Chemistry Demonstration MotherloadMother Lode

I stumbled upon this page today.  (gee, woulda been nice to know about it during my labs this semester…)

6 pages of video after video of chemistry demonstrations.  You’ll never get bored!

Kent’s Chemical Demonstrations Movies

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Chemistry Lab Demonstrations: Candy Chromatography

*For more cool stories, pictures, and videos of chemistry demonstrations, click here*

Last lab of the semester today. Next week is the lab final and checkout. This week the students practiced column chromatography. They purified their crude product mixture from last week’s nitration lab. I’ve talked about the theory behind column chromatography before, so I won’t rehash it here in any detail. Suffice it to say that different organic compounds have differing affinities for a stationary phase versus a mobile phase. These differing affinities allow for one compound of interest to be separated from a mixture through the use of column chromatography. Students were aided this week in that their product was bright yellow. They could physically watch it run down the column, then only collect the yellow fractions.

Last lab of the semester means last demo of the semester.  This one’s a do-it-yourself demo, if you’d like.  You can separate the colors contained in M&M shells (or Skittles, or Reese’s Pieces, or Sharpies, etc) through chromatography.  I got my M&M proceedure here.  If you’re interested, other proceedures are available here, and here.   Basic rundown: put drops of water on wax paper, and put a piece of candy on each drop.  Allow for the water to strip the color off the colorful candy shell.  Cut a coffee filter into a rectangle.  Use a toothpick to spot each color onto the coffee filter.  Put the coffee filter into a 1% solution of table salt and allow the water to rise through the coffee filter.  Watch the colors separate like magic!

Couple’a observations I noticed.  Quite interestingly… the stationary phase matters.  A lot.  I started by spotting the colors on my silica gel TLC plates .  I was quite disappointed because the red and yellow both travelled with the solvent front and there was little separation.  I tried several different solvents… no luck.  I also noticed that according to the websites I was looking at, red should have travel the shortest distance.  Then I switched over to filter paper, and all of a sudden I got the results I was expecting.  Who knew?  Also, you should put a crease in the coffee filter before placing it in the solvent.  The paper will start to buckle and it will droop and fall over if it is not creased first.  The more distance you give the colors to separate, the better the results.  I used the largest filter paper we had, and ran the chromatograph twice to get the results shown.

Pop quiz, hot shot: Do you know what the difference between Red 40 and Red 40 Lake are?  I didn’t either.  Turns out… nothing.  At least, not as far as the compound responsible for the hue is concerned.  It’s all in the formulation:

Color additives are available for use in food as either “dyes” or “lakes”.

Dyes dissolve in water, but are not soluble in oil. Dyes are manufactured as powders, granules, liquids or other special purpose forms. They can be used in beverages, dry mixes, baked goods, confections, dairy products, pet foods and a variety of other products. Dyes also have side effects which lakes do not, including the fact that large amounts of dyes ingested can color stools.

Lakes are the combination of dyes and insoluble material. Lakes tint by dispersion. Lakes are not oil soluble, but are oil dispersible. Lakes are more stable than dyes and are ideal for coloring products containing fats and oils or items lacking sufficient moisture to dissolve dyes. Typical uses include coated tablets, cake and donut mixes, hard candies and chewing gums, lipsticks, soaps, shampoos, talc, etc.

There are 5 food coloring agents in M&Ms: Red 40, Yellow 5, Yellow 6, Blue 1, and Blue 2.  As you might expect, green separates into blue and yellow, but surprising the red and yellow of the orange M&M do not separate.  Rather, there is one orange spot with a larger Rf than red.  Brown separates to blue, red and orange.   But it looks like the blue in the blue M&M is a different blue than the blue in the green and brown M&M.

I’ve got lots of pictures from my experience (click for larger).  Note how poorly silica works and how different the Rf’s are between silica and filter paper.  the video is of separating components of felt tip pens, but it’s also neat.

