Since the balloon itself is heavier than air, it must be filled with a large volume of something much lighter—either hot air or a very light gas, such as helium. Because the combined weight of the balloon and the gas is less than the weight of an equal volume of surrounding air, the balloon rises.
Hot air is a lot less dense for a few reasons. Hot air essentially means the particles have more kinetic energy, and move around a lot more. Cold air is dense because the particles move a lot less, have less energy, and are closer together.
