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Dr. Wendy Sue Universe

Dr. Universe,

Why does soap get bubbly?

— Samuel, 9, East Peoria, IL

When you wash your hands with soap and water, a few different things happen to make bubbles.

Just like you, water and soap are made up of parts called molecules. Water molecules really like to stick together.

If you’ve ever jumped in a puddle or a pool, you may have even observed how water splashes in the shape of little drops. As water sticks together, it likes to form spheres.

That’s what I found out from my friend David Thiessen, a chemical engineer at Washington State University. Thiessen is really curious about bubbles and droplets, especially how they work in different kinds of space technology.

If you took a straw and blew bubbles in a glass of water, you would see air bubbles form underwater. When they rise to the top of the water, they immediately pop. But if you added some soap to the water and blew into the straw, you’d see a lot of foam coming up out of the glass.

That happens because of the nature of the molecules in soap. They are called surfactant molecules and they spread themselves out evenly and sit on the surface of water.

This happens because surfactants have two ends. Thiessen said chemists usually talk about surfactants as having a “head” and a “tail.” The head likes water and wants to stick to the water. The tail doesn’t like water and likes to stay in the air.

When we see a bubble, there is also a force called surface tension at work. This force makes water behave a bit like a thin sheet of rubber. That’s how bugs can sometimes even stand on water without falling through.

The surface tension of water is really high, but when soap is added to water it lowers the tension. The surfactant molecules push their way between water molecules and in the process separate water molecules from one another, reducing their attractive force. The soap helps spread the water out into a thin film that forms a sphere: the bubble.

You can learn more about surface tension with a really simple activity. Pour some water on a plate. Sprinkle some pepper on top of the water. Then put a drop of soap on your finger and touch the middle of the pepper. The soap lowers the surface tension and the pepper scatters to the plate’s edge.

Soap and water molecules can not only help create bubbles but also help cut through grease on dirty dishes and even get rid of germs on your hands. Besides behaving in all kinds of interesting ways, bubbles can also make some really interesting colors.

When light hits the surface of a bubble and reflects off the two sides of the film, the light rays interfere with each other. It creates a phenomenon called iridescence and displays a rainbow of colors.

The next time you wash your hands or help out with the dishes, take a look at how many tiny bubbles you made and remember — it’s chemistry.

Have a science question? Ask Dr. Wendy Sue Universe, WSU’s resident science cat and writer, by email at Dr.Universe@wsu.edu.

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