Helium balloons float because they are less dense than air
Why does a balloon that is filled with your breath sink while a balloon that is filled with helium sink? You may have heard that the balloon filled with helium is lighter than air, however, this isn’t entirely the case. The best explanation for this phenomenon is that the balloon that is filled with your breath is more dense than air while a balloon filled with helium is less dense than air.
Density is a measure of mass per unit of volume. Most often you will find density expressed as g/ml, where g is grams and ml is millitres.
10 grams of air would fill 8 pop bottles while 10 grams of lead would be enough for a small nail
So density is really how much stuff is packed into an amount of space. Here’s an example. If you had 10 grams of air you would have enough air to fill more than eight 2 litre pop bottles. On the other hand, if you had 10 grams of lead you would have an object that is roughly the size as a small nail. Lead is much more dense than air.
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