Chemistry

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Materials

• 1 frying pan (or saucepan)
• Cooktop
• Water

Short explanation

When a drop of water comes in contact with a really hot surface, the underside of the drop evaporates instantly. This results in a protective "cushion" of water vapor which insulates the rest of the water drop from the heat.

Long explanation

When water boils, it changes from a liquid state to a gaseous state. During "normal" boiling of water, more or less the whole mass of water has time to heat up and small bubbles of water vapor are formed everywhere in the liquid water (but preferably at nucleation sites, i.e. at impurities or irregularities). But when the heating surface has reached a certain temperature, the so-called Leidenfrost point, the whole underside of the water mass boils at once and a film of water vapor gets trapped underneath the water and lifts it.

The film of water vapor conducts heat poorly, which means that the water mass above heats up very slowly and evaporates after a much longer time than you might expect. The film also causes the water droplet to move like liquid metal and "float" around on the surface, and perhaps also unite with other water droplets (yes, just like in Terminator 2).

The Leidenfrost point for a frying pan and water is around 200 °C (392 °F), a bit depending on, among other things, what the surface of the frying pan looks like and how big the water drop is.

The film of water vapor that forms is about 0.1 mm (1/256 inch) thick. However, the water vapor is constantly pushed away in some direction from under the water drop, which causes the water drop to become rocket-driven and move sideways. A "new" underside of the water drop then comes into contact with the frying pan and evaporates, and this is how the whole drop is slowly consumed.

The water drop has a spherical shape because the surface tension holds the water molecules as close to each other as possible. If the water drop becomes too large, however, it collapses due to gravity. Then the water vapor can not escape from under the drop through the sides, but makes one or more chimneys straight up through the water drop.

The water drop rotates all the time, at an unexpectedly high speed. You can see this if you place a dirt particle on the water drop.

This phenomenon of "water balls" is called the Leidenfrost effect, after Johann Gottlob Leidenfrost who wrote about this in a book in the 18th century, and can be used in cooking. It's a good indicator of when the frying pan is hot enough to add oil and food. With the help of its oily surface, the food, just like water drops, will "float" on the frying pan instead of sticking to it.

Experiment

You can turn this demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• What happens if you use milk instead?
• What happens if you place a dirt particle on the water ball?
• What happens if you create a really big water ball?
• What happens if you pour in a whole deciliter (1/2 cup) of water?
• What happens if you blow on the water ball?
• What happens if several water balls meet?

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