1.6: 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:
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 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.
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 LG). 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.
Part III: Cooling Curves A cooling curve is essentially a heating curve in reverse. Instead of showing the absorbance of heat, it shows it releasing it into the surroundings. This means instead of showing endothermic phase changes, it shows exothermic changes. Q4: What phase changes can be seen on the cooling curve? A4: condensation and freezing