It looks as though I’ve discovered that density bottles can be used to explore differences between heterogeneous and homogeneous mixtures! For those of you that are unfamiliar with this experiment, a density bottle contains two immiscible liquids enclosed in a bottle.1-4 The two liquids are usually some organic fluid (usually isopropyl alcohol or acetone) and a solution of an ionic salt (usually sodium chloride or potassium carbonate). When the bottle is shaken, the two liquids form an emulsion that separates back into the organic layer and salt water layer in about a minute. The layers can be colored using a variety of dyes, which can lead to some interesting effects.5,6 I have used density bottles to have my students investigate topics such as density, miscibility, polarity, and intermolecular forces. As stated previously, just this semester I have discovered that these bottles can also be used to demonstrate the difference between heterogeneous and homogeneous mixtures. Check it out in the video below:
To prepare the two liquids used in the video above, I dissolved 120 grams of potassium carbonate into 480 mL of water. After pouring the resulting solution of potassium carbonate into a 1 L soda bottle, I added 480 mL of 70% isopropyl alcohol. If you watch the above video all the way through, you will notice that light is scattered by a “solution” of soapy water. I don’t know about you, but I was quite surprised to see this result. I thought a soap solution was indeed a solution, so I did not expect soapy water to scatter light. I don’t know what the composition of the particles are in soapy water that causes light to be scattered. Do dirt and dust particles scatter the light in soapy water? Could it be extremely small air bubbles that are doing the trick? Or is it something else? If anyone has any insight into how this might be happening, please let me know in the comments. I look forward to hearing from you.
1. The Dynamic Density Bottle: A Make-and-Take, Guided Inquiry Activity on Density, Journal of Chemical Education, 2015 92 (9), 1503-1506.