Cambridge O Level Chemistry · Syllabus 5070 · States of Matter
Heating curve
What is Heating curve?
A graph of temperature against time for a substance that is being heated at a steady rate. For a pure substance at constant pressure it consists of sloping sections, where the temperature rises because the average kinetic energy of the particles is increasing, separated by horizontal sections called plateaus, where the temperature stays constant while a change of state takes place because the energy transferred is used to overcome the attractive forces between particles instead of increasing their average kinetic energy. The temperature of the first plateau is the melting point and the temperature of the second is the boiling point. A cooling curve is the same graph for a substance losing energy, with plateaus at the freezing point and the condensing temperature.
This definition is part of the States of Matter chapter in Cambridge O Level Chemistry.
Heating curve in context
A change of state is a physical change in which energy transferred to or from a substance lets its particles overcome, or be pulled back together by, the attractive forces between them. Melting, boiling and evaporating take energy in; condensing and freezing give energy out. The covalent bonds inside molecules are not broken, so steam and ice are both still \(\mathrm{H_2O}\). On a heating curve the flat plateaus mark these changes: the temperature stays constant because the energy being transferred is used to overcome the attractions between particles rather than to increase their average kinetic energy.
Questions students ask about Heating curve
Why does the temperature stay constant while a substance melts or boils?
The plateau on a heating curve does not mean heating has stopped: energy is still being transferred the whole time. During a change of state that energy is used to overcome the attractive forces holding the particles together and move them further apart, rather than to increase the average kinetic energy of the particles. Because temperature depends on the average kinetic energy, the temperature does not rise until the change of state is complete. The first plateau is the melting point and the second is the boiling point.