Short Notes:

At sea level, where atmospheric pressure is measured as 1 atmosphere, water boils at 100 °C, and becomes a gas. At high altitudes, where there is less pressure, water boils at a lower temperature. Similarly, dry ice, or frozen carbon dioxide, changes at 1 atmosphere and room temperature from a solid to a gas without melting first. Carbon dioxide is liquid only at more than 5.1 atmospheres, as in a fire-extinguisher. When it is released into the air at 1 atmosphere, carbon dioxide becomes a gas. Pressure determines whether, at a given temperature, a substance will be solid, liquid, or gas.