We did two labs today, generating heating and cooling curves for wax and making ‘wet’ dry ice. The first lab involved placing some wax in a test tube and then placing the test tube in a beaker full of water (water bath). The beaker was heated by a hot plate or butane burner until the wax melted. A thermometer is placed in the wax (we put a bulldog clip on the thermometer to keep it from touching the bottom of the test tube) and the test tube is removed from the heat. Students recorded the temperature of the wax for 30 minutes at 1 minute intervals. This data will produce a cooling curve. They also recorded the phase of the wax, liquid or solid or combination of the two. The temperature starts to drop pretty rapidly when removed from the hot water but as the liquid wax starts to solidify the temperature change slows down. Once the wax has finished solidifying the temperature will start to drop again.
After collecting the cooling data, students placed the test tube of solidified wax back into a hot water bath and recorded the temperature as it heated back up. This happens a bit quicker so students took data every 30 seconds for 5-10 minutes. One group found their water had cooled off too much so they never full liquified their wax and the other group had their water bath a little too hot and it liquified too fast.
We were having too much fun with the dry ice in the second lab to plot the data in class so I told them to use a computer/iPad to plot their heating and cooling curve at home.
The ‘Wet’ Dry Ice lab was taken from the Modern Chemistry curriculum. Dry ice is solid carbon dioxide and can be bought in grocery stores. Most people have never seen the liquid phase of carbon dioxide because it is a gas at our normal conditions (1 atm of pressure and 20-30 Celsius), which is why it goes straight to the gas phase when we pull the dry ice out of the cooler. To observe the liquid phase of dry ice we have to increase the pressure, so we cut the tip off a disposable pipette and filled the bulb half way with crushed dry ice (just put a towel over the block and bang on it with a hammer). Fold the narrow tip of the pipette over twice and close with a bulldog clip. Placing the pipette full of dry ice in a tank of water makes it easier to see what’s going on (no condensation) and makes a lot of nice bubbles when it pops.
Here’s a slow motion movie of a pipette with dry ice in it. You can see the dry ice begin to melt and then boil before the pipette starts to expand from the high pressure and eventually rupture. Once its ruptured the pressure drops and the dry ice goes back into the solid phase.
I did this using a centrifuge tube as well and just unscrewed the cap to relieve the pressure before it blew up. This was a little easier to see the carbon dioxide go back and forth between liquid and solid as I changed the pressure. I didn’t let the pressure get high enough to blow up because I wasn’t sure how bad that would be. I was the only one to use the centrifuge tube. The little plastic pipettes just make a big splash as they rip so I didn’t fear the kids getting hurt. We did have safety goggles on for this lab.
We also made carbon dioxide bubbles and used the gas to put out candles.
Here’s a great video from Flinn Scientific explaining the triple point and phase diagrams for carbon dioxide and it shows how to do this experiment in detail.