Density of gases:

The density of a gas is the ratio between mass and volume:

But this only applies for constant temperatures.

What will happen, if a gas is heated?

In a colder gas, the gas particles do not move very much:(short arrow)

If the gas is heated, the gas particles will move faster and faster(long arrow).
Since the gas particles move faster and faster, they need more space.
So the gas achieves a greater volume:

What happens with the density then?
As the number of particles stays the same, the mass does not change either. But the volume is greater, thus the density of the gas must decrease, as you can see in the formula for the densitiy.