When it come to busting up grease and dirt, few things work better than soap, thanks to the amazing structure of soap molecules! These molecules work by having two very different sides. One side is an oxygen (O) ion loosely hanging on to a sodium (Na) or potassium (K) ion. An ion is an electrically charged atom or group of atoms. Because this end is very similar to water molecules (H2O) it can mix well with water. The other end is a long chain of carbon and hydrocarbon (H) atoms linked together. This long end is very similar to the molecules that make up oils and fats and so it mixes very well with them, but not with water. Because soap can mix with both water and fats, it allows grease to dissolve into the water and wash off your dishes, body or clothes.
Although detergents and soaps do the same jobs and we often use the names interchangeably, there is a difference between the two. Soaps are made from a reaction between metal hydroxide (often sodium or potassium hydroxide, but even sometimes calcium or magnesium hydroxide) and fats or oils that come from animals or plants. With a shortage of plant and animal fats during World War I, scientists discovered ways to synthetically produce molecules similar to soap molecules - detergents. These are less likely to react with the minerals in hard water and most often make appearances in laundry and dish soaps, but are also a main ingredient in many shampoos and some "bar" soaps.
The following activity shows off the awesome property of soaps and detergents to make friends of the otherwise incompatible fat and water. If you don't have dairy milk around, you can use soy or nut milk that contains at least two grams of fat per cup. Nonfat milks will not work in this experiment. If you don't want to try it at home but are still curious, you can find the experiment as part of the Chemistry Lab set of experiments once every ten weeks or so.
Into one bowl or dish, put three tablespoons of water. In the other dish put three tablespoons of milk.
Drop three or four drops of food coloring into each dish. You can use a single color, or multiple colors. Make sure they do not mix!
How does the food coloring act in each dish?
Dip one end of a toothpick into the soap or detergent. Carefully touch it to the center of one of the food coloring drops in the water dish.
Dip a second toothpick into the detergent and touch the center of one of the food coloring spots in the milk dish.
What happens? Why do you think the food coloring acted differently in each dish?
You can try using a soapy toothpick to touch the food coloring again in each dish, but the results may not be as dramatic.
What's Going On?
Milk is an emulsion of fat and water. Very small droplets of fat are suspended throughout the liquid. The food coloring is water-based. Water and fat molecules are not very similar to each other and therefore do not mix well together; if not emulsified, they will separate into layers. Because of this the food coloring spread more easily in the water dish than in the milk dish before adding soap.
When you touched the soapy toothpick to the dishes, the soap allowed the water-based food coloring to mix with fatty milk, producing streams of color. The food coloring could already mix with the water so adding a soapy toothpick did not do much in the water dish.