Have you ever cooked a marshmallow in a microwave? In case you are not familiar with this experiment, when a marshmallow is heated in a microwave, gases trapped in the marshmallow expand and escape. When the gas molecules escape from the marshmallow, they push against the marshmallow, causing it to expand. To see this experiment, play the video below.
I often conduct this experiment in class when discussing the gas laws. I currently have a student in class named David Welsh who is interested in food science. When David saw me demonstrate this process during class, he wondered if he could carbonize the outer layer of an expanded marshmallow. David asked me if he could spend some time in the lab studying this for his end of the year project. I didn’t think this idea was particularly interesting, nor did I think it would produce any worthwhile results. However, because I didn’t want to stifle David’s curiosity, I told him to go ahead and work in the lab for a little while.
David immediately proceeded to the lab. He cooked a marshmallow in the microwave, quickly removed it, and heated it with a blow torch. He was unsatisfied with the amount of black carbon formed on the outer layer of the marshmallow. So, he proceeded to put the charred marshmallow into the microwave to expand it by heating it again. To see what he saw when he did this, play the video below.
Wow! A burnt marshmallow bursts into flame1 when cooked in a microwave! David immediately told me what he had observed. I was very skeptical of his claim, so I asked him to repeat the experiment for me. Sure enough, the charred marshmallow burst into flame when cooked in the microwave. When I saw this, David’s project was immediately promoted to intensely interesting to me. I wanted to know what was causing this effect. I encouraged him to try using a flame from a match to burn the marshmallow in place of the blow torch. Curiously, when the marshmallow was charred using a match flame, it would not ignite when cooked in the microwave. David began experimenting with many combinations of fuel and flame. He burned different brands of marshmallows, wooden sticks, and pine cones. He tried burning items with the blow torch, a Bunsen burner, a grill lighter, and matches. Through these experiments, David determined that carbon-containing materials burnt with a blow torch would ignite when heated in the microwave. Materials burnt using any other heat source would not ignite when heated in the microwave.
We wondered why it was that only materials that were singed with a blow torch would ignite when heated in a microwave. We speculated that carbon in the marshmallow (or other material) is converted to graphite upon vigorous heating. Graphite is a fair conductor of electricity, and might therefore behave like a metal when placed in the microwave2. We surmised that when the marshmallow is heated with the blow torch, it is heated to a high enough temperature to cause the conversion of carbon in the marshmallow to graphite. When heated with less intense flame sources (Bunsen burner, grill lighter, matches), the temperature of the marshmallow does not get high enough to allow this chemical conversion to occur.
I had David test this idea. Using a blow torch, he heated a marshmallow to glowing hot. He also used a blow torch to char a second marshmallow, taking care not to heat this other marshmallow to glowing. Play the video below to see a description of this experiment and its results.
We’re not completely sure that heating a marshmallow to high temperatures causes the carbon in marshmallows (or other organic material) to form graphite3. However, we have collected evidence that this process occurs, and we think it is a fascinating possibility. I enjoy working with David on these experiments, because together we are learning many new things. I am even more pleased that I encouraged David to attempt his original experimental idea, even though I thought his idea was dull and not likely to produce anything interesting. I certainly have learned that curiosity should be encouraged – not squelched – as much as possible!
Many thanks to David Welsh for conducting these experiments, filming the results and helping to make this blog entry possible.
To learn more about how a microwave works see: Sarah L. Cresswell and Stephen J. Haswell, J. Chem. Educ., 2001, 78 (7), pp. 900 – 904. http://pubs.acs.org/doi/abs/10.1021
To read more about graphite see: Peter A.H. Tee and Brian L. Tonge, J. Chem. Educ., 1963, 40 (3), pp. 117 – 123. http://pubs.acs.org/doi/pdf/10.1021/ed040p117
- The microwave oven used in this experiment is located in a hood. In addition, this microwave is never used to cook food. To prevent fire or damage to the microwave oven, anytime arcing is observed, the microwave is immediately turned off (within less than a second of observing sparks).
- When metals objects are heated in the microwave, they spark. I’m not completely sure how this works, but I am aware that electromagnetic microwaves induce currents in the metal. This is because the oscillating electric dipole of the electromagnetic field causes the electrons in the metal to oscillate as well. This can cause regions of high charge density to build up in the metal. Charge can build up on blunt or sharp ends on a microwaved metal object. If the charge build up is sufficient, arcing can occur.
- We’d love to hear from you regarding this experiment. Do you have any alternative explanations for what might be going on? Also, we are unaware of how we might test the marshmallows for the presence of graphite. Any ideas on how we might chemically test for graphite?