Most of us are aware that inhaling a mouthful of helium can give a person's voice a humorous, high-pitched tone that mimics a vocal recording played at high speed. But what some of you may not know is there is in fact another gas that has exactly the opposite effect, as a simple science lesson recently delivered by a high school teacher shows.
The Salt Lake City, Utah instructor, who goes by the name "dchummer" on YouTube, decided to give his Cottonwood High School chemistry class a little info-tainment by inhaling a small amount of sulfur hexafluoride (SF6). The inhalation immediately changed his voice into a deep baritone reminiscent of the voice of Star Wars character Darth Vader, much to the delight of the students.
Since the speed of sound depends on the density of the medium that it's passing through, sulfur hexaflouride (which is dense enough to lowers the speed of sound through it by nearly 60%) causes sounds resonating through the vocal tract to decrease in pitch, a demonstration the instructor says is an illustration of Graham's Law of effusion.
You can check out the sulfur hexafluoride demonstration in the video below.