HEATING AND COOLING CURVES TOPIC 7
The graph below shows the heating curve for H2O beginning at -20oC and ending at 160oC. The general shape is characteristic of all heating curves when temperature (average kinetic energy) is plotted along the y axis and time is plotted along the x axis while the substance is being heated at a constant rate.
The first line segment shows ice below its melting point being heated and its temperature rises. Once the temperature reaches 0oC, the melting point of ice, both solid and liquid phases are present and the temperature remains constant. All of the heat energy being added to the system is going to overcoming the forces of attraction between the solid particles as they become liquid particles. Once all of the solid particles are liquid particles, the temperature begins to rise again. The temperature continues to rise until the boiling point of water, 100oC is reached. Once the water is boiling, both liquid and gas phases are present and the temperature remains constant. Once again, all of the energy being added to the system goes to overcoming the forces of attraction between the liquid particles as they can become gas particles. Once all of the liquid particles are gas particles, the temperature begins to rise again.
*When one phase (s, l, g) is present, the temperature (KE) rises
*When one phase (s, l, g) is present, the applied heat increases the temp (KE) of the particles
*When two phases (s->l, l-> g) are present, the temperature remains constant
*When two phases are present, a phase change is occurring; s->l is melting; l-> g is boiling
Notice the first flat part of the line is 0oC, which is the melting point for water. Also notice that the second flat part of the graph is 100oC, which is the boiling point for water. Heating curves always flatten out at melting and boiling.
Line segment 1: KE increasing, PE remains the same
Line segment 2: KE remains the same, PE increasing
Line segment 3: KE increasing, PE remains the same
Line segment 4: KE remains the same, PE increasing
Line segment 5: KE increasing, PE remains the same
Energy is being added to the system; therefore, if the KE (temp) is remaining the same, the PE must be increasing. And if the KE is increasing, the PE must be staying the same.
A cooling curve is the mirror image of a heating curve. Instead of heat being applied, or added, heat is being removed at a constant rate. If a single phase is present, the temperature of the substance decreases. If two phases are present (g->l; l-> s) a phase change (condensing or freezing) is taking place and the temperature remains constant.
This graph shows water vapor (g) cooling from 140oC to 100oC. Once it reaches 100oC, the water vapor begins to condense (g->l) and the graph flattens out until the phase change is complete. Once all the particles are in the liquid phase, the liquid water cools until it reaches 0oC. At 0oC, the water begins to freeze (l->s) and the graph flattens out again. Once all of the water is in the solid phase, the temperature begins to decrease again as the solid is cooled.
Each time a single phase is present and the temperature is decreasing, or the KE of the particles is decreasing, and therefore the PE remains the same.
Each time two phases are present (a phase change is taking place) and the temperature remains constant, or the KE of the particles remains constant, and therefore the PE decreases.
Heating curves and cooling curves can both be read backwards. If you read a heating curve backwards, you get its cooling curve. If you read a cooling curve backwards, you get its heating curve.
Below illustrates how a heating curve is related to the heat formulas, q = mHf, q = mHv, and q = mC∆T. Remember, heat of fusion is for melting (and freezing), heat of vaporization is for boiling (and condensing), and q = mC∆T is for raising (or lower) the temperature.