A balloon will float when the gas inside the balloon is less dense than the air surrounding the balloon. Both helium and hydrogen have lower densities than air. This means that they weigh less than air when compared over the same volumes. A balloon filled with a certain volume of hydrogen will float because the hydrogen weighs less than that same volume of air that the filled balloon is displacing.
Floating a Balloon
Hydrogen can be generated when sodium hydroxide is placed in contact with aluminum. Sodium hydroxide is in some common household cleaning supplies found at the grocery store. The aluminum can be obtained from soda cans or kitchen foil. To fill a balloon with hydrogen, attach a balloon over the mouth of a bottle that holds a mixture of aluminum, sodium hydroxide, and water. The balloon will slowly fill with the hydrogen gas being produced from the mixture. Once the balloon is full and tied off, it will float.
Hydrogen gas is extremely flammable. Do not allow any open flames, sparks or hot objects near the balloon. Sodium hydroxide is a corrosive substance that is hazardous to the skin and eyes. Skin protection like gloves and eye protection like goggles or a face-shield should be used when handling sodium hydroxide.