2.5a: Heating Curves SPS7. Students will relate transformations and flow of energy within a system. b. Investigate molecular motion as it relates to thermal energy changes in terms of conduction, convection, and radiation. d. Explain the flow of energy in phase changes through the use of a phase diagram.

Part I: Heating Curve Basics  What happens to the temperature of a block of ice when you put a Bunsen burner underneath it?  You might think that the temperature goes up smoothly, but that's not what happens.  The graph of temperature vs. time is called a heating curve.  Let's look at the heating curve for water :

 Notice that, in general, the temperature goes up the longer the heating continues. However, there are two horizontal flat parts to the graph.  These happen when there is a change of phase.  These plateaus are also called phase changes.

 Notice that, in general, the temperature goes up the longer the heating continues. However, there are two horizontal flat parts to the graph.  These happen when there is a change of phase.  These plateaus are also called phase changes.  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.  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.  Different substances have different melting points and boiling points, but the shapes of their heating curves are very similar.

 For example, this is the heating curve for iron, a metal that melts at 1538°C and boils at 2861°C. Part II: Reading the Heating Curve (Practice)  Take a close look at this generic heat curve.  The letters on the graph represent where changes occur to the temperature of the substance over time.

Part II: Reading the Heating Curve (Practice)  Take a close look at this generic heating curve.  The letters on the graph represent where changes occur to the temperature of the substance over time.  Q1 : Identify the process that takes place during line segment DE of the heating curve.  Q2 : Identify a line segment in which the avg. kinetic energy is increasing.  Q3 : What is happening to the average kinetic energy of the particles during segment BC ? A1 : Boiling (phase change from L  G). NOT evaporation (that occurs below the boiling point). A2 : Avg. KE = temp, so AB, CD, or EF A3: Avg. KE = temp, so it remains the same.