Water dome

Procedure

• Pour water into the beaker.
• Add a couple of drops of food coloring (optional).
• Take a coin and wash it well to remove any dirt (dry it well for better results).
• Place the coin on a table and ask yourselves how many drops we could put on top without spilling. Let’s see who guesses or gets closest to the correct number.
• Slowly, keep adding the water drops and observe the incredible dome shape the water forms.
• Now, add a few drops of detergent to the water in the beaker and mix and repeat the experiment.

Theoretical explanation

This experiment highlights a characteristic property of liquids that is very interesting in water: surface tension. In general, in liquids, each molecule is attracted by the nearest molecules in all directions and with the same intensity. However, on the surface molecules, since there are no molecules above them to also attract them, a resulting force directed downwards appears, which tends to bring the molecule into the interior of the liquid, thereby reducing this surface to a minimum. The result is that the liquid appears as if it is surrounded by an elastic membrane, the surface tension, which is responsible for the resistance offered by the free surfaces of the liquids to their rupture.

The number of drops on the coin depends on the size of the drops, whether the coin is larger or smaller, and whether there is dirt on the coin.

However, soap breaks this surface tension. Therefore, in the experiment with soap, the formation of the dome is prevented. Only a very small one is formed, which breaks quickly.

**An experiment could also be done with alcohol instead of and see that fewer drops enter because the surface tension of alcohol is lower than that of water.