1. Melting Points and Mixed Melting Points
Experiment 1: Identify a compound by its melting
point and mixed melting points. oC oC oC oC oC oC oC oC
Acetamide
p-Aminobenzoic acid
Camphoric Acid
trans-Cinnamic Acid
Malonic Acid
p-Nitrophenol
Resorcinol
Succinic Acid
Urea

2. A sample is put in the bottom of a melting point tube.

3. Put a small amount of the compound in the open end of the melting point tube.
Turn over and tape the closed end on the desk top until the compound falls to the bottom.

4. Sample in the melting point tube.

5. Use a Thiele tube filled with mineral oil to heat your sample.

6. Attach the melting point tube to a thermometer.

7. Heat about 5o per minute until within about 10o of the melting point
Near the melting point heat at 1 - 2o per minute

8. Mel-Temp in the lab

9. Temperature
Finished melting
Starts to melt
m.p. = Start - Finish

10. Why is salt put on snow covered roads?

11. Ice melts! Ice melts at 0oC
What happens to the melting point if salt is added?
Ice melts!

12. Impurities such as salt lowers the melting point of water.
Putting salt on icy roads causes the ice to melt because it lowers the melting point of water.
Impure compounds usually melt lower than pure compounds so the melting point may be used as a measure of the purity of a compound

13. Acetic Acid, CH3COOH, is a colorless liquid that melts at 16.6oC.
Let’s look at the melting point of mixtures of water and acetic acid.

14. Plot of melting point vs
Plot of melting point vs. mole fraction water for mixtures of water and acetic acid.
Eutectic point

15. Cool a acetic acid - water solution with a mole fraction water of 0.9

16. Heat a acetic acid - water solution with a mole fraction water of 0.9

17. Heat a acetic acid - water solution with a mole fraction water of 0.2

18. Pure compounds usually melt over a narrow temperature range, often 1o or less.
Impure compounds melt lower than pure compounds and over a wider temperature range.

19. Melting points are a measure of purity
m.p. = 115o - 119o
m.p. = 118o - 120o
m.p. = 121o - 122o

20. How do you tell what is in each bottle?
Two of these bottles contain benzoic acid and one m-nitrophenylacetic acid.
m.p. = 120o - 122o
m.p. = 120o - 122o
m.p. = 120o - 122o
How do you tell what is in each bottle?

21. Mixed Melting Points
Grind samples together to be sure they are mixed and then measure the melting point.

22. Results m.p. = 120o - 122o Mixed 1 and 2 m.p. = 114o - 117o3
m.p. = 120o - 122o
Mixed 1 and 2
Mixed 1 and 3
m.p. = 114o - 117o
m.p. = 115o - 118o
Mixed 2 and 3

23. 1. Measure the melting point of your unknown
2. Run mixed melting points to confirm identification