A hot air balloon may look very different than a helium balloon, but the principle behind how the two types of balloon work is the same. It all comes down to buoyancy and the relationship of buoyancy to floating objects.
Why Helium Makes Things Float
The law of buoyancy, also known as Archimedes' principle, dictates that any body completely or partly submerged in a fluid (a gas or a liquid) at rest is acted upon by an upward, or buoyant, force, the magnitude of which is equal to the weight of the fluid displaced by the body. If you apply this to a helium balloon, the balloon is "submerged" in air (a mixture of gases). The balloon displaces an amount of air. Provided the displaced air is heavier than the weight of the helium (plus the material of the balloon), the balloon will float in the air. On the other hand, if you inhale a little helium you won't float like a balloon, because no amount of helium can make you lighter than the air around you.
Make Balloon Float Without Helium
It might not be an option for party decorations, but another way to make a balloon float is with hot air. A hot air balloon consists of a large bag, known as an envelope, with a wicker basket hanging underneath. An extremely powerful burner within the basket heats the air inside the envelope through a gap. Again, the buoyancy principle applies. For the hot air balloon to float, the weight of the balloon and the air inside it has to be less than the weight of the displaced ambient air. The hot air inside the balloon is lighter than the air surrounding the balloon, because when gas gets hot it expands, dispersing its individual molecules and making them less densely concentrated. The denser air outside the balloon buoys it up and makes it float.
When a hot air balloon needs to be brought back down to the ground, the air inside it is cooled, drawing the air molecules closer together. The more concentrated the molecules are, the heavier the inside air becomes, until it weighs more than the outside air and travels back down.
Problems With Helium
Fears of a global helium shortage have been circulating for years, with implications far more serious than a lack of party balloons. Helium is used in medicine and manufacturing in a wide range of machines and industrial activities, due to its stability and the fact that it doesn’t react easily with other chemicals. The good news is that researchers tracked and located a helium gas field in Tanzania in 2016, and they hope to find more.