Show Me Some Science: Balloon Buoyancy

There’s nothing intrinsically “floaty” about helium. You can make a helium balloon that sinks, if it’s cold enough, as we see in this video.

At normal room temperature, a helium balloon will rise, as you know. The weight of the rubber balloon plus the gas inside is less than the weight of the air that is displaced. Displacing a certain weight of air means that there will be an upward buoyant force. If this force is larger than the weight of the balloon plus the gas inside, there is a net upward force on the balloon, and it will rise.

But what happens if you cool off the gas? When we pour liquid nitrogen over a helium balloon, the gas inside contracts. The mass of the balloon and the gas inside is just the same, and so is the weight. But the smaller volume means less air is displaced—so the upward buoyant force is smaller. Just as large a downward force and a smaller upward forces means a net downward force—the balloon will sink.