Yuns needa learn me some kemostree

[ThankQ]

Chemistry was always my least favorite of the hard sciences, probably due to disappointment more than anything else. I had this "Mr. Wizard" impression of chemistry, all burners, beakers, flasks and fluorine. Then you finally take a chemistry class and it's all about the mole. No, not a tasty Mexican concoction, the unit that has to do with 6.02 sheets of carbon paper and 10^23rd of something else. Yes, I payed close attention

Anyway, I'm sitting here this morning drinking coffee and reading Jan 29 issue of "Science News," and there's an interesting article about making periodic table revisions in the atomic weights of several elements.

The atomic weights of oxygen, hydrogen, lithium, boron, carbon, nitrogen, silicon, sulfur, chlorine and thallium will now be noted as intervals with upper and lower bounds, rather than as specific values.
...
Until now, determining the atomic weight of an element with more than one stable isotope entailed averaging the relative amounts of the different versions, boosting the uncertainty around each number.

Okay, I'm with you so far. Simple enough.

Here's where my chemignorant question comes in:

Elements undergo what's called physical and chemical fractionation during processes such as going from liquid to solid. For example oxygen-18, or "heavy" oxygen, prefers cold things. So the ice cubes in you freezer will have considerably more oxygen-18 than the liquid water from which they formed.

Okay... from where does it gain these neutrons? Does it take them from "heavy" hydrogen in the water? Does it take them from whatever elements are in the air in the freezer? Will it take two 16^O and churn out a 14^O and an 18^O? There aren't a bunch of free neutrons just hanging about waiting for some Oxygen to get cold, right?

Teach me, oh wise ones.

 

[farmkid]

I don't think the atoms are gaining or losing neutrons. Rather, I think what's going on here is that the water molecules with the heavier isotopes of oxygen will freeze at a slightly higher temperature than those with lighter isotopes. My guess is that the heavier atoms move a little slower at the same temperature. This makes sense because temperature is essentially a measure of the average kinetic energy of the molecules in whatever is being measured. Heavier atoms/molecules will have a lower speed at the same kinetic energy compared to lighter atoms/molecules. Anyway, because of this lower speed, the heavier molecules will bond with other molecules to form a solid at a higher temperature than the lighter molecules will, so in water, molecules with heavier oxygen isotopes will freeze before those with lighter isotopes.

This makes sense to me, or I could completely wrong. I'm a biologist, after all. Any actual physicists or physical chemists want to weigh in?

 

[Jadzia]

It may be true that ice forming on top of a pond has more Oxygen-18 than the pond water. At zero degrees celsius, water containing that atom may crystalise more willingly than water molecules containing Oxygen-16.

But in an ice cube tray, where all water becomes ice, there isn't any molecular selection taking place overall.

If we're talking about oxygen atoms gaining and losing neutrons during the freezing of water... well those are nuclear reactions, and you'd have deadly gamma rays coming out of your freezer.

 

[iguana_tonante]

I don't think the atoms are gaining or losing neutrons. Rather, I think what's going on here is that the water molecules with the heavier isotopes of oxygen will freeze at a slightly higher temperature than those with lighter isotopes. My guess is that the heavier atoms move a little slower at the same temperature. This makes sense because temperature is essentially a measure of the average kinetic energy of the molecules in whatever is being measured. Heavier atoms/molecules will have a lower speed at the same kinetic energy compared to lighter atoms/molecules. Anyway, because of this lower speed, the heavier molecules will bond with other molecules to form a solid at a higher temperature than the lighter molecules will, so in water, molecules with heavier oxygen isotopes will freeze before those with lighter isotopes.

This makes sense to me, or I could completely wrong. I'm a biologist, after all. Any actual physicists or physical chemists want to weigh in?

Chemistry is far from my field of expertise, but that's how I interpreted it too. It would also make sense of the use of the term "physical fractionation", as the heavier fraction will freeze first.

 

[ThankQ]

^Okay, I'll buy all that, but that doesn't explain, to me anyway,:

So the ice cubes in you freezer will have considerably more oxygen-18 than the liquid water from which they formed.

The same water in solid form has more O-18 than in liquid form?

EDIT:

It may be true that ice forming on top of a pond has more Oxygen-18 than the pond water. At zero degrees celsius, water containing that atom may crystalise more willingly than water molecules containing Oxygen-16.

But in an ice cube tray, where all water becomes ice, there isn't any molecular selection taking place overall.

Ah, now that makes sense!

If we're talking about oxygen atoms gaining and losing neutrons during the freezing of water... well those are nuclear reactions, and you'd have deadly gamma rays coming out of your freezer.

Maybe I do. Mind your own freezin' business!

 

[sojourner]

I would think that in the case of the ice cube tray as the o-18 freezes first, this gives the o-16 more time to off gas from the water. So by the time the entire tray is frozen you end up with less o-16.

After all, they just said you would have less o-16, not how much less.

 

[MetalPants]

If we're talking about oxygen atoms gaining and losing neutrons during the freezing of water... well those are nuclear reactions, and you'd have deadly gamma rays coming out of your freezer.

Maybe I do. Mind your own freezin' business!

Have noticed any mutations happening lately? Can you shoot death rays out of your eyes?

 

[Robert Maxwell]

If we're talking about oxygen atoms gaining and losing neutrons during the freezing of water... well those are nuclear reactions, and you'd have deadly gamma rays coming out of your freezer.

Maybe I do. Mind your own freezin' business!

Have noticed any mutations happening lately? Can you shoot death rays out of your eyes?

The other day I woke up with a third penis. Is that normal?

 

[sojourner]

^Depends, how many men were in your bed?

 

[Robert Maxwell]

^Depends, how many men were in your bed?

There was only me, unless science has invented invisible men.

 

[sojourner]

Which can only lead to the question: Which came first, invisible men or the third penis?

 

[Asbo Zaprudder]

Elements undergo what's called physical and chemical fractionation during processes such as going from liquid to solid. For example oxygen-18, or "heavy" oxygen, prefers cold things. So the ice cubes in your freezer will have considerably more oxygen-18 than the liquid water from which they formed.

I suspect that they'll have to print a correction in the next edition. Popular science magazines screw up quite a lot. I object more to the use of the verb "prefers" as though the atoms are somehow sentient.

 

[Bisz]

Chemistry was always my least favorite of the hard sciences, probably due to disappointment more than anything else. I had this "Mr. Wizard" impression of chemistry, all burners, beakers, flasks and fluorine. Then you finally take a chemistry class and it's all about the mole.

Italicization mine. Great punch line, thank you for the genuine LOL

I think I'm with the majority in here on this, the guy writing the article didn't get it quite right.

 

[STR]

The other day I woke up with a third penis. Is that normal?

Not sure. Growing a second one is certainly odd, but once you have the second it might be inevitable that a 3rd one pops up.