirl – if all of the colors blend together into a new color the final product won't be as cool!
- Place a small paper card on the shaving cream mound and press lightly. Remove the card and use a squeegee to remove excess shaving cream. Look at the marbled paper product!
- Allow a kid to reuse their mound of shaving cream (re-swirling it, adding a new drop of food coloring, or as is) to color the back of their card or another card. Swirl the colors around some more to see how it chages the marbled pattern.
What's Happening: Hydrophobic substances repel water, and the name comes from the Greek stems "hydros" for water and "phobos" for fear. Hydrophilic srfaces attract water. Polarity is another way to describe molecules, and envelopes the concept of hydrophobicity and hydrophilicity. A polar molecule has a separation of positive and negative charges, while a non-polar molecule has no areas of prevalent positive or negative charges (is non-charged). Water is a polar molecule, having positive charge areas around the hydrogen and a negative charge area around the oxygen. Because water is polar, other polar molecules will be attracted to it; these are the molecules we describe as hydrophilic. Conversely, hydrophobic molecules are non-polar/non-charged.
Soap is an interesting molecule that is both hydrophilic and hydrophobic, something that chemists describe as amphipathic. A soap molecule looks a bit like a snake, in which the head is polar and hydrophilic and the tail is non-polar and hydrophobic. Shaving cream is a foam composed of soap and air.
Food coloring is just dye dissolved in water, and is therefore hydrophilic. When added to the shaving cream, the food coloring can only interact with the hydrophilic heads of the soap molecules and thus has limited mobility. In paper, on the other hand, the food coloring can move easily. Paper is composed of cellulose, a molecule with polar hydroxyl (oxygen and hydrogen) groups that make it hydrophilic. Because both food coloring and cellulose are hydrophilic, the food coloring can spread easily across the paper to create a more colorful pattern than it did in the shaving cream. This creates the marbling effect on the paper that is clearly distinct from the pattern seen on the shaving cream surface.
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