Structures and Densities

Structures and Densities

Things are made up of atoms. A body can be a soild, a liquid or a gas.

When a body is a solid, the atoms are packed closely together. They cannot move away, and can only vibrate about a fixed position.

If the body is heated, it can melt and become a liquid. In the liquid, the atoms are able to move around. They are usually slightly further apart than in the solid, so there is room to move. This means the volume increases slightly, so the density would decrease.

On further heating, the liquid can vaporise. In a gas, the atoms are far apart, with plenty of empty spaces in between. The volume is much larger than the liquid, So the density becomes very small.

There are some exceptions to the above picture. Ice is less dense than water, because the way the water molecules are arranged in ice creates more empty spaces in between. That is why ice floats on water. Iodine does not melt, but sublimes - i.e. the solid turns straight into the gas on heating. Iodine gas if of course less dense than the solid.

The Physics Notes