Doc,
Some parts of QM are really surprising and alien to the view of reality we form based on direct observations.
The influence of one additional neutron in the nucleus is strong in the case of superfluid helium. He-3 is really different from He-4 when the temperature is around 2 K or less. That difference is mainly due to the difference between a boson and a fermion.
The same effect applies, in principle, to other sets of identical atoms and molecules as well. H2O is different, when the oxygen atom is O-17, not O-16. The difference between ice crystals formed from these two varieties of H2O is, however, almost solely due to the different masses of the isotopes, while the property of being a boson or being a fermion is likely to be too small to observe.
The difference between fermions and bosons becomes observable when the QM eigenstates of the multiparticle system determined forgetting the fact that the particles are non-identifiable contains to a significant degree a mixture of the states where particles A and B appear in the original order and where their order is switched. For bosons contributions of these two orderings add up, for fermions they must be subtracted from each other. Therefore having two particles in the same state is enhanced for bosons, but forbidden for fermions. Both differ from the case of identifiable particles.
In quantum mechanics tunneling of the particles through a virtually impossible state is allowed. Thus a tunneling phenomenon, where two H2O molecules switch position in ice lattice is allowed in principle. This possibility leads to the difference between bosonic ice and fermionic ice. The frequency of this kind of tunneling event is, however, so extremely small that the influence on the properties of ice is totally negligible.
The only situation that I can imagine, where two or more water molecules can coexist under conditions where they can really interchange their places (or states) is the case of dimers in free space (perhaps also trimers, ..). Dimers in free space can rotate and the allowed rotational states are different for bosonic and fermionic H2O dimers.
Helium is different, because the interaction between two He atoms is so weak that He stays liquid down to temperatures where the quantum nature of the atoms makes a difference. The small mass of He helps also, while being a molecule rather than single atom has an opposite influence for H2O. These are, however, quantitative factors that determine the quantitative outcome, not fundamental.
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