This demonstration fits nicely into a unit on volcanism or density. It will help students realize that there are differences in the densities of gases, and help them understand how this can affect the behavior of gases.
In August of 1986 a lethal cloud of carbon dioxide gas erupted from the depths of Lake Nyos in Africa, killing over 1,700 people in a valley below. Tremendous amounts of the gas, dissolved in the cold, deep waters of the lake, were released as the water came to the surface. As the water rose, decreasing pressure caused bubbles to form just as they do when a bottle of cola is opened. Since carbon dioxide is more dense than air, the invisible gas descended into the populated valley, causing the deaths.
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