# Maximum density of heavy water at 11.6 degree Celsius?

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...

• 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". Apr 27 '17 at 10:03

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.