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When you breathe Helium instead of Air, the pitch of your voice
will be higher because of the lower molecular weight of helium. This effect
follows directly from the kinetic theory of gases. The mean kinetic
energy of a gas is given by:
Uk = (1/2) m v2 = (3/2)
k T
where Uk is the kinetic energy of the gas, m is the mass,
v is the mean velocity of the gas molecules, and T is the temperature. k is a constant,
given by R/NA (the universal gas constant divided by
Avogadro's number.)
The velocity of the gas molecules, and thus the speed of sound can now be calculated by:
v = √(3 RT/M)
where M is the molecular weight of the gas (gram/mol.)
The last formula shows that a lower molecular weight results in a
higher speed of sound. Note that also changes in temperature have an
effect, although this effect is small under "normal" conditions
(remember that the absolute temperature is used.)
Sound is produced in your vocal chords by resonance of sound
waves in your throat and mouth (i.e. a cavity). The
resonance of (sound) waves in a cavity is given by:
v = f × lambda
where f is the frequency (pitch) of the standing wave, and lambda
is the wavelength. The wavelength is essentially fixed for a given geometry of a cavity. Thus an increase in the speed of sound will result in
a higher frequency, or pitch of the resonated sound.
Air has an (average) molecular weight of 28.85 gram/mol. Helium has
a much lower molecular weight: 4.00 gram/mol, and will result in a much
higher pitched voice (A "Donald Duck" voice). On the other hand,
argon has a higher molecular weight than air: 39.95 gram/mol, and will
result in a lower pitched voice (A "Goofy" voice). The noble gas
krypton (83.8 gram/mol) would give you an even lower pitched voice.
Both helium and argon are noble gases, which means that
they do not react with any other elements (though rare exceptions
exist.) If you were dealing with pure gases, one hit of the gas
probably would not do any damage; the oxygen concentration in your blood
doesn't drop that fast. Also, the
argon will not displace all the air in your lungs, since both gases will mix.
However, I recommend that you do not attempt this,
since the gas cylinders that you buy for filling helium
balloons may contain toxic impurities. Gas manufacturers produce
many types of commercial gases, and will not guarantee absolute gas purity
unless this is desired by the customer (higher quality means higher
price.) Breathing even trace amounts of toxic gases such as carbon monoxide
can seriously harm you, and possibly kill you.
Not that it stopped me from trying this when I was younger...
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