Hot air balloons float because the air inside them is less dense than the air around them.
An object becomes buoyant when it is less dense, or compact, than another substance around it. For example, a feather floats on water not only because it is lighter than the water but also because the matter that makes it up is not as dense as the matter that makes up the water. This allows the less dense matter to rest on the more dense matter. You can prove this in a number of science projects with easy-to-find items.
Floating Balloon
Fill a balloon with helium and tie a string to it. On the other end of the string tie enough cardboard to keep the balloon's string touching the ground. Cut off parts of the cardboard until the balloon rises. Allow the balloon to rise a few feet off the ground and leave it suspended there. The balloon rises because helium is not as dense as the air, allowing it to rest on top of the more dense air. Expand the experiment by using other gases, such as simple hot air. Will the balloon rise?
Soda Cans
Show buoyancy in water with a 20-gallon tank, a cola can and a diet cola can. Fill the 20-gallon tank with water at room temperature. Put in the regular cola can. It should sink. Put in the diet cola can. It should float. The cola can sinks because its composition is more dense than the water. The diet cola can floats because its composition is less dense than the water. Try different kinds of soda and record which cans sink and float. Try changing the experiment by using cold soda cans.
Aluminum Boats
Construct five boats out of aluminum foil. Each boat should be square and have a half-inch lip. Build your first boat in a 3-by-3 inches square. Build each successive boat by adding an inch. Your last boat should be 7-by-7 inches. Fill a container with room-temperature water. The container should be big enough to comfortably hold the largest boat and allow it to sink. Set the smallest boat in the container. Put one penny in the boat at a time, placing each penny in a different location, until the boat sinks. Repeat this experiment with the other boats. Each larger boat should be able to handle more pennies because each square inch gives the boat more buoyancy by covering more surface of the water.
Pencil in Glass
Fill a glass about 3/4 full of tap water. Using a pencil that is shorter than the glass, stick a thumb tack into the pencil's eraser and put it eraser-down in the water. The thumb tack will cause the pencil to sink. Mark on the pencil with a permanent marker the water level. Refill the glass with warm water and stir in a teaspoon of salt until it dissolves. Put the pencil in again, eraser down. It should float higher than in the regular tap water since the salt water is more dense, causing the pencil to have more buoyancy. Mark the pencil again and stir in another teaspoon of salt. What happens? Try mixing in other substances, such as sugar and baking soda.
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