Melting Point Simulation
Percy Julian and Josef Pikl used the fact that melting pointthe temperature at which a substance changes from a solid to a liquidis a characteristic property of a substance to prove that the British chemist Robert Robinson could not have succeeded in synthesizing physostigmine. What do we mean by a “characteristic property”? Basically it means that every substance has a very specific melting point. For example, the melting point of ice (solid water) is 0°C. It’s not -1°C or +1°C, but always 0°C. If you have a substance that looks like ice but melts at -4°C, it’s not ice.
The next to last step in Robinson’s proposed synthesis resulted in a compound, called d,l-Eserethole, that he claimed could be changed into physostigmine with one final chemical reaction. Julian agreed with thishe too knew that d,l-Eserethole could be changed into physostigmine, and if Robinson’s synthesis to that point had in fact resulted in d,l-Eserethole, Julian knew he and Pikl had lost the race. But Robinson reported the melting point of his d,l-Eserethole in his paper, and Julian quickly noticed that it was not the correct melting point of d,l-Eserethole. Robinson had not in fact reached d,l-Eserethole in his next to last step! It was some other compound! And Julian could prove it, by conducting a simple melting point test.
This event illustrates the importance of simple identification tests, such as the melting point test, in chemical research. Substances have other characteristic properties besides melting point that help to identify themdensity and water solubility, for example.
An Interactive Simulation
When you are trying to identify a substance whose identity is unknown to you, one thing you can do is measure its melting point and then compare that melting point to known melting points. In the simulation that follows, you will try to determine which one of five "mystery" substances is physostigmine. Physostigmine’s melting point is given to you; it is between 105°C and 106°C. As you test all the samples, keep these questions in mind:
- If a sample’s melting point is not between 105°C and 106°C, what will that tell you?
- If a sample’s melting point is between 105°C and 106°C, what will that tell you? Will it tell you that the sample definitely is physostigmine? Why or why not?
(Note: The simulation will appear in a separate pop-up window. You will need to turn off your pop-up blocker to run the simulation.)