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Capillary Action Lesson for Kids

Instructor: Diane Sieverson

Diane has taught all subjects at the elementary level, was the principal of a K-8 private school and has a master's degree in Measurement and Evaluation.

Capillary action happens when a liquid, like water, moves up through a hollow tube or into a spongy, solid material. Come learn about capillary action, how it works, how it's helpful, and some other neat facts about this process.

What is Capillary Action?

You've just come in from playing in the hot sun and you decide to pour yourself a cold glass of water. Before you can drink it all, you accidentally knock over the glass and spill some water. You grab a paper towel and put it over the puddle. The water immediately soaks into the paper towel and the counter is dry. You've just used capillary action to clean up your mess!

Water moving up through paper towel due to capillary action
Water moving up through paper towel due to capillary action

Capillary action (pronounced ka-puh-LAIR-ee) is a process during which a liquid, like water, moves up something solid, like a tube or into a material with a lot of small holes.

This happens when 3 forces called cohesion, adhesion, and surface tension work together, so let's look at these forces and how they cooperate to cause capillary action.

What is Cohesion?

When you think of something sticky, you might think of a wad of gum stuck to the bottom of your shoe. But in science, water is considered sticky, too, even though it doesn't feel like gum!

Just like you hang out with your friends, water molecules, or small particles of water, stick to other water molecules. This is called cohesion (pronounced co-HE-zhun). But water molecules don't just stick to each other.

What is Adhesion?

Adhesion (pronounced ad-HE-zhun) happens when water molecules stick to a solid substance, like a paper towel or the sides of a hollow tube, and the water is pushed up.

Because water molecules stick together, when the first water molecule sticks to the side of the tube and moves upward, it pulls the next water molecule up with it and so on, like a chain.

What is Surface Tension?

Surface tension refers to water molecules that are more closely bound together at the surface, making the top of the water more tight and dense than the rest of the water. Surface tension holds the water together at the top as it moves up during capillary action, holding the water molecules together like a drawstring.

If you've ever seen an insect that was standing on top of the water in a pond instead of floating in it, you've seen surface tension in action!

Insect standing on water due to surface tension
Insect standing on water due to surface tension

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