2008
Science Experiment Receives Lots of Feedback
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A few weeks ago on my 9News segment I featured a Seven-Layer Density column. It’s a colorful way to talk about density in the classroom. Well… we thought we had all of our facts straight, but when we inadvertently put in the experiment write-up that oil and water don’t mix because they have different densities, my email box was flooded with concerned teachers, parents, administrators and more, who wanted to set the record straight. Yes, it’s true, oil and water don’t mix because of their intermolecular polarity, not because of density. I love when people actually get involved with the experiments we are posting and care enough about the information to let me know when it isn’t quite up to par.
So, we had our density problem solved, it would seem the Seven-Layer column had experience its fifteen minutes of fame, but, no… this one just wouldn’t die. The day after my news segment, some of our staff noticed that the vegetable oil and rubbing alcohol layers had switched places! Knowing the great response we received before, we opened it up to our readers for their thoughts on what was happening with our column. Posted below is what you had to say…
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July 30th, 2008 at 2:13 pm
As the alcohol in the alcohol solution evaporates what is left is water. This will eventually make the alcohol solution sink down to the water level. Of course by this time the alcohol solution won’t really be alcohol but water. So these are my thoughts on the seven layer density experiment. BTW my daughter did something very similar to this for her science project a few years ago. She had bystanders figure out solution was which by looking at the chart for density and seeing which solutions sank and which settled higher.
July 30th, 2008 at 2:13 pm
Likely water has been absorbed by the alcohol layer.
So the question is how to test this hypothesis?
We could use a small pipet to remove part of the alcohol layer and measure the density, but we already know the density changed.
Perhaps we could the color change of cobalt chloride (red = water, blue = no water). I think some silica packing still has the color crystals
to show this change.
July 30th, 2008 at 2:14 pm
It would seem the only way for the alcohol to change positions would be for it to change density……if something could be added or subtracted the density could change. If the alcohol is actually a mixture of alcohol and water, then the water and the alcohol could be evaporating from the mixture at different rates, that could change the density. That’s my hypothesis.
July 30th, 2008 at 2:14 pm
Hi, I just read about the alcohol and vegetable oils switching spots in your density demo. I am not sure this is a guess, but alcohol evaporates at a faster rate than many other liquids, right? I believe the carbon can’t bond to the hydrogen (not sure if that is exactly right, I thought I read that once somewhere), therefore the gas component in it is evaporating. If that’s the case, wouldn’t the liquid being left behind be more compact and have a heavier density?
Hope I’m not totally off base, if I am I will need to get a rag to wipe the egg off of my face…lol. I’d love to know the answer.
July 30th, 2008 at 2:14 pm
Try putting just vegetable oil and rubbing alcohol in a column and then capping the column. The idea here is to exclude water. I think some water may be diffusing into the rubbing alcohol which is usually a 70% solution of isopropyl alcohol in water, and increasing the density slightly.
I think you should rethink your opening statement “Everyone knows that vegetable oil and water don’t mix. That’s because the two liquids have different densities.” Oil and water don’t mix because they have different intermolecular forces and the oil molecules cannot force water molecules apart in order to mix. Alcohol and water have different densities and yet they mix quite well because they both rely mainly upon hydrogen bonding to hold them next to their neighbors.
July 30th, 2008 at 2:15 pm
OK, when you add alcohol and water, the alcohol fits in beween the water molecules, right? I wonder if the alcohol simply falls between the molecules of the other substances?
When alcohol and water mix the resulting volume of the two solutions is less than the total of the individual volumes. In this case “one plus one” does not equal two. The reason for this decrease in volume can be attributed to the hydrogen bonds which develop between the alcohol molecules and the water molecules (See “Surface Tension of Water” to see a further explanation of hydrogen bonding). This hydrogen bond pulls the molecules really close to each other and the small water molecules will fit nicely in the spaces between the alcohol molecules.
Having read the reference above, I have revised my thoughts. How about if the alcohol slips through each of the substance molecules, making temporary bonds as it passes down through the column?
July 30th, 2008 at 2:15 pm
Perhaps some of the vegetable oil dissolved into the rubbing alcohol, increasing the density of the new mixture enough that it became denser than the oil. Did something similar happen with the dawn or either sugar and the water layer?
July 30th, 2008 at 2:15 pm
Hello Spangler Team!!
Just a quick thought on why the rubbing alcohol and vegetable oil may have changed places in your density column. Rubbing alcohol typically contains acetone, methyl isobutyl ketone, water and denaturants. Some of these compounds evaporate quickly (the acetone, for example) causing the rubbing alcohol itself to change. Since the major compound in rubbing alcohol is water, it makes sense that the density of the rubbing alcohol as it evaporates would change to be very close to the density of pure water (1.00) and making it more dense than the vegetable oil (density 0.91.)
Keep the great ideas coming!!
July 30th, 2008 at 2:17 pm
My chemistry is quite old and rusty, but I do recall that alcohol loves to pick up water. Perhaps the alcohol is doing that slowly throught the oil layer, which would increase its density. That’s my guess.
July 30th, 2008 at 2:17 pm
I am curious…has the volume of either liquid changed? Specifically, is the same amount of oil present as was present before, or is there slightly less oil now then there was before? I’m not sure if it would even be measurable, actually, but If the level of the oil has lessened, that may support my theory, which is as follows and may be completely false:
The alternative fuel known as E85 is a mixture of ethanol (grain alcohol) and petroleum distillates. The alcohol mixes with the petroleum and bonds to it, creating a slightly more dense, higher-octane fuel. I am curious if perhaps the isopropyl alcohol behaved similarly with some amount of the vegetable oil, bonding to it and creating a denser alcohol/oil mix overall mix, in essence “sinking” the oil/alcohol mix below the remaining pure oil.
This may be WAAAAAYYYYYYY off, but who knows?
August 2nd, 2008 at 8:37 pm
I am reading in class 6.My teacher had told to do an science experiment on any topic of science obtained by scientific method and i can’t find any experiment.I am requesting that please send an easy experiment on my e-mail id that is akash04.2008@rediffmail.com
August 2nd, 2008 at 11:34 pm
Hi Akash,
I think our Science Fair Secrets section on our website would be really helpful for you. There are lots of free experiments and ideas on how to use the Scientific Method. Good luck with your project!
Steve