Naked Science Forum
General Science => Question of the Week => Topic started by: thedoc on 22/07/2008 20:03:03
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I was taught when I was young that we drink the same water today that the dinosaurs drank when they were alive, is that true?
Asked by Skye, Arkensaw
Hear this Question on our Podcast (http://www.thenakedscientists.com/HTML/podcasts/show/2008.07.27/)
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Professor Ken Carslaw, Atmospheric Science, University of Leeds:
[img float=right]http://www.thenakedscientists.com/HTML/uploads/RTEmagicC_800px-Water_droplet_blue_bg05_02.jpg.jpg[/img]Water is recycled through the water cycle. It evaporates from the oceans, forms clouds, it rains (or snows), the rivers return the water to the ocean. The longest timescale of water anywhere in the cycle is in the deep ocean (it stays there for several thousand years) and in deep ground water (perhaps 10,000 years). However, water is very slowly destroyed chemically in photosynthesis (plants converting carbon dioxide and water to sugars and oxygen) and recovered again in respiration (basically the reverse of photosynthesis to make energy and CO2). You can calculate how much water remains from the dinosaur age from the total amount of water on the planet and the amount of water taken up in photosynthesis per year. The Earth's plants take up about 12,000 billion kg of water per year (we know that roughly from the CO2 they take up). The total water on Earth is about 1400 billion billion kg. So within about 100 million years most of the water will have been chemically destroyed. Dinosaurs lived 65 million years ago. So, SOME of the water we drink is the same water, but more than half is different water.
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I think the water that's around today was around when dinosaurs were toodling about. Whether we are drinking the same water as them is a different matter. What they drank could be at the bottom of the Pacific Ocean by now.
mod edit - tweaked a little - I'm sure Dr Beaver can see why! - Others, you'll never know...
beaver edit - OI.. stop fiddling with my bits! [:(!]
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Some of the water excreted by a plant or animal will have been created in that organism by respiration.
So it is new water: a particular combination of hydrogen and oxygen atoms which did not previously exist.
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Given the water cycle, that is, it evaporates due to the heat of the sun, condences and forms clouds, then falls to the Earth as precipitation, it could be argued that it is the same water going round and round. Then again, it could equally be argued that, as it has been converted into gases (oxygen and hydrogen) and reformed, it is not the same. I think this depends on your point of view.
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Extremely unlikely. Water reacts in numerous ways in nature. It reacts with rocks and other materials to form oxide, hydroxides, etc. It also is consumed in biological systems and is incorporated into the living tissue in various molecular forms or excreted in some form back into nature. The odds of the exact H atoms recombining with the exact same O atom is extremely remote.
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Thanks Will, and welcome, by the way!
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Well that told me [:(]
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I had one more consideration after hearing the expert answer on your program yesterday. If you consider "one water molecule" to be the exact same three atoms bonded together, then the idea that water molecules are created and destroyed through photosynthesis and respiration is correct. That process also involves changing the oxidation state of the atoms - they gain and lose electrons O2-(formal oxidation state in water) ↔ 1/2 O2(g) + 2 e-.
If you go one further and consider /proton/ exchange, essentially the autoionization process that occurs in any water sample and gives neutral water a pH of 7 ([H3O+] = 10-7 M) but involves no change in the oxidation state of the atoms, it is even more unlikely that any individual water molecules have survived for 65 million years. I challenge someone to look up the kinetics of this process - it is very fast, and explains the very high conductivity of acid solutions - to obtain the half life of an individual water molecule at pH 7 in liquid water at 20 oC. With no data in front of me, I feel pretty confident that it will be less than a microsecond.
btw, I will certainly assign this as a homework problem in kinetics this fall!
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Nice point, I'd not considered that at all. If you do do the calculation, please let us know the result...
Chris
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All of the processes that replace old water molecules with new ones are dilution processes. What this means is that not every water molecule broken down through photosynthesis or some other chemical process needs to be one of the molecules that was around at the time of the dinosaurs. Many of the molecules will come from the countless generations of new molecules formed since the time of the dinosaurs. In fact, as time goes on and the total number of "new" molecules grows, the likelihood that a reaction involves a new molecule versus an dino-vintage molecule grows.
Looking at the figures given by Dr. Carslaw, we can calculate the percentage of old molecules that would remain if photosynthesis were the only change mechanism. Assuming the total volume of water and the rate of photosynthesis have remained relatively constant at the values given by Dr. Carslaw over the last 65 million years (a big assumption, I know) after one year we would have exchanged:
100 x (12 x 1012/1.4 x 1021) or 8.57 x 10 -7%
of the total water. In the second year, we would exchange the same percentage of the total water, but some of this exchanged water would be new water and some would be original water.
To calculate the total original water remaining after 65 million years we would use:
1.4 x 1021 x (1 - (12 x 1012/1.4 x 1021))65 x 106
This gives us about 802 x 1018 kg remaining, or 57.3% of the original water. If we assume all other cause contribute about the same amount of exchange each year, the figure drops to 32.8 %. So the actual percentage of original "dino water" still around depends on how accurate all of these assumptions are, but it is safe to say that we still have quite a lot of it.
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That's a fantastic answer, thanks. Can't believe it didn't occur to me.
Chris
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Thanks for all this fantastic Information. I can go and do my show at Hampshire water festival tomorrow and safely say that some of the Earth's water today was around when Dinosaurs roamed the planet.
Kids love this fact and I would have hated it to be a myth.
The show is called Wacky Water and is full of wet facts.
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Could it be argued that no water molecule is the same from one second to the next, even in a stable body of water, since water is subject to hydrogen bonding?
As I understand it, hydrogen bonding means that hydrogen atoms in a molecule of water will change places with the hydrogen atoms of the neighbouring atom, and the next, and the next ad infinitum. Thus, when in a large body of water, such as the oceans, one hydrogen atom from one molecule of water could have travelled many miles across countless water molecules every hour.
Is there ever a chance that the same two atoms of hydrogen will meet with the same atom of oxygen at the same time to make a molecule identical to one that existed before? Perhaps only in a small enough and perfectly stable body of water.
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Its true.I heard this news that water changes always.
Regards
http://www.bewellbuzz.com/ (http://www.bewellbuzz.com/)
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nooooooooooooooo
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NOT TRUE!!!!!!!!!