# a 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, there are 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, look for other files called cooling.

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 might think about
• 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 a similar 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.

### 8 Responses

1. tanuj says:

can the temperature go low when you heat things up?

• roger says:

Hello – in this setup, I can’t imagine that the temperature would go down when you heat things up. (If I fail to stir or agitate the tubes as they’re heated odd things CAN appear to happen). I am prepared to believe that there’s a chemical that does that somewhere in this world – but it’s not in any classroom or school exam.

2. vian says:

would anything change if you were to use more steric acid in the experiment?

• roger says:

I wonder what you think might change.
The science says that the shape of the graph stays the same. We never ever bothered to weigh the stearic acid.

Perhaps the times change. Maybe the temperatures do.

3. David says:

How do you decide how long you should continue with the investigation. Do I wait till the stearic acid reaches room temperature or is there a set time limit where I should stop the experiment?

• roger says:

You can stop when the stearic acid curve shows a plateau and then cools at different rate. That’ll happen when it’s dropped to 80′ degrees.
If you don’t see a plateau try again … I put the boiling tube, stearic acid and probe in a beaker get these all at say 95 -100′. Start recording and then remove the tube to the beaker water (at room temp).

4. tal says:

i was wondering if you could explain how the molecules change as stearic acid is changing from a liquid to a solid? and also, how the forces and bonds change with this (if they even do). its for a research project and i cannot find anything online that goes into detail about this

• roger says:

Thank you – you ask a really good question about what’s going on. I can guess at an answer but you deserve a better answer.