The Density Bottle Strikes Again

Test for homogeneous vs. heterogeneous mixtures

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.

References:

1. The Dynamic Density Bottle: A Make-and-Take, Guided Inquiry Activity on Density, Journal of Chemical Education, 2015 92 (9), 1503-1506.

2. https://www.chemedx.org/blog/chemistry-bottle

3. https://www.teachersource.com/product/poly-density-kit/density

4. https://www.flinnsci.com/salting-out---density-bottle-kit/ap7931/

5. https://www.chemedx.org/blog/solution-chemical-mystery-8-go-blue

6. https://www.chemedx.org/blog/chemical-mystery-8-go-blue

Join the conversation.

Comments 6

Michael Farabaugh | Fri, 12/01/2017 - 21:13

From the following website (http://www.scienceclarified.com/everyday/Real-Life-Chemistry-Vol-2/Mixtu...) it says, "When soap is mixed with water, it does not form a true homogeneous mixture or solution due to the presence of hydrocarbons. These attract one another, forming spherical aggregates called micelles. The lipophilic "tails" of the hydrocarbons are turned toward the interior of the micelle, while the hydrophilic heads remain facing toward the water that forms the external phase."

Tom Kuntzleman's picture
Tom Kuntzleman | Sun, 12/03/2017 - 09:29

Michael: Thank you so much for alerting me to this! Why didn't I think of this?! Of course the presence of micelles would cause soapy water to be heterogeneous! In any event, thank you for explaining to me the reasons for my observations with soapy water. 

Bob Worley's picture
Bob Worley | Sat, 12/02/2017 - 15:38

Soap in water is a colloid and therefore will scatter laser light. You cannot solvate the long tail hydrocarbon (hydrophobic) so they all come together as a sphere with the outside consisiting of the hydrophillic carboxylate section which is solvated by water. We did colloid chemistry in the 1960s but now it is called nanochemistry.

Tom Kuntzleman's picture
Tom Kuntzleman | Mon, 12/04/2017 - 09:08

How right you are! Thank you for helping me figure out what was going on. I wonder if this experiment could be used to estimate critical micelle concentration. Prepare samples of varying detergent concentration and see what concentration is necessary to observe scattering. Hmmm....maybe I'll get some of my students working on this!

Tom Kuntzleman's picture
Tom Kuntzleman | Fri, 12/08/2017 - 20:11

Hi Maureen, thank you for your question. Using density bottles to discuss polarity and intermolecular forces (IMFs) is a lot of fun. See, for example, this particular experiment I like to do for my students (Note: University of Michigan's colors are maize (yellow) and blue, while that of Michigan State is green and white).

Various mixtures of water, a salt, an organic fluid, and dye(s) can produce an assortment of interesting results. Forgive me if you've already heard this, but I'm going to briefly describe the way I view the experiment using IMF's (you can also read more here). Water is completely miscible with acetone or short chain alcohols due to hydrogen bonding (H-bonding with water and alcohols is easy enough to explain, with acetone and water: OH donor on water H-bonds to O acceptor on acetone). The introdution of salt into a mixture of water and organic fluid creates strong ion-dipole interactions between the water molecules and ions. These ion-dipole interactions are strong enough to overcome the hydrogen bonding interactions between the water molecules and organic molecules. Thus, addition of ions to a water/organic fluid mixture separates into two layers. If dyes are introduced into the mixture, then it is often the case that certain dyes dissolve in the salt water layer, while others dissolve in the organic layer. It is fun to try to use the chemical structure of the dye (if known) to explain which layer the dye dissolves in. Several different color effects are achieved by varying the salt used (NaCl, Na2CO3, K2CO3, etc.) the organic fluid used (acetone, methanol, ethanol, isopropanol) and dye (various food coloring, dyes from markers, dyes from glitter, etc). Be sure to let me know if you discover some new and interesting effects!