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Cooling curve

##### I expect that when something cools, it will get cooler in a plodding kind of way. That's true, but when something changes state, the cooling is not so steady.

The graph below shows how liquid stearic acid cools and turns solid. You'll notice a kink in the graph as it sets. The reason for the kink is that during the change of state, heat energy is released as the stearic acid turns solid. How do we know this? Hopefully this experiment will make that clearer.

Unusually, we've used two temperature sensors to measure the cooling. One is in the stearic acid, and one is in some water next to it.

What we did

We heated a tube of stearic acid till it melted. We then placed the tube in some cool water and measured the temperature of the tube and the water.

Our results

You will find some of the following questions easier if you put these results into your data logging software. Click here to get the results, then Open it in your data logging program.

Looking at the results
• Stearic acid is a liquid at 90 degrees Celcius and a solid at room temperature. At what temperature does it appear to setting into a solid?
• How does the graph show you that the stearic acid is getting cooler?
• The temperature continues to fall after it has set solid. Why?
• What happens to the temperature of the water all this time? Where does the water gain its heat from?
• Why doesn't the temperature of the stearic acid change as it sets solid? (Read the introduction above).
• The temperature of the stearic acid doesn't change as it sets solid. What evidence is there that it is actually losing heat?
• What do you think would happen to the graphs if we had kept taking results for another 10 minutes.
What you can do
• If you heated some ice, would it melt and produce a steady or a kinked graph?
• If you froze some water, would it freeze and produce a steady or a kinked graph?
Do our experiment but study the cooling curve of a mixture instead of pure stearic acid. Your graph will look quite different from this one, so give a reason for this.

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