Carbohydrates and Sulphuric Acid (or How to Dehydrate Carbohydrates)

One of the more spectacular chemical demonstrations getting around is sulphuric added to sugar.

What on earth happened? Suddenly out of the pristine white sugar we see a hideous black hissing column rising, like something out of a horror movie.

Well, as it happens, sulphuric acid is very dry.  It may seem and odd thing to say about a liquid, but concentrated sulphuric acid is only about 2% water.  It is a thick (kind of like runny honey) clear, odourless liquid.  If you get it on your skin it burns like hell, but the reason is not quite what you think.

All it’s doing is reacting with the water in your skin.  Because it is so dry, it gets very excited when it encounters some water and this makes it very hot.  As a matter of fact when handling sulphuric acid, students are often taught a rhyme:

May her rest be long and placid
she added the water to the acid
completely forgot what we had taught her
you should add the acid to the water

the reason for this is that if you add water to sulphuric acid it will generate so much heat so quickly it will explode in your face.  Even if you do it the right way – adding the acid to the water – the water will quickly boil.

I once had a corn on my little toe that I wanted to get rid of, as it was really annoying me.  I figured the best way to get it off would be to burn it off with sulphuric acid.  So I very carefully placed a drop of it onto the corn and was ready to wash it off as soon as it had burned through the corn.

But nothing happened – the drop just sat there.  I even tried some chromic acid on it (a souped up version of sulphuric acid) but it didn’t do anything either.

The reason is simply was that the corn was very dry skin and there was nothing for the sulphuric acid to react with.

Now you will recall that carbohydrates are just carbon and water.  Because sulphuric acid is so dry, and craves water so much, it will actually reach into the molecular structure and rip the waters off the carbons.

So what you see happening in the video is the sulphuric acid is ripping the water off the carbons, therefore leaving just the black carbon behind.  Because this generates so much energy, the water turns to steam.  This steam bubbles up through the carbon giving it a porous, honeycomb type structure, which increases the volume greatly.  Therefore you see the column rising out of the glass.

For the same reason if you spill even dilute sulphuric acid onto cotton clothing it will rip through it in an instant.   Anybody who has spilled battery acid onto their clothes will know that.  That’s why acid resistant safety gear is made out of synthetic fibres.

3370cookie-checkCarbohydrates and Sulphuric Acid (or How to Dehydrate Carbohydrates)