Liftoff! : Just A Lot of Hot Air

Adding heat energy to gas particles causes them to move faster, spread apart and occupy a larger amount of space (in science, called volume). Learn about how this applies to hot air balloons and why they float.


Adding heat energy to gas particles causes them to move faster, spread apart and occupy a larger amount of space (in science, called volume). In calculating the density of matter through the formula Density = Mass/Volume an increase in volume, by heating the sample and accelerating the particles, causes a decrease in the density of the sample. The heated, less dense gas is then displaced by the cooler, denser gases surrounding the sample (here contained in a thin metallic polyester sleeping bag).

“Containing” the sample by tying off or sealing the gas in the survival bag creates a sealed mass of warm gas that can be “launched” around the laboratory to move around on top of a layer of cooler ambient or surrounding gases. Heat energy, moving from hot to cold, will slowly leave the sample until its temperature equals the temperature of gases surrounding it. As the gas in the silver bag cools, the bag slowly drops to the floor.

More Science Please: Observant students will comment (correctly) that you are not simply heating the gas, but also mechanically pumping it into the sleeping bag. Ask the class to propose a test to see if the heat is in fact the factor affecting the density of the gas. One simple method of testing the effect may be to fill an identical bag with cool gas by adjusting the fan setting on the hair dryer to high, yet the temperature setting to low. The two silver bags, inflated side by side, should exhibit different densities according to the temperatures of the gases inside them. You may also simply repeat the experiment with the hairdryer set at a cool setting, yet allow the bag to return to room temperature before doing so. You may also test the cool gas, your control setup, first to omit the hidden variable of heat energy remaining in the bag (an insulator) after the heated trial.

If you are running this experiment, try to cool the classroom first to amplify the temperature gradient between the contained and ambient gases. Alternatively, you may take the class to an unheated space if possible, or outside (tie the bag with kite string to avoid losing it in the wind and creating a possible hazard if the sleeping bag drifts into traffic).

But what if…: If you can’t find a silver polyester emergency survival sleeping bag, substitute a large construction debris bag. If you are running this experiment on a cool but sunny day, the black plastic sack may launch even if it is inflated with cool gas because the infrared radiation of the sun will be absorbed by the dark surface of the bag. If you can find a silver polyester safety blanket, you may seal if on three sides to fabricate an inflatable structure, yet use a thin stream of adhesive to avoid adding too much additional mass to the structure and inadvertently making it too difficult to “launch” in the process. There are similar products marketed by science supply companies including a very long “solar” black tube, yet it’s quite expensive, fragile, and normally works only on the most calm, still and bright days.