1. Cooling Curves
Aim
Investigate cooling of different substances and use the data obtained from these experiments to construct cooling curves where the rate of cooling can be investigated.
You must calibrate the thermometers to ensure you collect accurate results.
You shall practise your technique with cold copper coins and boiling water. You shall then repeat the method with stearic acid and paraffin wax.
You shall construct cooling curves and find the rate of cooling at different points on the graph.

2. Calibration of thermometer

3. Cooling Curves Thermometer calibration
You must calibrate the thermometers to ensure you collect accurate results.
You should use ice water (00C) and boiling water (1000C). See the method on your practical sheet.
For the liquid filled thermometer, the graduations on the thermometer were at every 10C but you estimated to the nearest 0.50C
The digital thermometer read to the nearest 0.10C and was more accurate as the recording for ice-water was 0.00C and for boiling water it was (100.00C).
However, as you would be reading the thermometer every 60 seconds for the cooling curve experiments, it was hard to note down the exact temperature reading whilst the temperature was changing due to the fluctuating reading on the display. This would mean the actual reading would have to be estimated.
You decided to use the liquid filled thermometer and construct a calibration curve, where the actual temperature reading could be read off.

4. Cooling Curves Thermometer calibration curve
Plot a graph of thermometer readings with ice water and boiling water against actual values they should have been (00C and 1000C). Use this calibration curve to read off the corrected values for your experiments. x
Actual values (0C) x
Thermometer readings (0C)

5. Gradient from cooling curve
(copper coins)

6. Cooling Curves Gradient of cooling curve (copper coins)
Find the rate of cooling at the beginning and at the end of the experiment:
Draw a tangent to the curve at the point.
Gradient is change in temperature /change in time.
Gradient = y/x units: 0C/s
Corrected Temperature (0C) y x
Time (s)

7. Cooling Curves Gradient of cooling curve (copper coins)
Find the rate of cooling at the beginning and at the end of the experiment:
Draw a tangent to the curve at the point.
Gradient is change in temperature /change in time.
Gradient = y/x units: 0C/s
Temperature (0C) y x
Time (s)

8. (stearic acid & paraffin wax)
Gradient from cooling curve
(stearic acid & paraffin wax)

9. Cooling Curves Cooling curve (stearic acid)
Repeat the experiment with stearic acid. Find the rate of cooling at the start, end and where there was a plateau.

10. What you must include
in your write-up

11. Cooling Curves
Method
Write what you did for the experiment (method) in your own words.
Include suitability of equipment and method used, and how you knew how much solid was required. Heating in a safe way.
Graphs: calibration of thermometer, cooling curve for copper coins, cooling curves for stearic acid & paraffin wax.
See next slide for what you must include with each graph.

12. Cooling Curves Cooling curves For each graph you MUST: Have a title
Correctly labelled axes with units
Draw a smooth curve of best fit through the points
Find the rate of cooling at the start, end and where it seems to have change dramatically in-between.
Explain what parts of the graph correspond to the freezing point/melting point.
Use correct mathematical terminology throughout when describing the patterns and trends in the shape of the cooling curves (e.g. rapid increase, decrease, approximately constant etc).
Compare how close your melting points from your cooling curves are to literature and class values.
Link differences in melting points to accuracy in terms of where specific errors or problems with the given method or equipment may have led to inaccuracy.
Improvements that could be made to reduce levels of uncertainty.
Conclusions from the shape of the curves (see next slide).

13. Cooling Curves Conclusions and evaluation
Conclusions linking the rate of cooling to what is happening at a molecular level in terms of positions and velocity of molecules and the forces between them.
Accuracy (e.g. parallex errors, error in thermometer ±0.5cm3, did you use the same size boiling tubes each time as surface area affects rate of cooling, same volume of stearic acid and paraffin wax? How clean was the thermometer after it was removed from the stearic acid and put into the paraffin wax? etc)
Improvements (see the last 4 bullet points in the previous slide). You could have used a data logger which is very accurate and can produce a graph automatically…

14. Cooling Curves Cooling curves
Finding % difference: find the difference in melting point between yours and the literature value. Divide this by the literature value and multiply by 100 to find out the % difference between your value and the literature value. Remember to include the source of information for your literature value.
% difference = difference in melting points x 100
literature melting point

15. Cooling Curves Cooling curves Stearic acid was supplied by Timstar.
93% pure and melting point of 670C quoted.

16. Cooling Curves Cooling curves Paraffin wax was supplied by Timstar.
No data on purity was supplied, melting point of C quoted.

17. Change of state
From cooling curve

18. Cooling Curves
SOLID LIQUID GAS
Melt
Boil
Freeze
Condense

19. Cooling Curves
Cooling curve (stearic acid)

20. Cooling Curves Solid to liquid (melting)
Solid molecules are arranged in a regular shape with all molecules packed tightly together with intermolecular bonds between them. The molecules cannot move around but can vibrate when heated. As heat energy is applied the intermolecular bonds are broken and the molecules can begin to move around (kinetic energy). There is a change of state from solid to liquid. In the liquid the molecules are not so tightly packed and can move around but stay together due to some intermolecular forces still present between the molecules.

21. Cooling Curves Liquid to solid (freezing):
In the liquid phase the molecules are not tightly packed and can move around, although they stay together due to some intermolecular forces of attraction between the molecules. As the liquid cools down, the motion of the particles decreases so the temperature decreases. Where there is a plateau on the cooling curve and cooling temporarily stops as there is a change of state between liquid and solid. There is no change in temperature during this time because energy is released when the intermolecular bonds are formed between the molecules in the solid phase which is why the temperature remains constant. In the solid phase the molecules are arranged in a regular shape with all molecules packed tightly together. The molecules cannot move around but can vibrate when heated.
Once all of the intermolecular bonds have been formed between the molecules in the solid phase, cooling begins again, but shall start to flatten out at room temperature as it begins to stop cooling.