Changes of State

When heat is applied to a solid, it’s temperature and vapor pressure increase with increasing temperature, because the molecules of the solid have increasing kinetic energy. At the melting temperature, heat is absorbed as the solid is converted to liquid at a constant temperature. When melting is complete, the temperature of the liquid, and it’s vapor pressure, increase. At the boiling point, temperature remains constant as heat is absorbed to convert the liquid to the gas phase. When all the molecules are in the gas phase, the temperature again increases as the gas phase molecules acquire additional kinetic energy. On this graph of time verses temperature, the first segment represents heating of the solid. At point A, the solid melts. The temperature remains constant while melting occurs. At B, all the solid has melted, and the temperature again increases as the liquid is heated. At C, the liquid boils. Again, temperature remains constant with time until all the liquid has been converted to gas at point D. Then temperature again increases as gas is heated.