Quote:
Originally Posted by AndrewJ
One is made from heavy water
Andrew
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Is the correct answer. Congratulations, I owe you a Mars bar.
Yes, the normal ice cube is H2O and the other is D2O. Normal water has an atomic weight of 18 (16+1+1) but D20 has an atomic weight of 20 (16+2+2). Since both have the same number of molecules per mL the 'isotopically heavy' ice is also actually physically heavier. [I am assuming that the oxygen-16 : oxygen-18 ratio is normal but if it is also enriched then the ice would be even heavier.]
The photo was submitted on a mailing list in response to an request for a photo which demonstrates stable isotopes. There is another member of IIS who also measures stable-isotope ratios for a living so for him this would have been trivial and I think some others may also have a professional reason to understand this; they were the people who I wanted to stay mum.
Thanks to all who gave it a shot.