Paraffin Wax - an Exothermic Reaction: Caution

"What are we doing to help kids achieve?"

I met some amazing teachers at Chem Ed 2019. Two of these teachers are Yvonne Clifford and Sharon Geyer who ran a workshop about chemical demonstrations. One in particular caught my attention. The demonstration was an exothermic process with paraffin wax. Here is the demonstration.

The teacher takes a few grams of paraffin wax and places it in a small test tube. The demonstrator then carefully heats it until it melts and then gets the wax to a rigorous boil. The demonstrator turns off the flame and then sticks the test tube of boiling wax in a bath of ice water behind a safety shield. A big ball of fire shoots from the test tube.

Here is what I think is happening. The process of a phase change from a solid to liquid to gas is endothermic. It takes the heat from the burner to make this happen. The reverse process is exothermic. When the wax goes from a gas to a liquid to a solid energy is released. This energy, ideally, goes from the system of the wax into the ice water. Or, at least some of it does. There is a free side to the paraffin, the top side of the open test tube. In theory, energy can also exit this side as well. If it does, and it does, the heat could ignite any paraffin that happens to still be a vapor. Since this happens quickly, there is still enough vapor above the free surface. Essentially, the vapor ignites causing a flame. It is surprising in that people generally do not expect a flame to shoot out of a test tube when it is placed in ice water.  

Video 1: Parafinn wax demo on Chad Husting's ChemCast Science Videos YouTube Channel (accessed 8/f8/2019

This is important for a couple of reasons. First, if you ever plan on doing this as a demonstration with or without students, you must be extremely careful. You must practice this first and complete a risk assessment. Always wear goggles and use a safety shield. If any of this makes you nervous, do not do it. Also, I have known several teachers who have used paraffin with students in the past to have them do experiments with melting points and or phase change. It is simple, inexpensive and the stuff of candles. What could go wrong? Well, the vapor is also extremely flammable as you have now seen in the video. So....please...be carefull with parafinn. It does not always behave the way you think it will.