Experimenting With Surface Tension and Buoyancy

The start of this experiment will be exploring the concept of surface tension. Surface tension is the tension of the surface film exerted by a liquid caused by the attraction of the water molecules on the surface layer. If the conditions are right, the molecules will hold tight enough to support your paper clip and make it seem as if it is floating even though it is not. There is a visible film you will be able to see if you look closely enough that is supporting the paperclip. Once the tension is broken with the pencil, the paperclip will sink down as it is not buoyant enough to float on the surface.

The second part of this experiment is discussing buoyancy, which is an upward force exerted by a fluid that opposes the weight of an immersed object. The foil boat will able to float because it is holding air which is lighter than water, in turn making it buoyant.

When you take 2 pieces of foil relatively the same size and change how they are molded, you also change the buoyancy. One that is rolled into a ball traps air, making it lighter and allowing it to float. The piece of foil that is folded over and over, has all the air pushed out and will sink. The foil pieces will have the same mass and density, but will have different buoyancies.

Key Terms

Surface Tension – the tension of the surface film of a liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid, which tends to minimize surface area

Buoyancy – the ability for an object to float within a fluid

Displacement – the occupation of a submerged body or part of a body of a volume which would otherwise be occupied by a fluid

Sink – when the entire volume of an object has become submerged in a fluid

Float – to rest or remain on the surface of a fluid

- Tin Foil Square (min. 3x3 inches [8x8 cm] piece)

- Paper Clip

- 1 Square of Toilet Paper or 1/4th of a Full Size Tissue

- Pencil

- Bowl

- Water

Notice how the tissue stays on a single layer atop the water.

At this point notice how the paperclip is atop an invisible layer. Also, notice how a slightly raised area has formed around the paperclip. The invisible layer and raised edge are the water molecules being held together by the hydrogen (H+) in H2O (water) forming hydrogen bonds. Just imagine all the hydrogen holding together in a net to hold the paperclip up. This is also how some bugs can walk on top of the water.

Adding the next paperclip should make it sink.

Try different designs. Make one with high sides, one with low sides, one with a flat bottom, and one with a v-shaped bottom.

You can also place paperclips into the boat. Notice how individually the paperclips could only float one at a time, but now with the boat increasing the surface area it allows for more paperclips to float atop the water.

Note how the two are still the same mass, but different densities. The foil that was folded tightly leaves no room for air. The foil loosely crumpled into the ball has air pockets within its' structure.

Notice that the neatly folded foil mass sinks due to not having any air within. Notice how the crumpled ball of foil is buoyant and floating on top of the water due to the air trapped in its' folds. Also, try to compact the foil ball tighter and tighter. Each time that you compact the ball tighter forcing air out you should notice that it sinks a little more into the water becoming less buoyant.