CHARLES D. WINTERS / SCIENCE PHOTO LIBRARY CHARLES D. WINTERS / SCIENCE PHOTO LIBRARY
Dry ice. A fog-like vapour spills off a dish of dry ice. Carbon dioxide is a colourless gas at room temperature but becomes a liquid under com- pression. If the liquid is allowed to expand rapidly to atmospheric pressure, it cools suddenly and freezes to dry ice - a white snow-like solid. Dry ice sublimes (turns straight into a gas from the solid state) at -78.5 degrees Celsius (195 degrees Kelvin). If mixed with water it produces thick clouds of fog, as seen in theatres and discotheques.
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