Archive for the ‘Basic chemistry’ Category

Fingernail growth

Friday, December 12th, 2008

A little while ago, about 384 hours if you want to be exact, I was in the lab doing an experiment with some of my students.  The experiment involved isolating proteins from milk, then doing some chemical tests to show that what the students had isolated was actually protein.

One of the tests the students used on their isolated protein was the xanthoproteic test.  This is a simple test for proteins that works on proteins containing an aromatic ring, and involves the addition of a nitro group to the ring by reaction with nitric acid.  If a protein or amino acid with an aromatic ring is present, the test will give a yellow color.

You may, if you’ve ever been in an introductory chemistry lab, heard your teacher warn you about nitric acid.  In addition to causing you some pain, it will also turn your skin or fingernails yellow.  This is the same sort of chemistry in the xanthoproteic test.  Your skin and nails, after all, contain proteins.

Now, back to 384 hours ago.  While I was cleaning up the lab after my students had left, I spilled a small amount of nitric acid onto the top of my finger.  Sure enough, my finger turned yellow, along with a small part of the fingernail.

Here's the nitric acid stain.  I originally spilled the acid at the point where my skin meets the fingernail.  It's moved a bit, now.

Here's the nitric acid stain. I originally spilled the acid at the point where my skin meets the fingernail. It's moved a bit, now.

384 hours after the spill, all the yellow skin had been replaced.  But the nail has to grow out.  We can find out how fast my fingernails are growing with a simple measurement.

Here's a (somewhat crude) neasurment using a ruler from the lab.  The nitric acid stain has moved about 0.18 cm.

Here's a (somewhat crude) neasurment using a ruler from the lab. The nitric acid stain has moved about 0.18 cm.

So, my fingernail has been growing at a rate of (0.18 cm) / (384 hr) = 0.00047 cm per hour.  That works out to be 3.4 mm per month, which is right about what Wikipedia claims for the average rate of fingernail growth.

Now who says you never learned anything useful from chemistry?

Units!

Friday, August 22nd, 2008

Tired of reading about politics?  Don’t have anything else to do on a Friday afternoon?  Why not brush up on metric units and unit conversions?

No, really!  Click below for more.

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The aluminum bromide reaction

Wednesday, August 13th, 2008

[This is an update of an earlier post on this blog.  This version includes streaming video]

Let’s say you don’t want to do the thermite reaction, but you still want to see some flashy chemistry. The reaction between aluminum and bromine might fit the bill.

2Al(s) + 3Br2(l) –> 2AlBr3(s)

It’s a very simple-looking reaction – a little electron transfer from aluminum to bromine.  Like lots of these reactions, it’s exothermic.  Exothermic enough to put on an impressive show.

Enough heat is generated by the reaction to vaporize some of the unreacted bromine – throwing off orange smoke.  On top of that, the aluminum gets hot enough to melt and spark.  For obvious reasons, this reaction should only be done where you’ve got very good ventilation. I used my hood for these pictures and this video.

Here’s a still image of the reaction vessel containing only liquid bromine and its vapor.

Liquid bromine and its vapor

Liquid bromine and its vapor

Bromine is the dark red liquid at the bottom. Bromine is quite volatile, and you can see orange bromine vapor in the top of the beaker.

About ten seconds after adding some torn aluminum foil, things look more like this.

Aluminum bromide reaction, 10 seconds after adding aluminum

Aluminum bromide reaction, 10 seconds after adding aluminum

A little later …

After a few more seconds, it's hot enough to spark

After a few more seconds, it's hot enough to spark

And then …

Oh yeah!

Oh yeah!

… but you didn’t read all this way for still pictures, did you?  How about a video?

After the reaction’s over, you’ll want to buy a new beaker.  The melted aluminum foil fuses with the bottom of the beaker.  (The sand bath is there to catch anything that gets through the bottom of the beaker when it breaks!)

Aluminum melted into the glass at the bottom of the beaker

Aluminum melted into the glass at the bottom of the beaker

So where’s the aluminum bromide?  Some of it has stuck to the sides of the beaker.

Aluminum bromide (white / yellowish solid) on the beaker

Aluminum bromide (white / yellowish solid) on the beaker

Aluminum bromide formed will react with water, causing the release of acidic hydrogen bromide vapors, so you need to be careful disposing of the product! That reaction is also very exothermic, so touching the product or adding water to it is not recommended. Leave it out long enough, though, and it will absorb water from the air on its own.

Ain’t chemistry neat?

Disclaimer: Do not try this reaction at home. In fact, do not try this reaction at all! You were warned.