Sink or Float?

Basic gas laws rule how gas particles to change behavior based on internal pressure, external pressure, temperature and volume.When heat is applied to a helium balloon the helium particles will bounce around inside the balloon more often and with a bigger force, increasing the internal pressure. This increases the volume of the flexible container and decreases the density of the balloon causing it to rise.

The opposite will happen if you apply something cold to a helium balloon. The helium particles inside a balloon will slow down a considerable amount and hit the sides less causing the internal pressure to decrease. This lowers the volume, shrinks the balloon, and increases the density making the balloon to fall to the ground.