Name: Heating and Cooling Curves
Uploaded: 2020-09-24T15:06:49
Description: Learn by watching this video about Heating and Cooling Curves at JoVE.com

Heating or cooling a substance leads to temperature changes, followed by phase changes.&#160;
Heating a substance increases its molecules&#39; thermal energy, which is reflected as a rise in temperature until it reaches a transition point.
When the substance has absorbed enough heat, the attractive forces between its molecules are overcome, leading to a phase transition at a constant temperature.&#160;
Once the transition is complete, heating results in a rise in temperature again.
When heat is removed from the substance, the decrease in its molecules&#8217; thermal energy corresponds to a drop in temperature, until it reaches a transition point.
Then, the temperature remains constant as stronger intermolecular forces are re-established during the phase change.
The behavior of a substance in response to temperature changes can be modeled using heating curves or cooling curves, where the temperature change is plotted as a function of the heat added or heat removed.
Consider a beaker filled with ice cubes, initially at &#8722;20 &#176;C. As heat flows in, the temperature of the ice rises steadily. The amount of heat absorbed in warming the ice depends on the specific heat capacity of ice.
Once the melting point of ice has been reached, the temperature stops rising despite the continually flowing heat. The effects of the absorbed heat weaken the intermolecular forces until the ice completely melts into liquid water.&#160;
The plateau of solid&#8211;liquid equilibrium is characteristic of the phase transition at a constant temperature. The enthalpy change between the beginning and end of the plateau indicates the amount of heat needed for the melting process, or in other words, the enthalpy of fusion of water.&#160;
After the melting process is complete, the absorbed heat results in a corresponding linear rise in temperature. The specific heat capacity of water dictates the amount of heat absorbed.
At the boiling point, the temperature stops rising. The absorbed heat instead contributes to overcoming the attractive forces between water molecules until the water completely vaporizes.
The plateau of liquid&#8211;gas equilibrium represents the phase transition at a constant temperature. The change in enthalpy between the beginning and end of the plateau is the enthalpy of vaporization of water.
After all the liquid has transformed into vapor, additional heat leads to the temperature rising again.&#160;

heating and cooling curves

When a substance&#8212;isolated from its environment&#8212;is subjected to heat changes, corresponding changes in temperature and phase of the substance is observed; this is graphically represented by heating and cooling curves.
For instance, the addition of heat raises the temperature of a solid; the amount of heat absorbed depends on the heat capacity of the solid (q = mcsolid&#916;T). According to thermochemistry, the relation between the amount of heat absorbed or released by a substance, q, and its accompanying temperature change, &#916;T, is:
<img alt="Eq1" src="/files/ftp_upload/11351/11351_Equation_1.png" style="text-align: center; height: 50px;" />
where m is the mass of the substance, and c is its specific heat. The relation applies to matter being heated or cooled, but not changing state.
When the temperature is high enough, the solid begins to melt (Figure 1, point A). The heat absorbed depends on the solid&#8217;s heat capacity (q = mcsolid&#916;T), and a plateau is observed at its melting point. The plateau indicates a change of state from solid to liquid, during which the temperature does not rise due to the heat of fusion (q = m&#916;Hfusion). In other words, further heat gain is a result of diminishing intermolecular attractions, instead of increasing molecular kinetic energies. Consequently, while a substance is changing state, its temperature remains constant.
Once the solid has completely melted (Figure 1, point B), the liquid starts warming and experiences a rise in temperature. The heat absorbed depends on the liquid&#39;s heat capacity (q = mcliquid&#916;T). When the liquid reaches its boiling point, the liquid begins to vaporize (Figure 1, point C) and the temperature remains constant despite the continued input of heat. Another plateau (constant temperature) is observed at the liquid&#39;s boiling point during the liquid to gas transition due to the heat of vaporization (q = m&#916;Hvap). This same temperature is maintained by the liquid as long as it is boiling. If heat is provided at a greater rate, the liquid&#39;s temperature does not rise, but instead, the boiling becomes more vigorous (rapid). After all the liquid has vaporized (Figure 1, point D), the temperature of the gas increases.
<img alt="Image1" src="/files/ftp_upload/11351/11351_Image_1.png" style="text-align: center; height: 330px;" />
Figure 1. The representative heating curve for a substance depicts changes in temperature that result as the substance absorbs increasing amounts of heat. Plateaus in the curve (regions of constant temperature) are exhibited when the substance undergoes phase transitions.
This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/10-3-phase-transitions">Openstax, Chemistry 2e, Section 10.3: Phase Transitions</a>.

Learn by watching this video about Heating and Cooling Curves at JoVE.com

Heating and Cooling Curves

When a substance&#8212;isolated from its environment&#8212;is subjected to heat changes, corresponding changes in temperature and phase of the substance ...