by STEM Scouts
- Two plastic cups
- Bundle of string
- Scotch tape
- Cut a 24-inch (61 centimeter) piece of string
- Soak the string in water for a few seconds; remove and squeeze out excess water
- Tape one end of the string to the bottom of the side of one cup; repeat with the other cup
- Fill one cup halfway with water
- Hold the cup with water up, leaving the empty cup on the table; pull the string tight by positioning the water cup at a 45-degree angle from the empty cup
- Slowly pour the water down the string
- What happens? If you can’t do this experiment over the sink or outdoors, you may need the towel to clean up any spills
- How far away do you think you could pour the water without spilling it?
How Does It Work?
This experiment works, thanks to cohesion and adhesion. Steve Davala explains it well:
“[Water] has both strong ‘cohesive’ and ‘adhesive’ properties. What this means is that water can stick to itself (cohesion) and other things (adhesion). This goes down to the molecular level in how a water molecule looks: it has two hydrogen atoms and one oxygen atom. These atoms end up having positive and negative charges which cause them to stick to one another . . .
“At first, you soak the string in the water, and the water adheres to it. Even though water may drip out of it, it is still soaked in there. That is adhesion. When you pour water along it, you notice the water clings to other water molecules attached to the string, and you see it form a little stream underneath the string. That is cohesion!”