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Tuesday, November 24, 2015

The One With Magic Sand

Unit 1.C: Intermolecular Structure 

"Why did the bear dissolve in water? It was a polar bear."

Practice Problems:

   For this cycle, the problems we were given all required us to identify the polarity of certain compounds and how this would affect how they reacted with other compounds. I was originally a little confused with which element of a covalent compound would be partially negative. After learning that the element farther on the right of the periodic table would be partially negative, I had a much easier time working on the practice problems. Another aspect of the practice problems that I struggled with was when the compound was 3D, because the forces within the compound would be different compared to those of 2D compounds.
Ex. of Understood Problems
  1. Draw an IMF interaction diagram that shows the mixing of water with ethanol (CH3CH2OH)

Ex. of Challenging Problems
Draw an IMF interaction diagram for BH3
(why is this compound 3D and not 2D? isn't the B fulfilled by the 3 bonds and therefore needs no lone pairs?)

     On the first day of the cycle, we were asked what was in a bubble of boiling water. My group was stumped. At first, we thought that the H2 and O would separate. But we realized this would just result in a bunch of explosions, which obviously doesn't happen when you boil water. We remembered that the difference between gases and liquids was the space between molecules. So we hypothesized that the bubble still contained H2O as a gas, with its molecules farther apart than the ones in the liquid. This turned out to be correct!

Our drawing depicting what is in bubbles of boiling water.
     Our next activity ended up becoming one of my most favorite labs we've done so far. We put water and hexane in a test tube and found that the two didn't mix. Next, we put in copper sulfide and iodine and shook the mixture, only to find that the purple iodine stayed at the top with the hexane while the blue copper stayed at the bottom with the water. As a class, we learned that this was because the positively charged copper ions were attracted to the partially negative oxygen in water. This explained why the water and copper stayed at the bottom no matter how much we shook the tubes.

The test tube with copper sulfide and iodine
Water and hexane (top) in the test tube

     Another activity we did for this cycle was comparing the evaporation times of four different liquids: hexane, ethanol, propanol, and acetone. We found that liquids with weaker IMFs evaporated faster since the forces holding together each molecule weren't as strong.

The chemical on the left has weaker IMFs, therefore it evaporates more quickly

     We also did a lab where we compared the effects of holding a charged rod by streams of different liquids: hexane, water, and ethanol. Just like the last activity, the IMFs dictated how each liquid behaved. Compared to hexane (which was non-polar, thus wasn't attracted to the rod) and ethanol (which only had one partially positive hydrogen that could be attracted to the rod), water had two partially positive hydrogens and was drawn to the rod the most.
The water is strongly attracted to the negatively charged rod

     This cycle was full of really interesting labs, especially the next one. We compared the behavior of normal sand and "magic sand" in water. While normal sand behaved just as we thought it would, magic sand stayed dry even when we mixed it in water. This was because it was sprayed with chemicals that had london dispersion IMFs.
Notice how the sand mixes with the water and becomes clumpy
The magic sand stayed dry!
     Two other activities we did during this cycle showed the effect of dish soap when mixed in water. The first activity used pepper and water. When we dipped a small amount of detergent in the center of the dish, the pepper showed how the dish soap mixed with the water and weakened the IMFs between the water. In the second activity, we used milk instead of water and food coloring instead of pepper, but the results were still the same.

The pepper and water activity

The milk and food coloring activity (skip to 1:05)

     Finally, the class got to try chromatography and analyze some pen samples in order to figure out who left Mr. Musallam a certain note. We got a sample of each pen on filter paper and dipped the end of each piece in water. For one sample, as the water traveled up the paper so did the ink. For the other sample, this did not happen.

A time lapse of the chromatography activity

Quiz: (To Be Edited After The Tests Get Returned):
     I was really happy that we got to have a group quiz for this cycle because I liked having others to work the problems out with. All the problems on the test had similar solutions and dealt with the fact that compounds with similar IMFs were soluble with each other, while unlike IMFs were not. The final problem concerning why the certain components in bubble solution made bubbles was confusing for me, and I'd like to find out the solution.

  • Why do the chemicals in bubble solution make bubbles?
  • Are there any exceptions to the "like IMFs are soluble with each other, unlike IMFs are not" rule?

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