A balloon rises because it has buoyancy, a lifting power that gives it the gas inside. Some gases are heavier than air and some are lighter.
It is only when a balloon is filled with a gas lighter than the air that it will begin to rise. The most common gas to fill balloons is helium.
A balloon filled with helium will remain suspended in the air as long as the gas inside and the equipment that is attached to it (basket, ropes, etc.) weigh less than the air in which they are floating.
The balloon will rise to a height, where the weight of the air outside the balloon is in balance with the weight of the air inside it. At that moment, it begins to float.
Gradually, however, the gas inside begins to escape and the balloon begins to descend to the ground.
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