Why does the concrete cube have greater mass than the equal-size air cube? Density!
Lighter-than-air flight has much to do with density. The density of a substance is its mass divided by its volume. Various substances differ in density. Which has the greatest mass: a cubic yard of air, or a cubic yard of concrete? The concrete, of course. Therefore, the density of the concrete is also greater.
More Heat, Less Density
The density of a gas changes when its temperature changes. When air is heated, it expands, or takes up more space. As the air expands, its density decreases — hot air is less dense than cool air.
Low Density Gases
Some gases are less dense than air even without the application of heat. Helium and hydrogen are two examples. Helium’s low density makes it excellent for use in balloons and blimps.