Gases, like helium and oxygen, are compared in many different ways, one of which is by density. Density refers to the relative heaviness of the gas in a constant volume. Balloons can be filled with each gas and tested to see which is lighter than the other by how much they float or sink.

Helium is the second most abundant element in the universe. It's an odorless, colorless, tasteless natural gas. This gas requires special drilling procedures to extract it from the Earth. Helium takes up about .0005 percent of the Earth’s atmosphere, but it isn’t bound to our planet by its gravitational pull so we’re constantly losing helium to space. The helium we lose is constantly replaced by the decay of radioactive elements that are released from the Earth's crust.

Oxygen is the third most abundant element found in the universe. It is highly reactive and can be combined with most other elements. Oxygen makes up 21 percent of the Earth’s atmosphere and a large portion of your own body. This element is required by most living creatures on this planet to sustain life. Oxygen can be extracted from liquefied air. It is also produced through electrolysis of water or heating potassium chlorate.

When comparing the density of helium and oxygen, you can fill a balloon with each and see which floats higher. Helium has a density of 0.0001785 per cubic centimeter, while oxygen is 0.001429 per cubic centimeter. Therefore, helium is lighter than oxygen and it will rise higher than the oxygen-filled balloon. The oxygen-filled balloon will sink, with the balloon’s material weighing it down.

Air-filled balloons are not the same as oxygen-filled balloons, so they shouldn’t be confused for each other. Air-filled balloons are comprised of 78.1 percent nitrogen and 20.9 percent of oxygen, with a small amount of trace gases. Nitrogen is actually slightly heavier than oxygen, so an oxygen-filled balloon is lighter than an air-filled one. The difference is not much, but it's there.