Some chemical formulas, like H2O (a.k.a. water), are almost as familiar to us as the words themselves. But why do we write them the way we do? The answer goes back to ...

For example, the formula H 2 O shows that a water molecule contains two hydrogen atoms and one oxygen atom.

Melting and freezing are both physical changes, which are often easy to reverse. To reverse this process, the water would need to be cooled enough until it freezes back into ice. This works because ...