Evaporative cooling

Water leaves, and heat goes missing!

15 minutes


  • Carefully review the general safety advice on the back of the box cover before starting the experiment.
  • Observe safety precautions when working with boiling water.
  • Avoid touching heated objects with bare hands.


  • Dispose of solid waste together with household garbage.

Scientific description

What happens when you heat an object? Everything is made up of tiny particles called molecules . Yet, the same object can be both hot and cold—what does this mean for its molecules? They are constantly moving  at different speeds. Heating an object accelerates its molecules and increases the object’s temperature.

In solid objects, the molecules are held tightly in place , and when heated, they merely begin to vibrate  faster, until the point that they vibrate so intensely they begin to break the bonds  holding them in place. When this happens, the object will gradually transition from its solid to its liquid form—in other words, it will melt.

Molecules in liquids move more freely. Leave a drop of water on a table—and it dries up over time. However, its molecules do not disappear, but become vapor—gaseous water—and drift away , mixing with the air.

The process of converting a liquid into a gas is called evaporation. During evaporation, the fastest-moving (i.e. hottest) molecules  gradually abandon the liquid for the air. Hot water has a lot of fast molecules that are eager to leave the liquid. Once they do, mostly slow molecules remain—that’s why the temperature of the remaining liquid lowers. And the thermochromic screen reflects this cooling process.