Activity 5C (Interactive)
Melting Point Simulation
In this computer simulation students try to identify physostigmine in a set of unknown samples by measuring the melting points of the unknown samples. Students will learn that melting point is a characteristic property of a substance, that is, it is a property that helps to identify a substance. They will also learn that, while the melting point is a characteristic property, melting points are not unique, in that many compounds share the same melting point. Students should understand that, given this fact, additional tests besides melting point determination are usually needed to determine the identity of a compound.
To give students the experience of using a melting point determination to identify a compound and to give them practice in the kinds of reasoning needed to draw conclusions from experimental data.
Basic computer skills.
No special safety measures.
|Student Ability Level and Grouping||This activity is appropriate for average middle school students working alone or in small groups.|
Students should be made familiar with the concept of a melting point before beginning the simulation.
In this simulation two of the five unknown compounds have melting points matching that of physostigmine (105.5 degrees Celsius). This is meant to illustrate the fact that while the melting point is a characteristic property of a compound, melting points are not unique, and in the real world many compounds have the same melting point. Given this fact, additional tests besides melting point determination are usually needed to determine the identity of a compound.
The simulation was inspired by an event in the quest for physostigmine. Before Percy Julian and Josef Pikl succeeded in synthesizing physostigmine, a British team led by Sir Robert Robinson published claims of having achieved the first total synthesis of physostigmine. His claimed synthesis took many steps, and Julian and Pikl believed that Robinson had misidentified the reaction product (d,l-Eserethole) in the next to last step. To prove this, Julian and Pikl showed that the melting point of the supposed physostigmine precursor did not match the melting point of the compound Robinson claimed it to be. What’s more, they showed it was a compound that could not be converted into physostigmine, thereby proving that Robinson had not succeeded in achieving the first total synthesis of physostigmine. This event illustrates the importance of simple identification tests in real chemical research.
The melting point apparatus pictured in the simulation is taken from an illustration in a textbook that would have been used in the time period when Julian and Pikl were investigating physostigmine. (Holleman, A. F. A Text-Book of Organic Chemistry. New York: Wiley, 1930.)
This activity meets the following National Science Education Standards (Grades 5-8) and Curriculum Standards for Social Studies.
National Science Education Standards (Grades 5-8)
Unifying Concepts and Processes
Science As Inquiry
History and Nature of Science
Curriculum Standards for Social Studies
|For Further Reading||Julian, Percy L. and Pikl, Josef. “Studies in the Indole Series. IV. The Synthesis of d,l-Eserethole,” Journal of the American Chemical Society, 1935, 57, 563. (Julian and Pikl’s critical 1935 paper.)