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Water Exists as 2 Different Liquids

S u m m a r y :
Water is not just one liquid: it actually exists as two different phases, shows new study published in the journal Proceedings of the National Academy of Science (PNAS).

A Tale of Two Liquids

Liquid water is not what we initially thought of it, suggests a discovery made by researchers from Stockholm University. The study broadens our opinion of water: instead of thinking of liquid water as disorderly molecules, consider its occurrence as being in two phases that display differences in both structure and density, as shown by X-ray experiments.

The newly-discovered property allows water to exist as two liquids at conditions of low temperatures whereby the rate of ice crystallisation is also low, explains study author Anders Nilsson.

Using X-Rays to Show 2 Phases of Water

Nilsson and colleagues made use of X-rays at US-based Argonne National Laboratory to show the two different structures. The team then demonstrated the dynamics thereof at an X-ray lab in Hamburg, Germany; the two phases were confirmed to be two liquid phases, thus showing that water can exist as two liquids.

An artist’s impression of the two phases of water, at different densities, one more viscous than the other. The background depicts the X-ray speckle pattern taken from actual data of high-density amorphous ice, which is produced by pressurizing water at very low temperatures. Photo credits: Mattias Karlén.

The X-ray experiments were meant to determine the positions of molecules with respect to one another at different times, explains Fivos Perakis.

“We have in particular been able to follow the transformation of the sample at low temperatures between the two phases and demonstrated that there is diffusion as is typical for liquids,” says Perakis.

Water vs Most Liquids

It is known that water has anomalous and weird properties which make it stand out from other liquids. For instance, its melting point and density are in stark contrast with the majority of liquids. These special features, in turn, are essential for life to occur.

As for its solid state, ice, it also mostly exists as a disorderly arrangement of molecules; though, it is more commonly described as having an ordered, crystalline phase. Ice actually can occur as two forms of amorphous ice with low and high density, and the two can change from one to the other. This pair of ice phases was suspected to be linked with forms of liquid water of high and low densities—this hypothesis is what led the Stockholm University scientists to pursue their studies.

“I have studied amorphous ices for a long time with the goal to determine whether they can be considered a glassy state representing a frozen liquid,” says co-author Katrin Amann-Winkel.

A Dream Come True: Two Viscous Liquids

For Amann-Winkel, it is a dream come true to have studied in detail the transition from a glassy phase of water into a viscous liquid which then turns into a different, more viscous, low-density liquid in no time.

We might think that we already know all that there is to be known about wate, but we couldn’t be more wrong. Daniel Mariedahl from Stockholm University attests to this fact, describing the possibility of making new discoveries concerning water to be fascinating, and the more so because X-rays were used to do so when the father of X-ray radiation, Wilhelm Röntgen, had mentioned the possibility of water existing in two different forms.

A Non-Complicated Liquid

The new study brings more clarity to previous observations: the two phases help explain the anomalous properties of the compound. For instance, at room temperature, water cannot ‘decide’ if it has to be of high or low density, and so it occurs as local fluctuations between the two phases. Furthermore, the results also help to build a clearer picture of how water is influenced by temperature and pressure as well as salts and other molecules crucial for life. Moreover, the findings can assist researchers in purifying and desalinating water.

“In a nutshell: Water is not a complicated liquid, but two simple liquids with a complicated relationship,” says Lars G.M. Pettersson.


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