After taking the first measurements of liquid water at temperatures much colder than its typical freezing point, a research team at the US Department of Energy’s Pacific Northwest National Laboratory has concluded that the water, supercooled , it is actually two liquids in one.
The finding, published Sept. 17 in the journal Science , provides long-sought experimental data to explain some of the strangest behaviors that water exhibits at extremely cold temperatures, found in outer space and on the fringes of the planet. Earth’s own atmosphere.
Water, a substance as common as it is peculiar
Despite being one of the most abundant and studied substances on the planet, and despite its apparent simplicity (two hydrogen atoms and one oxygen atom per molecule), H2O is deceptively complicated. Water has a number of anomalous properties, including its density . Its solid form is less dense than the liquid, a characteristic that has preserved life and regulated the planet’s climate.
Until now, some scientists have wondered if it is possible for water to actually exist as a liquid at extremely low temperatures, –83 ° C ; or if such strange behavior is simply water reorganizing itself on its inevitable path to a solid.
It’s surprisingly difficult for water to freeze just below its melting point – water resists freezing unless it has some other solid to stick to. It takes an energetic push to push the molecules into the special arrangement needed to freeze. And water expands when it freezes, which is strange behavior compared to other liquids. But that rarity is what sustains life on Earth. If the ice cubes were to sink or the water vapor in the atmosphere did not retain heat, life on Earth as we know it would not exist.
The new study has shown that liquid water at extremely cold temperatures is not only relatively stable, but exists in two distinct structures , according to Greg Kimmel, a chemical physicist at the Pacific Northwest National Laboratory (PNNL): “The findings explain a controversy long-standing on whether deeply supercooled water always crystallizes before it can balance out. The answer is no”.
Supercooled water: two liquids in one
Various models have been proposed to explain the unusual properties of water. New data obtained using a kind of stop motion snapshot of supercooled water shows that it can condense into a high-density liquid-like structure. This higher density form coexists with a lower density structure that is more in line with the typical bond expected for water.
The proportion of high-density liquid decreases rapidly as temperature increases, supporting ‘mixing’ model predictions for supercooled water. In other words, water at this temperature is actually two liquids.
The researchers used infrared spectroscopy to monitor water molecules trapped in a kind of stop motion when a thin film of ice was hit with a laser, creating supercooled liquid water for a few nanoseconds.
Additionally, this research may help explain graupel , a phenomenon also known as ice chips or often hail , which sometimes falls during cold weather storms. It forms when a snowflake interacts with supercooled liquid water in the upper atmosphere.
These studies can also help understand how liquid water can exist on very cold planets (such as Jupiter, Saturn, Uranus, and Neptune) in our solar system and beyond. The supercooled water vapor would also create the tails trailing behind comets.
However, there is still much to learn, and these measurements will help pave the way to a better understanding of the most abundant liquid on Earth.
Source: Phys.org statement
References: “Reversible structural transformations in supercooled liquid water from 135 to 245 K” Science (2020) https://science.sciencemag.org/content/369/6510/1490
