Experiment water, surface tension, paper clip

If you put a paper clip on water, it goes down – of course. However, under certain circumstances, it can also float on water, as can be seen in this experiment. The question of what happens when you add a little detergent to the water is also interesting. (More experiments with water)

What is needed for this experiment?

  • a glass of water
  • paperclip
  • a piece of tissue or blotting paper
  • some dish soap or soap

What to do in this experiment?

Take the piece of paper handkerchief or blotting paper that is just big enough to fit the paper clip on it, put it on the water and carefully put the paper clip on it. Lo and behold: the paper sinks, the paper clip floats. You can also try to fill the glass with water so that the water level arches slightly over the edge of the glass. Then you carefully slide the paper clip onto the "water hill". If none of this works, it can sometimes help to grease the paper clip very easily – for example, by rubbing it on the forehead or nose. If the paper clip then floats, you can take one small Add drops of detergent or some soap to the water and watch what happens.

What happened in this experiment??

The paper clip is floating due to its Surface tension on the water. Detergent or soap destroy this surface tension and the paper clip goes under. Depending on how high the water level was in the glass, this also overflows.

The surface tension of the water arises because water molecules interact with each other attract. For a molecule inside the water, these attractive forces act from all sides – top, bottom, right and left. However, the molecules of the water surface are only pulled from below and from the side, so that the top water layer has a special stability and rests on the water like a skin. This can be clearly seen in this experiment.

Dishwashing liquid as well as soap consist of so-called tensides. Your molecules have a water-loving and a water-repellent part. If you add detergent or soap to water, the water-loving part turns to the water molecules and squeezes between them. This reduces the surface tension of the water. The rest of the surfactant molecule looks out of the water. Only when there is no more space on the surface of the water will the surfactants be distributed in the water.

This everyday phenomenon and the physics of the soap bubble

Probably the most beautiful aspect of the surface tension is foam and soap bubbles. A soap bubble is nothing more than a thin layer of water, on the outside and inside of which a soap film is applied. The water-loving parts of the soap face the water, the rest hangs in the air. The soap reduces the surface tension of the water so much that the bubbles get sufficient stability. If you mix soap bubble liquid with too little soap, the surface tension is too high and the bubbles burst immediately.

The soap molecules also prevent the water from evaporating too quickly, because drying out is the death of the soap bubble. Anyone who has made soap bubbles on a rainy day knows that they live particularly long in high humidity. Fog or drizzle is ideal soap bubble weather.

Surface tension is also the reason why a leaky tap drips and the water does not simply run out in a thin stream. Water always strives to keep its surface as small as possible and therefore forms drops. If there is so much water hanging from the tap opening that the weight that pulls the water drop down is greater than the adhesive force that holds the water at the tap, the drop drops.

If you wash greasy dishes with detergent, the surfactant molecules hold their water-repellent part in the fat, while the water-loving part remains in the water. If a fat droplet is completely surrounded by a layer of surfactant, it can be easily washed away.

Further experiments with water

Physical mini-experiment with water: Magic finger in the pepper water
If you sprinkle ground pepper in a soup plate with water and then dip your fingertip into the water with a little soap or detergent, the pepper will break apart.


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Christina Cherry
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