Observatoire de Paris
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Water molecule.
Copyright : Meudon Observatory / UFE

Once made by stars, atoms are almost eternal. On the other hand, they can combine to form molecules, which are organized differently, depending on the temperature and pressure of the medium. Thus, oxygen and hydrogen atoms are a billion years old. On the other hand, water molecules (H20) can easily break up to give back hydrogen and oxygen.

We know three principal states of matter :

  • The solid state : atoms (or molecules) are very tight.
  • The liquid state : atoms (or molecules) can move around each other.
  • The gaseous state : atoms (or molecules) are distant from each other.

The atoms can also lose electrons and become ions. When the matter is made up of separate nuclei and electrons, we speak of plasma.

Heating a body shakes up atoms/molecules, and makes matter pass from the solid state to the liquid and then gaseous ones. The temperature, which makes the matter pass from one state to another, depends on the matter and on the pressure conditions. The same amount of matter (i.e. the same number of atoms/molecules) takes up more space in the gaseous state than in the liquid or solid state.