Regular ice is made of frozen water, but dry ice is made of frozen carbon dioxide. As ice melts, it turns back into water, and as dry ice melts, it turns back into carbon dioxide gas. Water normally freezes at 32 degrees Fahrenheit (0 degrees Celsius) and boils at 212 degrees Fahrenheit (100 degrees Celsius), but if you take water above or below sea level, the boiling and freezing points change according to the different levels of atmospheric pressure. Carbon dioxide switches directly from solid to gas at normal pressures. The only way to get liquid carbon dioxide is to expose it to very high pressures - - like those in fire extinguishers. Dry ice is a product of expanding liquid carbon dioxide; as the liquid quickly evaporates, the rest of it cools and freezes into a solid. That solid then melts into a gas.
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