Have you ever seen a bug skate across the surface of water? This is made possible by the cohesion of water molecules on the surface of the water. When we say that water molecules are cohesive, we mean that they stick tightly to one another—so much so that they create surface tension. Because bugs weigh very little, they can rest their weight on the top of the water without breaking the tension—so to them, it is almost as if they’re standing on a solid surface! In this experiment, we’ll take something relatively heavy (a paperclip) and balance it on the surface of the water in order to demonstrate just how powerful surface tension can be.
Make a paperclip float in a bowl of water.
Can you make a paperclip float? Will adding detergent to the water help you? Why or why not?
- Tap water
- Paper towel
- Paper clip
- Cotton swab
- Small bowl
- Laundry or dish detergent (or both!)
- Fill a small bowl ¾ of the way full with water.
- Use the scissors to cut a small square out of a paper towel.
- Place the paperclip in the center of the small square.
- Place the small paper towel square on the surface of the water. When doing this, try to lay the paper towel down as evenly as possible. If the center gets wet first, your paperclip may not float.
- Use a cotton swab to gently push the paper towel down into the water, away from the paperclip. What happens? Record your observations
- Repeat the experiment, but before adding your paper towel and paperclip, add ½ a teaspoon of laundry or dish detergent to the water and stir until it is well mixed. What happens? Record your observations.
When there is no detergent in the water, you'll achieve a floating paper clip! When detergent is added, the paperclip may still float, but it is far less likely to do so.
The paperclip will float due to surface tension. Surface tension is the attraction of like particles to one another on a surface—in our case, a water-air surface. Water molecules on a surface experience cohesion, which is the sticking together of one molecule to another molecule of the same material. The paperclip is less likely to float in the water with detergent because the detergent acts as a surfactant—a material that reduces the surface tension of the liquid it is dissolved in.