Science Spotlight

Capillary Action

When you water plants, you water their roots. But how does the water travel all the way from the roots of the plant to the leaves? They are standing upright: wouldn’t that defy gravity? In fact, there is a phenomenon known as capillary action that helps plants accomplish this feat, without having to use any magic. Capillary action is the ability for a liquid to flow in very narrow spaces without the assistance of, or even the opposition to, an external force like gravity. Capillary action is possible because of two properties of water molecules – adhesion and cohesion. Water molecules are very attracted to other objects, this is known as adhesion. However, they are also extremely attracted to each other and are known to stick together, this is called cohesion. Plants have a long tubed known as a xylem, which water travels through. Water molecules are more attracted to the xylem than they are to each other. This causes the molecules to  adhere to the tube. Because of adhesion, the molecules are strongly attracted to the tube. But, the molecules still want to bring along other water molecules because of cohesion. Cohesion pulls the water upward, as if the molecules were in a chain. Watch water defy gravity and view capillary action yourself with this experiment you can do at home!

Materials: Two glasses, cool water, paper towel, food coloring.

Directions: Fill one glass about ¾ with cool water and add a few drops of food coloring. Take the paper towel, roll it up and place part of it in the glass within the water. Fold it over into the other glass. What do you think will happen with the water?

How It Works: The adhesive force between the paper towel and the water is more attractive than the cohesive force between the water molecules themselves. This pulls the water up and over the bridge! 

Want more? Research it! Does this experiment work the same with warm water? Why or why not?