In a closed system Density is the inverse of volume.
When you heat up the air the molecules begin moving faster which results in them spreading out. Density is the number of molecules per volume, so if you increase the volume without increasing the number of molecules the density becomes less.
When you heat up the air the molecules begin moving faster which results in them spreading out. Density is the number of molecules per volume, so if you increase the volume without increasing the number of molecules the density becomes less.
