11.12
Heating and Cooling Curves
When a substance—isolated from its environment—is subjected to heat changes, corresponding changes in temperature and phase of the substance is observ…
Heating or cooling a substance leads to temperature changes, followed by phase changes.
Heating a substance increases its molecules' 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.
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’ 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 −20 °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.
The plateau of solid–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.
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–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.
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Q1: Why does temperature remain constant during a phase transition?
During a phase transition, absorbed heat overcomes intermolecular forces rather than increasing molecular kinetic energy. The temperature stays constant until the transition completes because all heat energy goes toward breaking attractive forces between molecules. Once the phase change finishes, additional heat resumes raising temperature.
Q2: What does the plateau on a heating curve represent?
A plateau represents a phase transition occurring at constant temperature. During melting or boiling, the temperature does not rise despite continuous heat input. The plateau's length indicates the duration of the phase change, while its vertical position shows the melting or boiling point temperature.
Q3: How does specific heat capacity affect temperature change in a heating curve?
Specific heat capacity determines how much heat a substance absorbs to change temperature by one degree. Materials with higher specific heat capacities require more heat for the same temperature increase, resulting in steeper slopes on heating curves. The relationship is expressed as q equals mass times specific heat times temperature change.
Q4: What is the difference between enthalpy of fusion and enthalpy of vaporization?
Enthalpy of fusion measures heat required to melt a solid into liquid at constant temperature. Enthalpy of vaporization measures heat required to vaporize a liquid into gas at constant temperature. Vaporization typically requires significantly more energy than fusion because gas molecules are farther apart and intermolecular forces are completely overcome.
Q5: How does a cooling curve differ from a heating curve?
A cooling curve plots temperature decrease as heat is removed from a substance, creating a mirror image of a heating curve. Like heating curves, cooling curves show plateaus at phase transitions where temperature remains constant as intermolecular forces re-establish. The slopes reflect specific heat capacities of each phase during cooling.
Q6: Why does boiling become more vigorous when heat is applied faster?
When heat is supplied at a greater rate during boiling, the temperature remains constant because the substance is already at its boiling point. Instead of raising temperature, excess heat accelerates the vaporization process, causing more vigorous bubble formation. The temperature plateau persists until all liquid has transformed into gas.
Q7: What determines the slope of a heating curve between phase transitions?
The slope between phase transitions depends on the specific heat capacity of the phase being heated. Solids, liquids, and gases have different specific heat capacities, producing different slopes. A steeper slope indicates lower specific heat capacity, meaning less heat is needed to raise temperature by one degree.