There are no more demos planned, since the lab course is over.  Hope you enjoyed my miniseries.

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Chemistry Lab Demonstrations: Silver Nitrate/Copper Wire

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This week in lab, students performed electrophilic aromatic substitution (and here).  Dissolution of 4-methylacetanilide in 70% nitric acid gives mono nitration.  There are two possible products.  The acetamide is a better ortho/para director than the methyl group, so 2-nitro-4-methylacetanilide is the major product of the reaction.  The reaction was a bit touchy.  In my lab, for the most part the reactions were carried out at room temperature.  This resulted in the reaction not occuring!  Nearly every student got back unreacted starting material, instead of product.  The product is supposed to be a bright, crayon yellow solid.  Other labs ran the reaction on low heat and got excellent results…  But some students left the reaction on the heat too long or at too high of a temperature and the reaction decomposed into this ugly brown oil.  So there is a very small window for success in this reaction.  For the first time this semester (!) students analyzed the reaction mixture by Thin Layer Chromatography.

A brief safety warning.  Nitric acid is a very strong oxidizer.  It reacts explosively with readiliy-oxidizable small organic molecules like alcohols and acetone… acetone being of course what all good lab students rinse their glassware with before they start lab.  There was an explosion in our department last year as a result of improperly mixing nitric acid waste and acetone waste.  Nitric acid MSDS here, incident report on nitric acid explosion (not from our department) here (it’s probably worth glancing through the other incidents on that page as well), and video showing gas evolution from nitric acid oxidation (this time of copper) here – note how much gas is produced in a short amount of time.  Imagine this in a closed container.  Keep watching the video till the end, as it actually makes for a neat demo in itself.

The demo for the week was the silver nitrate/copper wire demo.  Silver nitrate (nitrate being the conjugate base of nitric acid… which is how I’m relating this demo to lab this week…) dissolves readily in water to give a solution of silver nitrate.  Just about everything silver nitrate touches gets stained black.  Not immediately… only after exposure to light.  For this reason, silver nitrate used to be used in early photography.  No stains for me, though it is always a concern.

Dropping copper wire into the silver nitrate solution initiates a redox reaction between the silver ion and copper metal.  The silver is reduced to elemental silver and the copper is oxidized to copper(II):

Cu(0) + 2AgNO₃ = Cu(NO₃)₂ + 2Ag(0)

The silver crystallizes at the surface of the copper and the copper wire quickly becomes coated with a bunch of elemental silver.  At the same time, the copper ions go into solution and the colorless solution turns a characteristic blue as the concentration of copper ions builds.  It’s a pretty dramatic demo of a neat redox reaction.  I was testing the speed of the reaction before I went to lab (silver started to become visible within 30 seconds to a minute), and I ended up leaving the copper wire in the silver nitrate while I went to lab.  I came back several hours later and the crystals had plenty of time to grow and were very nice looking.  I let it go overnight to see how big the crystals would get.  I took a picture when I got to lab the next morning (click for larger).  And then I collected the silver in a scintillation vial to take home with me.  The pictures are from my experience, Noel posted a picture a while back as well, and there is a nice video below.

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Chemisry Lab Demonstrations: Nylon Rope Trick

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This week’s lab involved taking the unknown carboxylic acid from experiments 1 (acid/base extraction) and 2 (purification by recrystallization) and converting it to a primary amide. The acid was refluxed in neat thionyl chloride and added dropwise to 30% ammonium hydroxide. Extract with dichloromethane and evaporate.  The fumes from this lab are pretty nasty.  The reagents are bad enough – the lab smelled like ammonia all day – but sulfur dioxide and HCl gas are liberated during the reaction.  The “fume hoods” in the ugrad labs aren’t so hot, so the students attach a long-stem funnel (where do I sign up to be a glassware photographer?!) to the water aspirator and invert the funnel over the reaction mixture as it refluxes.  This collects the fumes as an extra “mini fume hood” during the reaction.

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