If we fill a glass with water to the brim and carefully place a needle on its surface, we will verify that, miraculously, it does not sink. The truth is that it is not a miracle but a diaphanous example of what in physics is known as surface tension. Inside the glass, a water molecule is completely surrounded by other molecules and all of them are linked together by molecular bonds as if they were linked by springs. On the surface, this molecule does not have any above it, but it does have one next to it and below it.
So if we pull up on it, the “springs” come into action pulling the molecule down. In the same way, when placing the needle we push the water molecules to the bottom and the adjacent molecules push them up to restore them to their initial position. Of course these forces are weak. As far as it is known, only one case is known of someone who has been able to stand on the surface of a lake … Of course, if they were only 2% more intense, we would be unable to get into the pool.
The surface tension is what makes the water form drops and does not spill. And it is that the water is wet, but not much . This is inconvenient when it comes to washing. We need the water to soak the clothes well. This is the second objective of the soap: to reduce the surface tension so that it works more effectively. That’s what happens in our first experiment, the tangible proof that the surfactant components of detergent do their job by lowering the surface tension of water.
The following experiment is what chemists would call the explosive decomposition of hydrogen peroxide (H2O2) , which, very dilute, is what we know as hydrogen peroxide. What happens is that when introducing a catalyst such as potassium iodide into the mixture of detergent (to make foam) and a food coloring (to give it color), the following reaction occurs:
2 H 2 O 2 (liquid) — 2 H 2 O (liquid) + O 2 (gas)
Hydrogen peroxide decomposes in water and oxygen is released. That release happens explosively , causing all that foam to come up into what some funny scientists call elephant toothpaste .
Miguel Angel Sabadell
