Heating/Cooling Curves
From APChempedia, the free AP prepbook
Heating and cooling curves are a graph of a substance's phase changes over time as heat (energy) is added or lost. In a heating curve, an upward sloping line represents the solid, liquid, and gaseous phases, while a horizontal line represents the melting and vaporizing processes. In a cooling curve (the opposite of a heating curve), a downward sloping line passes through the substance's gaseous, liquid, and solid phases. A horizontal line shows the time it takes for the substance to condense and solidify. Heating and cooling curves show that no temperature change takes place while a substance is in the process of a phase change.
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Heating/Cooling Curves Explained
A heating/cooling curve is a graph of a substance ' s transformation from solid to liquid to gas with respect to time.
As you can see from the graph, the line is the heating curve of water. The brown line shows water as a solid. The process of melting occurs in the green line, so both solid and liquid H2O coexist. During the blue line, H2O is a liquid. During the red line, H2O is boiling, so both liquid and gaseous H2O are present. Finally, during the orange line, H2O is gaseous. The green line in which H2O is melting and the red line in which H2O is boiling are parallel. As the graph shows, the temperature of H2O only increases when it is a pure solid, liquid, or gas. The temperature of water does not increase during its transformation from solid to liquid or liquid to gas. Instead, all of the energy that would normally go into heating the water is used to break apart intermolecular bonds that hold the molecules together. The energy to break apart the bonds (heat of fusion/vaporization) is far greater than the energy required to raise the temperature of water by a few degrees.
A cooling curve is the opposite of a heating curve. It shows the state of a substance as it is cooled from a gas to liquid to solid. Instead of melting and then vaporizing, the substance condenses and finally solidifies into a solid. Each substance's heating and cooling curves are direct symmetries of each other. The same amount of energy is gained or loss by the substance during the phase changes and temperature changes.
Experiment
We decided to create an experiment so that we could visually see the phase changes of a substance. So, we used water as the substance that we were going to observe as it is readily available in its solid form. We performed an experiment to gather data to create water's heating curve.
Materials:
Procedure: Heating Curve of H2O
- Put your goggles on and prepare the necessary materials.
- Set up the Macbook, temperature probe, and Logger Pro application.
- Add 150-mL of ice to the 250-mL beaker.
- Place the beaker on ring stand.
- Suspend the probe in the water without touching the sides of the beaker. Use a test tube clamp to hold it in place.
- Start recording temperature with Logger Pro at 30 second intervals.
- Continue collecting data until water begins to boil.
Data Table
Heating Curve Graph:
Our heating curve does not exactly fit that of H2O , but it does illustrate the essential trends. For the first few seconds, temperature did not change as energy was added to the ice. This is because the energy is necessary to break IMFs that exist between molecules of H2O . When the temperature approaches boiling point, more energy is need to overcome IMF's between the water molecules. It takes more energy to form water vapor from liquid, than it does to form liquid from solid ice.
Pictures of our sample of H2O as heat was added:
| In this image, our sample of H2O is a solid ice. Some water is accumulating at the bottom of the beaker because of exposure to heat in the room. |
| Now, as heat has been added to the H2O, the solid ice melts into a liquid. Some ice still remains and will require more energy to melt into liquid. |
| All of the H2O has melted. The water has also heated up. Bubbles have begun to form along the glass. However, it has not begun to boil yet. |
| The water is boiling. It might be difficult to see the bubbles of water bursting at the surface. However, note the decrease in water level that has occurred between this image and the previous one. This change in water level reveals that water has begun to enter the gas phase as water vapor. Please refer to the video clips to see the water boiling. |
This animation shows the changes in phases that occur as energy is added to a substance. Also, it illustrates changes between the phases at the molecular level as well as the relative difference in the strength of intermolecular forces between the phases.
Heating/ Cooling Curve Practice Problems
1. Which phase comes first in a heating curve?
A. Solid
B. Liquid
C. Gas
D. Vapor
2. Does it take more energy to heat one gram of liquid water by one degrees Celsius or vaporize one gram of water at 100 degrees Celsius?
A. More energy to heat the one gram of the liquid water.
B. More energy to vaporize the one gram of liquid water.
C. Equal energy for both.
3. Short Answer Question: Why are heating curves and cooling curves exact symmetries of each other?
1) Answer: A
Solids, on the left side of the graph, are the first phase to in a heating curve.
2) Answer: B
The process of breaking bonds takes far more energy than simply increasing the energy of a liquid without breaking the bonds.
3) Answer: Energy cannot be lost (it can only change forms). It takes the same amount of energy to heat a substance as a substance releases when it is cooled. The amount of energy released/consumed to create or break bonds is also the same. The only difference is that the energy is consumed by the substance during heating and released during cooling, making the sign on the amount of energy different.
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