Today’s lab went great, it was fairly simple but had pretty impressive results and the students did a great job with it. The lab, Temperature Changes in Dissolving, is from Chapter 5, Lesson 9 of the American Chemical Society’s free middle school chemistry curriculum. Before we started the lab I wrote ‘exothermic’ and ‘endothermic’ on the white board and asked if anybody knew what the words meant. Nobody answered, so I asked if they could look for clues in the words and immediately there were shouts of ‘temperature’ for thermic and ‘outside’ for exo. One students said bugs have exoskeletons which means its on the outside and we have endoskeletons – skeleton is on the inside. So I explained how exothermic means something is giving off or releasing heat and would be warm to the touch while endothermic means heat is being absorbed. I then did a few demos, including ‘hot ice’, which is super saturated sodium acetate. There are many recipes for this on the internet/youtube, basically you have a small amount of water and dissolve a huge amount of sodium acetate into the water at high temperature. Then you let it cool. I poured the super saturated solution on to a petri dish containing a few crystals of solid sodium acetate so that the solution crystalized immediately upon contact. You can see the broken stalagmite in the photo below and the glass jar behind it where the solution crystalized before I could finish pouring it. The reason I did this demo today is that when the sodium acetate turns back into a solid it becomes very hot, hence the term ‘hot ice’ for this demo. Its actually the same stuff you find in hand warmers and instant hot packs. The heart shaped HotSnapZ in the photo below is an example of an instant hot pack. I also had an instant cold pack on hand and you’re supposed to activate it by squeezing and popping the internal bag of water inside the cold pack but we just cut it open so we could see what was going on and dumped it in a large beaker. It became very cold when the pellets (probably ammonium nitrate) dissolved in the water.
The lab itself involved dissolving 2 grams of different chemicals (potassium chloride,
calcium chloride, sodium bicarbonate and sodium carbonate) in 10 ml of water and recording the initial temperature and the maximum or minimum temperature reached as each of the chemical dissolved. Two of the solutes were exothermic and two were endothermic and the changes in temperature were very noticeable, one increased by almost 20 degrees Celsius! So overall a very nice lab.
I showed the following Ted talk on cold packs at the end of class.