When and how does salt water freeze

Chemistry: Why water only freezes at minus 48.3 degrees

Water freezes at zero degrees Celsius. In theory at least. Due to its salt content, sea water may not solidify until minus 1.9 degrees or, in the extreme case of the Dead Sea, even at minus 21 degrees. But even salt-free water only freezes well below zero if it is extremely clean. Then the crystallization nuclei, which the H 2 O-molecules need to attach and insert into an ordered lattice. But how deep is it?
Researchers at the University of Utah have examined it more closely: water must freeze at minus 48.3 degrees Celsius at the latest, no matter how salty or clean it is. Liquid water of minus 40 degrees had already been found in clouds, so now the theoretically achievable lower limit has been found.

At first sight, one could think that the decreasing temperature has then reached a point at which the molecules' own motion is so reduced that their buzzing can no longer overcome the forces of attraction and they come together in the solid crystal lattice. But rather, at minus 48.3 degrees, a completely new spatial structure of the water arises.

No turning back

The floating molecules are arranged in a tetrahedron shape, i.e. in the form of pyramids with a three-sided base, in the middle of the pyramid there is also a molecule. While each molecule in the liquid state of aggregation was initially connected to two others (via loose so-called hydrogen bonds), there are now four partners. In a sense, there is now no turning back, and the network of bonds becomes so tight that a regular crystal lattice is formed.

Knowing this is of interest not only to spiritual theorists, but also to meteorologists and climate researchers. Because the circumstances, when which molecular water structures merge, also influence how irradiated sunlight is absorbed and bound in the atmosphere.