When
things get hot the atoms and molecules vibrate so they take up more room – in other words
the volume of the solid of which they are part gets bigger.
Now as you know
Density = Mass/Volume
The volume has got bigger due to the greater vibration of
the particles but the mass has stayed the same – there is still the same number of
particles.
So bigger volume and the same mass means that the density will go
down.
The same thing usually happens when a material is melted. With the
exception of ice, which contracts when it melts and turns to water, generally solids will
expand when they turn to the liquid. The bonds between the molecules break and so they
can move about more.
Remember that DURING the actual melting the
temperature of the material does not change.
There is a really nice diagram of the variation of the
forces between molecules in the 16-19/Thermal Physics section of the site.
(See:
Forces between molecules)