Pyrolysis: Sugar on Fire!

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Heating our sugar cubes to 186 degrees Celsius with our butane torch leads to a chemical reaction called pyrolysis and the formation of caramel, carbon dioxide and water, which we see as a pale brown steaming syrup with a familiar, pleasant nutty odor.

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The version shown here is an easy way to demonstrate several indicators of a chemical reaction for students who are learning to differentiate between chemical and physical (or reversible) changes. We offer the blowtorched sugar cube as a clear example of a chemical change because students can see a clear color change and sense the odor of a chemically distinct substance that is produced by the breaking and reconfiguration of chemical bonds in the sucrose.

On the other hand, a physical change may involve simply dissolving the sugar cubes into hot water to create a supersaturated sucrose solution, and observing the crystals formed as the water evaporates in the span of a few days. The crystals retain their chemical identity when simply dissolved, yet take on new properties when burned and heated above 186 degrees. The temperature at which this pyrolysis reaction occurs is a property that can be used to identify an unknown sample of sucrose in a lab with older students.