Heating and Cooling Curves
The graph of temperature against time is called a heating curve. What happens to the temperature
of a solid substance when you put a burner underneath it? You might think that the temperature
goes up smoothly, but that's not what happens. Different substances have different melting points
and boiling points, but the shapes of their heating curves are very similar. See the hotplate activity
for examples.
Notice that, in general, the temperature goes up the longer the heating continues. However, there
are two horizontal flat parts to the graph. These plateaus happen when there is a phase change
Kinetic Energy (diagonal portion)
Since temperature is a measure of "Average Kinetic Energy", any change in temperature is a
change in Kinetic Energy. This is reflected in the diagonal portions of a heating/cooling curve.
Potential Energy (plateau, phase change: solid ↔liquid ↔ gas)
1. The first change of state is melting (changing from a solid to a liquid). The temperature
stays the same while a substance melts. For water, this temperature is 0°C because the
melting point for water is 0°C. At the melting point, the kinetic energy of the molecules is
large enough that the attractive forces can no longer hold the molecules in a rigid crystal
structure and the substance melts.
2. The second change of state is boiling (changing from a liquid to a gas). The temperature
stays the same while a substance boils. For water, this temperature is 100°C because the
boiling point for water is 100°C. At the boiling point, the kinetic energy of the molecules is
now greater than the forces that hold the molecules together. The molecules separate as the
substance goes to the gas phase.
Let's examine the heating curve for water.
Cooling Curves
Heating curves show how the temperature changes as a substance is heated up. Cooling curves are
the opposite. They show how the temperature changes as a substance is cooled down. Just like
heating curves, cooling curves have horizontal flat parts where the state changes from gas to liquid,
or from liquid to solid.
Cooling Curve for Stearic Acid
***Note- The melting and freezing occur at the same temperature. During freezing, energy is
removed and during melting, energy is absorbed.
Try This:
1. A sample of water is heated from a liquid at 40oC to a gas at 110oC.
a) On the heating curve diagram provided above, label each of the following regions:
(Liquid only ; Gas only; Phase change)
b) For section QR of the graph, state what is happening to the water molecules as heat is added.
c) For section RS of the graph, state what is happening to the water molecules as heat is added.
2. The graph below represents the heating curve of a substance that starts as a solid below its freezing point.
What is the melting point of this substance?
Base your answers to questions 3 and 4 on the heating curve below, which represents a substance starting as a
solid below its melting point and being heated at a constant rate over a period of time.
3. What is happening to the average kinetic energy of the particles during segment BC?
4. How does this heating curve illustrate that the heat of vaporization is greater than the heat of fusion?
Base your answers to questions 5 through8 on the following heating curve where substance X starts as a solid
below its melting point and is heated uniformly:
5 Identify the process that takes place during line segment DE of the heating curve.
6 Identify a line segment in which the average kinetic energy is increasing.
7 Using "o" to represent particles of substance X, draw at least five particles as they would appear in the
substance at point F.
8. Describe, in terms of particle behavior or energy, what is happening to substance X during line segment BC.
The above notes were modified from http://www.kentchemistry.com/links/Matter/HeatingCurve.htm