A choice of waters: still, sparkling, para or ortho?


The world’s best known molecule actually comes in two types – with different behaviours. Phil Dooley reports.


Water: at once familiar and strange.
Water: at once familiar and strange.
Raeff Miles/Getty Images

We all know water is H2O. Now researchers have discovered if you stick one H from the pair on upside down, it changes how water behaves – a surprising new insight into the chemistry of one of the most important substances on our planet.

Hydrogen atoms have a magnetic orientation, called spin. When two hydrogens form water by attaching to either side of an oxygen atom, like ears on a Mickey Mouse cap, they can end up in one of two configurations: with their two spins opposite ways up, a form called para-water; or with spins symmetrical, called ortho-water.

Although researchers knew all the water on our planet is a mixture of both, they have only recently found a way to separate the two forms because they behave almost identically at room temperature.

But new experiments reported in the journal Nature Communications show there are differences in the behaviour when the molecules are cooled to freezing. These temperatures are similar to those found in outer space, suggesting the discovery could provide insights into reactions that might form the building blocks of life in interstellar regions.

Stefan Willitsch from the University of Basel in Switzerland used a technique pioneered at Germany’s Hamburg Centre for Free-Electron Laser Science to separate out streams of para- and ortho-water molecules.

Willitsch and his team then shot the results at nitrogen molecules to which hydrogen ions had been added – creating a species known as a diazenylium ion, which occurs in interstellar space.

The team was looking for a reaction that occurs in space, in which a water molecule collides with a diazenylium ion and steals its hydrogen, forming hydronium (H3O+).

The surprise was that the para-water was 25% more likely to make a successful theft than ortho-water.

“That we can really show they have different chemical reactivities is a pretty fundamental insight,” said Willitsch.

“It’s one of the most fundamental molecules on this planet – in the entire universe, so this is quite an important piece in the whole puzzle.”

Willitsch doesn’t believe the finding will have a big impact on most chemistry, however. While water molecules in isolation stay in their ortho or para form, in water’s liquid, room-temperature state, there are constant molecular collisions between molecules that quickly mix the two forms up.

A disappointment perhaps – it seems that for we humans at terrestrial temperatures, there will never be a chance to compare the tastes of pure para-water and ortho-water.

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Contrib phildooley new.jpg?ixlib=rails 2.1
Phil Dooley is an Australian freelance writer, presenter, musician and videomaker. He has a PhD in laser physics, has been a science communicator for the world's largest fusion experiment JET and has performed in science shows and festivals from Adelaide to Glasgow. Under the banner of Phil Up On Science he runs science pub nights around the country and a YouTube channel.
  1. https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201405986
  2. http://dx.doi.org/10.1038/s41467-018-04483-3
  3. https://link.springer.com/referenceworkentry/10.1007%2F978-3-642-27833-4_1816-5
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