Three balloons are filled with gases. Two of the balloons rise to the ends of their strings, while the third hangs down. When a small flame is applied to the balloons:
First, whether a balloon rises or sinks in air depends on the density of the gas within it. One balloon contained oxygen gas (O2), which is more dense than air. That balloon sank. A second balloon contained hydrogen gas (H2), which is less dense than air, and it rose. The third balloon contained a mixture of H2 and O2. The mixture is also less dense than air, so it rose.
Second, hydrogen gas is combustible, that is, when a flame or spark is applied, it will react rapidly with oxygen to form water and produce heat and a flame. The reaction is so fast it creates an explosion.
Oxygen itself, contrary to popular belief, is not combustible! It's what other things react with when they combust. So when a flame was applied to the balloon containing only oxygen, it just popped.
When the flame was applied to the balloon containing only hydrogen, it combusted, but because it takes a little time for the hydrogen in the balloon to mix with the oxygen in the air before it can react, that combustion was relatively slow. It made a low-pitched boom.
Finally, when the balloon containing both hydrogen and oxygen was ignited, it exploded with a very loud bang! That's because when the hydrogen and oxygen are intimately mixed, the reaction happens much faster!
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explanation furnished by UCCS's Dr. David Anderson