Why is soap so slippery?
Why is soap so slippery? asks Laryssa Ouro-Rodrigues, a student in Brookville, NY.
Stepping on a bar of soap is more risky than stepping on a banana peel-but only if the bar is wet. Open up a bar of Ivory or Dove or Dial, and it’s unlikely that the dry bar will shoot out of your hand and land on the bathroom floor. But add water, and voila: your soap has leapt into the air, landed behind the sink, and is covered in a fine layer of grit and cat hair.
Why do soap and water make such a good team? Soaps are surfactants, “surface-active agents” that turn down the surface tension of water. Surfactants actually make water wetter, by weakening the attraction between the water molecules. Which is a good thing, if you’re looking to get clean.
The basic recipe for soap is fat or vegetable oil plus an alkali solution, such as lye. Think of soap molecules as tiny tadpoles, with round “heads” and narrow “tails.” The heads have a negative electrical charge. Likewise, water molecules have a slight positive charge on one end. Since (electrical) opposites attract, the soap molecule heads are hydrophilic–water lovers.
The greasy tails are hydrophobic–water haters. Oil and water don’t mix. But the greasy tails glom right onto fat and oil. When you use a bar of soap and a streaming shower to get clean, the tails of the soap molecules dissolve into the mixture of oil and dirt on your skin. The water-loving heads pull the greasy dirt out into the water, where it washes down the drain.
So why is soap so slippery? Many surfaces-from kitchen floors to asphalt roads-are “Slippery When Wet.” That’s because water acts on a tiled floor just as it acts on the slides at a water park: It reduces friction. When one surface rubs against another, their nooks and crannies catch, producing a dragging effect. But add water, and your shoe (or skin) glides on a liquid film, reducing your contact with the floor’s (or slide’s) imperfections.
Greases and oils are also champion friction fighters, which is why they’re used to lubricate moving parts in bikes, cars, and other machinery, as well as to keep cakes from sticking to the pan. Now think about an already-slippery oil-slicked road in the rain. Unless drivers are careful, the road can become a demolition derby, its friction reduced to a minimum by the combined effects of oil and water.
According to chemists, a wet bar of soap is coated in a thin film of water, like an icy pond whose surface is melting a bit. This reduces the friction between your skin and the soap, just as the friction is reduced between the blade of ice skates and the pond. But the friction-dampening effect of water is magnified on a fatty bar of soap, since the bar is also greasy. (Ever use soap to grease the runners of a stuck drawer?) The combination makes soap a slippery menace in the shower.
