What is ice cream 7 1

Ice cream is not just ice cream

Water freezes into ice. You can use it to cool drinks or skate on them. But the ice we are so familiar with is actually just a variant of frozen water. Scientists currently know about twenty other types of ice. One thing is clear: The H2O-Molecule has one or two surprises in store.

Water plays a central role on our planet: It fills the oceans, we ourselves consist of over fifty percent water and die of thirst if there is a lack of water within a very short time. Like other substances, it can exist in different aggregate states. But the H2O molecules, each composed of one oxygen atom and two hydrogen atoms, show some extraordinary properties. The most prominent example of this is likely to be its reduced density when frozen. This ensures, for example, that ice cubes float on liquid water. But not all ice cream is created equal. "You can't give a clear definition," says Christoph Salzmann from University College London in the UK. "That's because water - or more precisely ice - is such a multi-layered material."

Structure of hexagonal ice

Common household ice cubes or snowflakes are, for example, what is known as ice I, also known as ice I.H. The “h” stands for hexagonal, because the molecules are arranged hexagonally on a microscopic level. The oxygen atoms are located at the corners of a hexagon and are connected to the other water molecules via hydrogen bonds. Each water molecule can form a total of four such hydrogen bonds and arrange itself in space in six different ways.

Ice crystals are often mentioned in everyday life, but: “Strictly speaking, normal ice I is not a crystal at all,” says Salzmann. A typical feature of a crystal is that its components are arranged on a microscopic level like on a grid - and that regularly. So every atom has its own fixed place. In the case of hexagonal ice, however, this is only the case for the oxygen atoms. “The hydrogen atoms are not arranged periodically,” explains Salzmann. "They are randomly distributed, the whole water molecule rotates, so to speak, on its lattice place."

Ice cream I.H is so familiar and omnipresent to us because it forms at atmospheric pressure and below zero degrees Celsius - for example in the ice compartment or in the clouds. "This hexagonal stacking recipe can be imagined as taking a layer of ice crystals and putting the mirror image on top," explains Salzmann. “But there is also the cubic stacking recipe, in which you move the entire layer in relation to the layer below.” The so-called ice cream Ic - "c" stands for the English word cubic - can form at temperatures below minus 22 degrees Celsius.

“Until recently, it was assumed that there was ice that was one hundred percent hexagonal or cubic stacking sequences,” reports Salzmann. “But if you try to make cubic ice in the laboratory, it is always mixed with hexagonal stacking sequences.” The record is currently 73 percent of cubic ice, set up by Salzmann and his colleagues. Your trick is to cool small drops of water very quickly. The scientists want to continue trying to produce purely cubic ice in the future. “That would be the perfect ice cube,” says the researcher.

In principle, the same conditions prevail in the earth's atmosphere that the ice with mixed stacking sequences needs to form. It is therefore entirely possible that not only hexagonal but also stacked, disordered ice will form there. An indication of this is provided by reports and observations of snowflakes, which instead of the usual six-fold symmetry only show a three-fold symmetry. The internal structure of ice can be examined with the help of X-rays, which are bent in a characteristic way due to the regular arrangement of the atoms. Such analyzes are not possible on site in the clouds. Thus the snowflakes with threefold symmetry provide an indication, but a definitive proof is still pending. “But in principle it should just exist in our atmosphere,” says Salzmann.

Phase diagram of water

In addition to Ice I - whether cubic or hexagonal - the phase diagram of water has numerous other ice variants with different structures and properties to offer: Researchers currently know 17 different crystalline forms of ice that form depending on pressure and temperature. “And these are completely different materials,” says Salzmann. This great difference in the phases, which all arise from the same basic building blocks, is particularly impressive with graphite and diamonds. Both the pencil lead and the gem are made of pure carbon, but couldn't be more different - and not just in terms of price. The inner structure is decisive: while a diamond, just like ice Ic has a cubic crystal structure, graphite is similar in its structure to an ordinary snowflake, because its crystal structure is hexagonal.

In addition to the different microscopic structure, the different forms of ice mainly differ in their density. "If you go to the higher pressures under which it is manufactured, you get much denser materials," explains Salzmann. “But there are also big differences in terms of hardness, melting point or crystal structure.” The crystalline phases of ice can also be divided into hydrogen-ordered and hydrogen-disordered forms - depending on whether the hydrogen atoms are firmly arranged in the crystal lattice or not. “There are always pairs of ice cream. Ice XIII is the hydrogen-ordered counterpart to ice V, and ice XV is the counterpart to hydrogen-ordered ice VI, ”explains Salzmann. The hydrogen-ordered forms Ice XIII, Ice XIV and Ice XV were discovered by the scientist and his colleagues. They only succeeded in doing this by adding tiny amounts of hydrochloric acid to the ice. The hydrochloric acid helped the hydrogen atoms arrange themselves regularly before freezing in their places in the lattice. The hydrogen-ordered phase has not yet been detected for ice IV.

Iceberg in Antarctica

While ice I can be found all over the world, the other phases of ice have so far only existed in the laboratory. The only exception is ice VI, which has been found in diamonds. The various ice shapes can be created artificially with the help of hydraulic presses and punches - but these too reach their limits at some point. "Ice VIII is the densest form of ice," says Salzmann. In principle, it can be made from ice VII at temperatures below five degrees Celsius. However: “For production we need a pressure of 25,000 atmospheres. We can create it once, but then our equipment is broken, ”says the scientist.

Beyond Earth, however, it would be entirely possible for exotic ice forms to occur naturally. Ice moons such as Ganymede around Jupiter have layers of ice up to 600 kilometers thick, which generate enormous pressure despite the celestial body's lower gravity. In the rest of the universe, too, water is represented quite often in the form of ice - after all, H.2O the third most abundant molecule in the universe. It forms on comets or small grains of dust, but then has an amorphous, i.e. disordered structure. So far, researchers are familiar with at least two different amorphous forms of ice. But even with 17 crystalline and two amorphous forms, the phase diagram of ice does not seem to be complete. Christoph Salzmann agrees: “There is no end in sight”.