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.

Carbohydrates and Ironing Aids

What is in ironing aids and how do they work?

Ironing aids (such as Fabulon) have four components:

1.  Water.  This provides a steam cushion as the hot iron glides over it.

2.  Gliding agent.  This is typically a silicon oil that is colourless and odourless and provides a lubrication for the iron as it glides across the fabric

3.  Fragrance.

4.  Starch (either natural or synthetic).  Starch of course is the component that stiffens the fabric when it is ironed. But it’s not the actual starch that stiffens the clothes. If you just sprayed some starch on your clothes and let it dry, it wouldn’t make a scrap of difference to the stiffness of the fabric.

What is needed to stiffen the fabric is starch plus heat.  This produces a process called hydrolysis which is essentially where the starch molecule is broken into bits and water is added across the bonds at which the molecule is severed.

These bits of starch are simply short chain carbohydrates and they are termed dextrins.  Dextrins have a number of properties in the food and pharmaceutical industries, and one of them is the stiffening of clothes.  This occurs as the dextrin binds to the fabric (particularly cotton) and helps straight and out the physical structure of the weave.

Interestingly, dextrins are also the stiffening agents in the crust of a nice crispy bread.

Carbohydrates and recycling

Carbohydrates are the entirely composed of recycled materials – carbon and water.

Water we all know about – we know about rain and evaporation, the two factors principally responsible for the recycling of water in our environment.

Carbon is also recycled, but most people are not sure how.

Let’s consider the simple case of wood.  Wood ultimately is a carbohydrate.  When it is burned it releases CO2 into the atmosphere.  Later on the same CO2 molecule is converted back to wood by photosynthesis.

We could write the reaction this way:

carbon dioxide + water = trees + oxygen

in simplified chemical terms, it would look like this:

CO2 + H2O = CH2O + O2

There are of course other ways of returning carbon to the atmosphere.  When we breathe is, for example, we breathe in oxygen and we breathe out carbon dioxide.  The carbon ultimately comes from the carbohydrates in the food that we eat. So we breathe in oxygen, we breathe out carbon dioxide, it is converted to a banana which we then eat a year later, and the carbon is returned to our bodies.

So for plants to grow they need water and carbon dioxide.  Most people know about the first one, but few people realise how much plants love carbon dioxide – check out this video

More on carbohydrates later.


The Chemistry of Carbohydrates

Let’s start with a quiz.

Based purely on the name, what do you reckon a carbohydrate is? Hint: carbohydrates are one of the few classes of chemical whose name tells you exactly what they are.

Well, if “carbo” stands for carbon, and of course “hydrate” refers to water, then we arrive at the conclusion that a carbohydrate is carbon + water.

In fact, you’d be right. Let’s write it as a chemical symbol and see how we go: CH2O

As it happens this is quite right – every carbohydrate can br educed to this simple formula. Let’s consider glucose – the simplest carbohydrate:

Its formula is C6H12O6 – if we divide everything by 6 we get back to CH2O

In fact carbohydrate molecules occur in blocks of 6 carbons (called “saccharide” units) – so glucose, a monosaccharide is C6H12O6, and sucrose, a disaccharide is C12H24O12

And they just get bigger, with units up into the hundreds or thousands. The mono and di-saccharides are simple carbohydrates and the higher numbers are complex carbohydrates.

As it happens, glucose (ie glycogen) is the fuel that our body runs on. Before anything that we eat can be used as fuel, it must first be converted to glucose. The ease with which this process happens is recognised as being a major factor in determining whether we put on weight or not and is linked with diabetes.

Not all carbohydrates are digestible, however. Like trees. Or cotton. But more on this later – and how they help stiffen your clothes when ironed.