Re: Why doesn't air or water follow the life cycle?

Hello Jesse, your question was: Everything on Earth had a birth, life and 
death. The stuff in rocks, same thing. So why doesn't air or water have 
that birth, life, and death cycle? They get polluted and everything, but 
the molecules don't die? Why?

A thought-provoking question! I could address your question by asking one. 
My question would be – what is the difference between life and death? I’m 
sure we all have a good idea of the answer – and that it is easy for us 
all to recognise the difference between living, growing objects which 
includes plants, animals, even bacteria and microbes of all sorts, and 
inanimate objects such as rocks, sand, water and air. Cycles of birth, 
life and death are easy to understand in living objects – but what does 
that concept mean (if anything) for inanimate objects.

I’m going to guess what underlies your question, in that I think you are 
recognising that water and air molecules can go through repetitive cycles 
in which they reappear in the same way that they were previously, despite 
having been processed by all sorts of complex mechanisms in the meantime. 
Water falls as rain – lots of it back into the sea, but a significant 
fraction onto land. There it may get absorbed by plants, get trapped 
underground for years get drunk by animals including ourselves, then 
absorbed into our bodies (we are mostly water), but will eventually find 
its way back to the sea and atmosphere one way or the other. Recycling of 
air is a little less obvious, but air, mostly nitrogen, oxygen and some 
carbon dioxide is dissolved in water – the sea and fresh water, is 
breathed by animals and plants and is in fact often chemically altered in 
the process. An equilibrium is often established – for instance we breathe 
in oxygen and breathe out carbon dioxide. Plants breathe in carbon dioxide 
and breathe out oxygen – and a balance is achieved. (Other complex 
processes contribute to this too.)

While chemical reactions can cause the molecules of air to become altered 
and bound to other atoms or molecules, the atoms are never destroyed. The 
same is true for the atoms that make up the molecules in us. When we die, 
our molecules will get broken down into simpler ones, but the atoms are 
never lost. Perhaps if you could label individual molecules in air and 
water you might find that some get changed for ever into something else – 
although for ever is probably the wrong phrase. Similarly rocks which get 
broken down to form sand, do not lose any atoms in the process, and indeed 
many of the molecules can be unaltered for huge lengths of time.
In the end, I would say that the world, the universe, even, is a 
chemically dynamic place where nothing lasts unchanged for ever. Except 
through nuclear fission, (and fusion) atoms never die (or are born). In 
that sense there is no life cycle for inanimate objects – they merely go 
through transformations, sometimes reversibly, and sometimes not very 
reversibly.

Thanks for your thought-provoking question – and I hope my answer makes 
sense. If not, or if it leaves you wanting to ask more questions – fire 
away!