Question: Why does your voice change pitch when you enhale the contents of a helium baloon?

Jarvist Moore Frost answered on 9 Mar 2012:

Dear Sausageman,

This happens because the speed of sound is higher in Helium than in normal air.

The wavelength of the sound that your vocal cords make is still the same, but as the speed of sound is higher, the frequency has to increase.

The Helium atoms are really lightweight so they move much more quickly at the same temperature than the heavy Nitrogen and Oxygen that makes up most of air.
Helium is also a Noble gas and so it monatomic (just individual atoms flying around), whereas the Oxygen and Nitrogen in air is made out of molecules of two atoms (diatomic), for complicated reasons involving statistical mechanics and degrees of freedom (which you get to learn in your 2nd year of a physics degree!) this also makes the speed of sound slightly higher.

For the same reason, Helium is really conductive to heat! We use helium in our cryostats (things that get really really cold, and then hold the temperature) so that the temperature is transferred really efficiently from the liquid nitrogen to what we’re cooling.
Helium is also sometimes used for a breathing gas when diving. You have to be careful if you’re using helium not to use it to inflate your drysuit, otherwise it’s really cold.