Sunday 3 February 2008

Elements and reactions

Chemistry doesn't have a great public profile. Biology's pretty straightforward - it's the wet, squishy, science of life, so it's easy to see what it ties into. Everything else is pretty much either physics (big cool space things) or maths (the equation for the perfect cheese sandwich and such). Chemistry only really comes up as "chemicals" of one sort or another, be it drugs or poisons or preservatives or other artificial things that you don't want to spend too much time around. So with this in mind I decided to start off this blog with an explaination of what chemistry is, so that non-scientists could follow what I'm saying. Hopefully. Enjoy!

Chemistry is the science of the structure of matter, and how substances can be changed from one to another. Of course this is a very rough definition. You can change the structure of a Lego house by taking it to bits and reassembling it as a car, or turn a pile of uranium into a pile of plutonium by bombarding it with whooshy Science Particles, but those aren't chemistry. Chemistry deals with figuring out, and messing with, the structure at a particular scale, the scale which makes life work day-to-day.

It was alchemy, which amoungst other things frittered with trying to turn boring old lead into retirement-tastic gold, which gave birth to chemistry. The alchemists were doing what we'd call chemical reactions, in a very controlled setting - they put aqua regia (a nasty brew of scary acids) and some gold into a pot, heat it a little, and see what happens (the gold dissolves). Alas they didn't know that turning metals into one another falls outside of the realm of chemistry, so they were doomed from the outset. The problem lies with the notion of chemical elements.

Elements are a very old idea which is fairly intuitive. Everything in the world is a mixture of elements, the theory says - earth, fire, wind and water for example. The way they combine makes the difference between a cat and a tree and a rock. When you set wood alight, the fire gets out and it turns into ash, say. The elements themselves are basic and universal, like Lego bricks. Alchemy, as with chemistry, broke things down and put them together in order to find out what the real building blocks were (clue: not Lego) and how you could put them together. As it turns out, gold - and most metals, for that matter - are chemical elements. No amount of messing about with ever-hotter fires will break gold down into something else.1

It turns out that matter has the same sort of granularity as Lego, too. Imagine that you've got a big block made from yellow Lego bricks. You can break it down into individual yellow bricks. Likewise, you can chemically break down a lump of gold into individual grains of gold, called atoms.

With this atom theory, everything starts to come together. You can't break atoms down - each atom is an indivisible, particular chemical element, be it gold or carbon or lead or oxygen. Those atoms combine in various ways to give the materials around us. When they come apart from eachother and recombine in a different way, you have a chemical reaction, which turns material into another, and this happens under the sort of conditions you see in the everday world. As an example, your body takes in food, rich in carbon atoms, as well as air, rich in oxygen. You breathe out carbon dioxide gas, which a mixture of the carbon and oxygen.

How, exactly, do these elements combine together to give the substances we know? Why does the body bother changing food and oxygen into a pile of troublesome carbon dioxide gas? I'll discuss those in the next couple of introductory posts, on molecules and energy.

1Actually, you can break down elements and turn them into eachother, but this is very difficult. It wasn't really possible until around the 20th century, and is part of the field of nuclear physics. I'll bring this up when I write about the structure of the atoms themselves next time.

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