Iron floats

Shortly after the paper with the paper clip is placed on the surface of the water, it soaks up water and sinks. However, the paper clip continues to float on the surface of the water. If you add a drop of washing-up liquid to the water in the next step, the paper clip will also sink.

Place a piece of blotting paper or kitchen roll paper that is slightly larger than the paper clip on the surface of the water. Then carefully place the paper clip on top. In the second part of the experiment, a drop(!) of washing-up liquid is added.

• several containers with water,
• washing-up liquid,
• paper clips,
• blotting paper, "kitchen roll paper" or similar,

• The paper clip should be as flat as possible.
• If nothing works at all, it helps to grease the paper clip a little.
• In the experiment above, a Tempot handkerchief was used. This soaks up very quickly and sinks. You therefore have to be very quick when placing the clip. Less absorbent paper is certainly more suitable at school.

Plastic paper clips were used for the recordings because they are more visible. For teaching purposes, however, all-metal staples are better because they are even heavier and therefore appear less buoyant.

The technical background to the experiment

The floating of the paper clip is due to surface tension. This is a result of the forces between the water molecules. The individual water molecules exert attractive forces on each other (cohesive forces). For a molecule further down in the water, the cohesive forces from the different directions cancel each other out. At the surface of the water, the upper partners are missing, i.e. there is an overall downward force. One can imagine that the molecules further down are constantly pulling on those at the surface of the water. Consequently, it can be said that the water surface is like being stretched over the water. In order to bring a molecule to the surface of the water, it must be transported to the surface against the pulling forces. Each expansion of the surface therefore costs energy. For this reason, the water "resists" an increase in the water surface. This "resistance" ultimately leads to the paper clip being carried on a rubber membrane. The detergent reduces the surface tension. This means that "soap molecules" are deposited between the water molecules and thus reduce the surface tension. As a teacher, care should be taken to comment appropriately on the obvious idea of a "skin of water". A naive understanding of the "skin" supports the idea that the "skin of water" could be destroyed not only with detergent, but also with a needle, etc.

What the experiment has to do with everyday life

The perfect example of a user of the surface tension of water in nature is the water strider. It can easily walk on water without sinking. The observation of such a water dweller is very suitable for introducing and explaining surface tension.

Which experiments go well with this

With a little skill, you can also place a paper clip and a sewing needle on the water. Greasing helps here too.

Carolin Schneider & Bastian Fleck