Researchers Explain the Transformation of Water into Ice

Making ice from liquid water is simple enough. Figuring out the molecular processes underlying the transformation, however, has proved far more difficult. But in a report published today in the journal Nature scientists describe having used a supercomputer to pinpoint the specific conditions required to start the crystallization of water into solid ice.

Previous simulations of the freezing of water had imposed artificial conditions on the system or resulted in a crystallized state different than the one that forms at atmospheric pressure. A detailed understanding of water’s freezing processes is complicated by the substance’s inclination to achieve a supercooled state, in which it remains a liquid despite being cooled well below its freezing point. Supercooled water turns to ice only after the formation of a so-called critical nucleus that grows until the entire sample crystallizes. In the new work, Masakazu Matsumoto and colleagues at Nagoya University in Japan set out to uncover the molecular-level details of how such a critical nucleus forms in liquid water. They computed changes in the relative orientations of 512 water molecules using calculations of the forces acting on and between them. The spontaneous appearance of several unusually long-lasting hydrogen bonds between adjacent water molecules, they report, is the first step of ice formation. “The initial nucleus then slowly changes shape and size until it reaches a stage that allows rapid expansion,” the authors write, “resulting in crystallization of the entire system.”

Though the newfound understanding of the processes at work in a freezer won’t change the ice that comes out of it, the findings do offer more general insight into crystallization. According to Srikanth Sastry of the Jawaharlal Nehru Center for Advanced Scientific Research in India, “understanding the behavior of water could help us to tackle other questions, such as when a liquid readily forms a disordered glass state�an important issue in designing non-crystalline materials.”

Why doesn’t the ocean ever freeze?

There are a few different reasons why the water in the ocean doesn’t freeze all over, like certain lakes and ponds. (Which sometimes freeze enough that you can go ice-skating on them!)

One reason ocean water doesn’t freeze all the way through is because it’s so salty! Salt lowers the freezing point of water, meaning that freshwater (without salt) will freeze sooner than salt-water; salt-water like the ocean has to get even colder than freshwater before it will freeze, and it doesn’t always necessarily reach that cold enough temperature.

In addition to all of its salt, ocean water also undergoes more movement than the freshwater in lakes and ponds. The movement of ocean water (like from currents or wind) helps it retain more heat than smaller lakes and ponds where the water is generally much more still. 
In certain cold parts of the ocean, some water will freeze on top and then float like big sheets on the water below it! The amount of frozen water in the ocean depends on the season, but generally at least 15% of ocean water is frozen at any given time. – That’s several million square miles!

What happens to ocean water when it freezes

The freezing temperature of water depends on the amount of dissolved salts (salinity). Normal ocean water, with a salinity of about 3.4%, begins to freeze when temperatures reach about -1.9°C. At its maximum extent in the winter, sea ice covers about 19 million square kilometres of the Southern Ocean near Antarctica. Approximately 80% of this ice melts each summer, contributing to the global mixing of ocean water.
Vertical mixing of ocean water, known as ‘overturning circulation’ or ‘thermohaline circulation’, is an important aspect of the global current system that is driven primarily by rising and sinking of water masses at high latitudes in both the northern and southern hemispheres.
The seasonal formation and melt of sea ice is the dominant factor controlling the salinity and density of surface ocean waters in the polar regions. Salt is expelled as the ocean water freezes to form sea ice. This creates dense brine that sinks and flows down the continental shelf of Antarctica to form Antarctic Bottom Water – the densest water in the open ocean. This water flows outward from the Southern Ocean and through other ocean basins as part of the global ocean circulation ‘conveyor belt’ that distributes heat, nutrients and gases around the world. There are only a few areas in the world where this dense water is formed.
Sea ice extent in the Arctic has been declining rapidly over the past few decades. Although there are currently no clear trends in the extent of Antarctic sea ice, with the exception of the area around the Antarctic Peninsula, numerous climate models agree that Antarctic sea ice will decline in the future. A decrease in the amount of sea ice may slow the overturning circulation, decreasing the ocean’s ability to absorb atmospheric carbon dioxide and affecting the distribution of heat around the globe.