The speed of sound is given by v =rad(gp/p). Since the density p is proportional to the molecular weight A at STP, then v = 1/ rad(A) for monatomic gasses. (In comparing with air we also have to take into account the different g.) For a light gas like helium the speed of sound is higher, and more wavelengths can fit in a fixed distance like an organ pipe or human voice box. The pitch is therefore raised by this same factor 1/ rad(A), about 3 for helium. You can breathe some helium and demonstrate the Donald-Duck-like effect, or blow some into an organ pipe to raise its audible frequency.
What gas shall we use to lower the pitch of the voice?- a gas of high molecular weight, but another most important property is that it be non-toxic. Some possibilities are listed below:
|Gas||Molecular weight||Approximate price for 1 mole (22.41) at 99.995 purity|
|air||29||free for now|
|radon||222||trace quantities supplied free in some municipalities|
Radon is the heaviest gas, and, chemically it is completely inert. However, since its longest lived isotope has a half?life of only 3.8 days, sulfur hexafluoride is the gas of choice. This gas is colorless, odorless, tasteless, water-insoluble, thermally stable, non toxic, and non-reactive. As shown in the diagram, the six fluorine atoms completely surround the central sulfur atom protecting it from attack. The strongly electronegative fluorine atoms make this gas very difficult to ionize. It is a much better gas-phase insulator than air; thus, it is used in high voltage electrical switches. You might wish to demonstrate the heaviness of this gas by filling a balloon and then throwing the "lead balloon" across the room.
Our formula above predicts that the speed of sound in sulfur hexafluoride is about half as fast as in air, so the pitch is halved.
Lecture bottles of He are available for demonstration. It is a good idea to allow some breathing space between the demonstrations to make sure you keep getting plenty of oxygen.