Xenon (atomic number 54) is colourless and odourless. It is one of the 7 noble gases that inhabit group 18 of the periodic table - the Magnificent Seven as my school chemistry teacher used to call them. Among other things, xenon is used as a general anaesthetic, in spacecraft propulsion and arc lamps (like those that are used in IMAX projectors).

As you might have seen/heard, speaking with a lungful of helium changes the sound of the human voice. This is due to that fact that the sound waves are moving through a gas that is less dense than air, which changes the speed of sound (but not the frequency/pitch).

Under most conditions, xenon is considerably denser than air. We did a couple of experiments to see what effect xenon had on the sound of the human voice.

The pitch of the voice was not altered, but the timbre of the voice was. There is more extensive explanation of the physics behind this at the University of New South Wales School of Physics site.

Please do not try this at home. Breathing anything other than air can be dangerous and gases that are denser than air can settle in the base of your lungs, leading to asphyxiation.