Which is denser ice or water

Water is made up of two liquids

Hidden double life: Physicists have discovered that liquid water exists in two different forms. The supposed uniform liquid actually consists of two different, interacting liquids. So far, these two water variants have only been detected at very low minus temperatures, but the researchers believe it is likely that they also exist at room temperature.

Water is not only a prerequisite for life, it is also an extremely exotic substance: it differs from most other liquids in at least 70 properties. Perhaps the best known are heat capacity and density anomaly - the fact that water expands when it freezes. In its liquid state, water also forms a highly complex mixture of "molecule clumps" that change their structure and arrangement in a flash.

Density differences in amorphous ice

But it gets even stranger: some time ago researchers discovered that the so-called amorphous water ice occurs in two variants. This non-crystalline, glass-like ice is rare on Earth, but is common in the solar system. It occurs, among other things, when liquid water is cooled so quickly that the molecules do not have time to form an ordered crystal structure. Analysis reveals that this water ice exists in a more dense and a 25 percent less dense form.

“Scientists have long been wondering whether these two types of ice do not have the corresponding variants in liquid water,” explains co-author Felix Lehmkühler from the German Electron Synchrotron (DESY) in Hamburg. “However, this is very difficult to measure. Even if there are both variants in liquid water, they are constantly mixing, transforming into one another, and there is no way of separating the two. "

So far, the two variants of water have only been detected at low temperatures, but they could also exist at room temperature. © Gesine Born / DESY

Discrepancy even in the liquid state

However, the physicists have now overcome this hurdle. Their trick: They first cooled ultrapure water down to such an extent that it turned into amorphous water ice. At minus 150 degrees it was in the form of dense HDA ice. Then they slowly heated the sample to minus 140 degrees and observed the structure of the water with the help of high-energy X-ray light.

It was shown that at these temperatures the amorphous HDA ice not only becomes liquid, two different structures of liquid water are also formed - analogous to the two types of ice. First, the HDA ice cream turns into a high-density liquid form (HDL), then this is converted into a lower-density form (LDL), as the researchers report.

Double existence even at room temperature?

"We have thus discovered a remarkable new property of water: We have established that water is present as two different liquids at low temperatures," says study leader Anders Nilsson from Stockholm University. His colleague Lars Pettersson adds: "In short: water is not a complicated liquid, but two simple liquids in a complicated relationship."

But what does that mean for water in our everyday life? It has not yet been proven that the water consists of these two variants even at room temperature, but the researchers consider it very likely. “The new results suggest that water at room temperature cannot decide which of the two forms to take - a high or a lower density,” explains Pettersson. "That leads to local fluctuations between the two forms."

With future studies, the scientists hope to be able to demonstrate the “double life” of water at room temperature too. In any case, the results provide new insights into the strange world of the life molecule H2O and its strange properties. (Proceedings of the National Academy of Sciences, 2017; doi: 10.1073 / pnas.1705303114)

(German Electron Synchrotron DESY / Stockholm University, June 27, 2017 - NPO)

June 27, 2017