Floating Paperclips

As you may already know, water can hold light things like a piece of paper, but what about the additional weight of a paperclip? In this experiment, we will try this out and watch the effect of surface tension!

Materials

  • A bowl of water

  • Dish soap

  • Paperclips

  • Scissors

  • A piece of paper

Time Estimated:

10 minutes

Directions: 

  1. Try dropping a paperclip or two into the bowl of water. 

  2. You will probably notice that the paperclips sink right to the bottom. 

  3. Now, cut a square of paper and place it under the paperclip. Try setting both on the surface of the water. 

  4. Gently nudge the paper so that it gets wet and sinks to the bottom, leaving the paperclip floating on the surface of the water. You can use sticks to help you get the paper wet and detached from the paperclip.  

  5. Squirt a little soap into the bowl and observe what happens to the paperclip.

Think Like a Scientist!

  1. Both the dropped and gently placed paper clips are roughly the same weight. What makes the difference in whether or not they can float?

  2. Why does soap interfere with the paperclip’s ability to float?

How Does it Work? 

Surface tension, a property of the surface of a liquid that resists external force, is what allows water to hold the weight of light objects like paperclips. Liquids have this property because water molecules have a cohesive nature, in that they attract one another to form strong hydrogen bonds, and create a kind of “skin” at the liquid’s surface. Unlike the water molecules inside the liquid that can bond with other hydrogen molecules all around it, a water molecule at the surface does not have a topper layer of hydrogen molecules to interact with. As a result, the top water molecules tend to bond with neighboring ones at the surface more strongly, and thus create surface tension. As water molecules at the surface bond less strongly with air molecules above them, this surface layer also creates a considerable barrier between the atmosphere and the water. In fact, water has the second-greatest surface tension of any liquid. When the paperclip is placed gently on top of the water, it does not break through and separate this top layer of “skin” and, as a result, can float on top. 

You might have noticed that the paperclip will sink to the bottom right away when soap is added. Soap or detergent molecules arrange themselves into a ball shape, whose outside layer is comprised of polar molecules that interact strongly with water molecules and thus reduce the hydrogen bonds at the surface. In this way, soap breaks the surface tension, so we observe that the top layer can no longer hold the paper clip.

Further Exploration: 

Surface tension can be useful to us in many ways! Tents are an example in which surface tension can bridge the woven materials' pores and make them waterproof. Other examples include soap and detergent, as we used in this experiment, that use this property of water to clean dishes and clothes! 

Anticipated Concerns: 

Not all students may be able to get the paperclip to float.

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