ASK A SCIENTIST
Question: Why does helium make a baloon float?
Before addressing your question about helium balloons floating in air, let's first think about the more noticeable observation that certain things float on water while others sink. Some substances, like cooking oils, are less dense (not packed together as closely at the level of individual molecules) than the water, and therefore they form a layer that floats on top of the water layer. Other substances, like steel, have a density far greater than that of water, and would therefore be expected to sink. Very large ships made of steel, however, are able to float on water despite this material's greater density. The issue in the case of a steel ship floating on water or a helium-filled balloon rising in air involves a consideration of the downward force of gravity on the object and the buoyancy (the upward force from the displaced water or air) experienced rather than the individual density of any one part of our system.
A simple experiment that you can try at home is to take two small squares (3" x 3") of aluminum foil and press one into a very tight ball while shaping the other into a boat. Now test each in a cup of water to determine which shape will float and which will sink. For each shape, the mass of the object is nearly identical because each started with roughly the same size square of foil. The mass of water moved aside or displaced will be equal to the mass of the foil. In the case of the ball, the mass of the aluminum is in the compact form of a sphere, so when an equal mass of water is pushed away by the sphere there is not enough buoyancy to counteract the force of gravity and therefore the ball sinks. In the case of the boat, the mass of the foil is spread over a much greater area and the air inside the boat has a very small mass compared to the water displaced. Thus the buoyancy from the displacement of water is greater than the force of gravity on the boat and the boat floats. If the walls are not high enough or if you have a hole in your boat, water will rush in and fill the space occupied by the air. This makes the ship's mass too great to be supported by the buoyancy from the water it displaces and, as with the ball, the foil will sink in the water. Archimedes of Syracuse (c. 287 – c. 212 BC), a very famous scientist and mathematician of the ancient world, is credited with being the first to understand and explain why an object floats or sinks in a liquid or gas; the finding is often referred to as Archimedes' Principle in his honor.
Having had a chance to experiment firsthand with buoyancy in water, let's address your original question. The sea of water that supported our boat is now replaced by a sea of air. It is so easy to forget that we live at the bottom of a column of air since it is invisible and not something that we generally feel when the air is still. Nevertheless, it is always present and it is the key to answering your question about why helium-filled balloons will float while balloons that you fill yourself with air from your lungs will sink. An empty balloon experiences a force due to gravity that is much greater than the buoyancy from the air it displaces, and it therefore sinks. An air-filled balloon will also sink, but not as dramatically as the empty balloon because its gravitational force is closer to the buoyancy it experiences. Due to the very small mass of helium gas, a helium-filled balloon experiences greater buoyancy than the force exerted by gravity and it floats up in the sea of air. Hopefully now you can understand why even very large objects like blimps can float in air when enough helium is added to allow for the buoyancy of the air to overcome the force of gravity on the object.
Ask a Scientist appears Thursdays. Questions are answered by faculty at Binghamton University. Teachers in the greater Binghamton area who wish to participate in the program are asked to write to Ask A Scientist, c/o Binghamton University, Office of Communications and Marketing, PO Box 6000, Binghamton, NY 13902-6000 or e-mail firstname.lastname@example.org. Check out the Ask a Scientist Web site at askascientist.binghamton.edu. To submit a question, download the submission form(.pdf, 460kb).