I came across a question asking me the temperature at which $\ce{D2O}$ will have maximum density. I didn't really know, so I checked the answer, and I found that the answer is $\pu{11.6^oC}$. But why ? Why is the density of $\ce{D2O}$ maximum at $\pu{11.6^oC}$? I can't find the reason anywhere...
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2$\begingroup$ It's possible to explain why there is a maximum at all, but justifying why the maximum occurs specifically at the temperature it does is almost hopeless. The best you can do is probably "because that's how our Universe is set up", or more precisely, "because the Standard Model has the dimensionless constants that it does". $\endgroup$– Nicolau Saker NetoApr 27, 2017 at 10:03
1 Answer
The maximum density of $\ce{H2O}$ occurs when cooled to about $\pu{4^oC}$. At this point, the thermal motion of the molecules has slowed down enough such that water molecules can start to orient themselves in a manner more resembling that found in the expanded crystalline ice structure, and density thus starts to decrease with decreasing temperature. This pre-freezing expansion happens at a somewhat higher temperature for $\ce{D2O}$ due to the greater hydrogen bonding strength of $\ce{D2O}$ at any given temperature.
Also note that $\ce{D2O}$ freezes at about $\pu{4^oC}$, so clearly the maximum density of $\ce{D2O}$ must occur at a greater temperature than that of $\ce{H2O}$. In the case of $\ce{H2O}$, the temperature of maximum density occurs at about $\pu{4^oC}$ above it's freezing point, where for $\ce{D2O}$ the temperature of maximum density occurs at about $\pu{7^oC}$ above it's freezing point.
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2$\begingroup$ I understand, what you are trying to explain when you say 'molecular vibrations have slowed down' but this is not correct; the frequency is the same irrespective of temperature. What would be more accurate is to explain that as the freezing temperature is approached regions of structured H bonds have a chance to form without thermal motions disrupting them by too much. On balance (H bonds expansion vs. thermal motion causing contraction) the density is increased. On freezing the H bonds are now 'fixed' into a larger lattice and then occupy a larger volume than in the liquid. $\endgroup$ Apr 27, 2017 at 8:15
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$\begingroup$ @porphyrin , yea, I didn't have that all right. I made some changes, thx for the heads up. $\endgroup$– airhuffApr 27, 2017 at 17:36