A Moment of Science

The Surprising Chemical Structure Of Soap

Soap has been used for cleaning for many years, but it wasn't until modern chemists began to understand its structure that anyone knew how soap worked.

colorfully wrapped soaps

Photo: Thomas Hawk (Flickr)

Be careful what kind of soap you pick. Some soaps can leave build-up in your sink.

Soap has been used for cleaning for thousands of years, but it was not until modern chemists began to understand its molecular structure that anyone knew how soap worked its magic.

How Soap Works

The long soap molecule has one end that is attracted to fats and oils, the other end is attracted to water. When soap is added to the wash water, one end of its molecule attaches to the oily dirt and pulls it away from the fabric or your skin.

The other end stays attached to the water, and when the water is washed down the drain the dirt attached to the other end of the molecule follows.

Hard Water

The problem with soap is that it doesn’t work well in hard water. Hard water contains a lot of calcium, and before soap begins to clean you or your clothes it separates the calcium from the water.

This is what makes the scum of the bathtub ring. After the soap has removed all the calcium in the water, it starts to clean. That’s why it takes more soap to clean in hard water—the first soap gets rid of the calcium, then more is needed to get rid of the oily dirt.

Chemical Attraction

After World War II washing machines became very popular, resulting in large demands for soap. However, the public wasn’t satisfied with the grungy film left on clothes.

When chemists began working on a cleaner that wouldn’t leave a film, they knew they needed to keep the basic structure of soap, that is a molecule with one end that was attracted to oil, and the other attracted to water.

Detergents Were Born

To eliminate the film they developed a substance whose water-attractive end would not have an affinity for calcium.

These detergents did not separate the calcium which formed the ring, but left it in to be washed away with the dirt. And these are the detergents we use today.

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