What are Heating and Cooling Curves?

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Lesson Transcript
Instructor: LaRita Williams

LaRita holds a master's degree and is currently an adjunct professor of Chemistry.

In this lesson, we will define heating and cooling curves. We will discuss how to understand and interpret such curves from both the heating and cooling perspectives by studying a generic example.

What are Heating and Cooling Curves

What happens when you let a cup of ice sit out on the counter for several minutes? It melts, of course! The increase in temperature causes the water to change from a solid to a liquid. What happens when you pour that cup of water into a pot on the stove and let it cook for several minutes? It boils! Once again, the increase in temperature causes the water to change phases, this time from a liquid state to a gaseous state.

In the science world, we use heating and cooling curves to model such physical changes. A heating or cooling curve is a simple line graph that shows the phase changes a given substance undergoes with increasing or decreasing temperature.

Interpreting the Curve: Heating

Example Heating/Cooling Curve

On a heating and cooling curve, such as the generic one shown above, temperature is measured on the vertical y-axis and the amount of heat added over time is measured on the horizontal x-axis. For the heating and cooling curve of any given substance, the solid phase of that substance will be represented in the lower left corner of the graph, where the temperature is at its lowest and the amount of heat added is also relatively small.

Solid Phase: If we start at the solid phase and follow the graph from left to right along the x-axis (in the direction of the red arrows), we notice that as more heat is added over time, the temperature steadily increases until the graph eventually reaches its first plateau. In the figure, this constant rise in temperature is marked by the area of the graph with a positive, increasing slope labeled 'solid.' Here, although heat is being added and the temperature of the substance is going up, the substance remains in the solid phase until the temperature becomes hot enough to begin melting the solid.

Melting: The temperature at which a substance changes from its solid phase to its liquid phase is known as the melting point. In the figure above, the melting point is observed at the first plateau, or flat line, above the solid phase. Here, the substance exists as a mixture of both the solid and liquid phases, and the temperature remains unchanged (even as heat is being added) until all of the substance has melted.

Liquid Phase: When all of the substance has melted, the only phase present is liquid, and the temperature will once again continue to rise as heat is added. In the figure above, this second steady rise in temperature is marked by the area of the graph with a positive, increasing slope labeled 'liquid'. Here, although heat is being added and the temperature of the substance is rising, the substance remains in the liquid phase until the temperature becomes hot enough to begin vaporizing the liquid.

Vaporization: The temperature at which a substance changes from its liquid phase to its gaseous phase is known as the boiling point. The boiling point is observed at the level line above the liquid phase. Here, the substance exists as a mixture of both the liquid and gaseous phases, and the temperature remains unchanged (even as heat is being added) until all of the substance has been evaporated.

Gas Phase: When all of the substance has evaporated, the only phase present is gas, and the temperature will once again continue to rise as heat is added. In the figure above, this third steady increase in temperature is marked by the area of the graph with a positive, increasing slope labeled 'gas'.

Interpreting the Curve: Cooling

Studying the same curve from the cooling perspective, we'll observe those same phase changes, just in the reverse direction.

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