This is a question that comes up a lot in science classes, but it’s one of those things that you can’t really understand until you do it for yourself.
The first thing to understand is that the reason balloons go up when they’re heated is because hot air rises. If you heat the air inside a balloon, it expands and as it expands it has a lower density than the surrounding air. This makes it buoyant, which means that it will float upwards until it reaches an equilibrium with its surrounding air.
The second thing to bear in mind is that warm air rises because there are fewer molecules in each cubic centimeter of warm air than cold air. So imagine filling your balloon with cold air at sea level (20°C) and then heating it up by putting it near an open flame or something similar. As soon as you heat up the gas inside your balloon, its temperature will rise above 20°C and start rising upwards towards the cooler atmosphere above where there are fewer molecules per unit volume of space.
Last modified: September 7, 2022