Why does sound travel faster in helium that is lighter than air but also travels faster in water that is more dense than air?
In: Physics
The speed of sound is related to the speed at which the molecules move, as it is the molecules bumping into each other that allows the pressure wave to propagate. The speed of the molecules depends on the temperature and the mass of the molecule. Temperature is the energy the molecules have, and for a given amount of energy, something lighter moves faster than something heavier.
Air is mostly nitrogen and oxygen, which are formed of diatomic molecules, so one nitrogen molecule has two atoms, same for oxygen. Oxygen has a mass of 16 and nitrogen 14 per atom, so air is composed of molecules of mass 28 (a bit above 70%) or 32 (most of the rest). Helium atoms have a mass of 4, and don’t form molecules. Water is 1 oxygen and two hydrogen (mass 1 each), so a water molecule has a mass of 18.
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