Why does water form lakes and oceans underneath a vast expanse of airy sky? The answer to that, and many more questions, is density! Density is most easily described mathematically by the following equation:
This means that density depends on only these two quantities: Mass, or the amount of matter that something is made up of, and volume, a measure of how much space it takes up.
Most of us have heard of density in relation to things floating in water, but it also works for air. Seeing whether or not something floats in another thing is a great way to compare the density of two substances. Here, we can see that a balloon filled with nitrogen is more dense than air. Air is mostly made up of nitrogen, so it is mostly the additional weight of the balloon itself that causes it to fall to the ground. Alternatively, the helium balloon floats upwards as it is so much less dense than the surrounding air that the comparatively heavy rubber balloon is dragged up along with it. These two objects have the same volume, but their densities differ because of their mass.
The other quantity that we can look at is volume. If the volume becomes smaller, but contains the same mass, the density will increase. Alternatively, increasing the volume will decrease the density. In this demonstration, we take advantage of a property of gases described by Charles’s Law, which states that if we change the temperature of a gas, its volume will change proportionally. By lowering the temperature of the helium balloon with liquid nitrogen, we decrease its volume. This raises its density to be above that of air…until it warms up and expands back to a low enough density to again take flight!
This concept can help to explain many things, even if they seem odd at first. Clouds are made of water, but hang in the sky! It might seem like this is just because clouds are light, but the average cumulus cloud weighs over 1 million pounds! That is definitely not light, but they are light for how big they are, and they are big. Clouds are made up of water, but this water is very spread out. The volume is so big that this 1 million pounds of cloud actually weighs less than an equal amount of air would, even though we always think of air as being pretty light!
However, as this air rises it cools and gets closer together just like the balloon dunked in liquid nitrogen. Eventually, these water molecules band together as they get cold. When enough of the molecules stick together, they get to be more dense than the cloud around it, and plummet back to earth as precipitation.
By the way, the average person is a little less than a thousand times more dense than air. This means if you were 10 times taller, wider, and thicker but not any heavier, you would float like a balloon! Please don’t try this at home.
Written By: Scott Alton