Heat Changes Associated with Physical Changes; Heating Curves

Heating curves and cooling curves are graphic tools that show chemists the temperature at which a substance undergoes a phase change. As heat is added to a solid, the molecules begin to vibrate and gain energy. The solid will melt, becoming a liquid. As even more heat is added, the liquid will vaporize and turn into a gas. Of course, substances will eventually cool if removed from the heat source. Gases will condense back into liquids. As temperatures drop, the liquids will eventually freeze back into a solid. Once again, the molecules of the substance will be arranged in fixed positions.