Water Gets Weirder as New Properties Come to Light

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CC BY-SA 2.0. Andrew Newill

As if it's not enough that water covers over two-thirds of the earth and is the basis for the existence of life, water continues to amaze us.

Water has many weird properties, including that fact that water ice floats in liquid water - the crystalline form of most substances is denser and sinks; can you imagine what would happen to life if lakes froze from the bottom up? Water can absorb a tremendous amount of heat before it boils, and has an unusually high surface tension. Water also acts as a sort of "universal solvent," able to dissolve many substances. Some scientists are investigating whether water might even be two different liquids in one.

Now scientists are adding a new property to the list of water weirdness. Most everyone knows that water is H2O, or two hydrogen atoms linked with one oxygen atom. Less well known is the fact that the H2O is continually breaking apart into OH- and H+ bits, hydroxide and hydrogen ions.

These OH- and H+ ions are constantly moving about through water. For a long time, it was assumed that they both jump around at the same speeds, using mechanisms that effectively mirrored each other. Then, surprisingly, computer models predicted an asymmetry in the mechanisms of transport.

Proving this suspicion required some novel scientific thinking, which a team at New York University believe they have achieved. Their approach required cooling water to its temperature of maximum density, where the asymmetry is expected to be most pronounced. They then used nuclear magnetic resonance imaging to see what was happening with the hydroxide and hydrogen pieces (NMR is the chemists' name for the instrument doctors call MRI, magnetic resonance imaging; it has nothing to do with scary nuclear radiation but instead uses properties of the atomic nucleus to make pictures).

The approach yielded two breakthroughs: first, the team demonstrated that the OH- ions do have a longer lifetime at that temperature - meaning they are moving more slowly to the place where they can quit being OH- and join up into other water molecules again. The evidence supports the asymmetry hypothesis.

Second, the team posits that the asymmetry is actually the reason water has its maximum density at this temperature (4°C or 39°F) before becoming less dense as the crystalline structure of ice forms. The longer-living OH- ions are forming their own complexes, contributing to the unusual density properties of water.

Two mysteries solved for the price of one! The study's lead author, Professor Alexej Jerschow says,

“The new finding is quite surprising and may enable deeper understanding of water's properties as well as its role as a fluid in many of nature’s phenomena.”

Because understanding the weird properties of water helps engineers harness it for clean energy, aids biochemists in understanding the way our cells work, and sheds light on the nature and evolution of life on earth, any new science in the weirdness of water is welcome.