Sat. Feb 29th, 2020

Ice that is hot and does not float

Introducing ice cubes in a drink is such an everyday action that we don’t usually stop to think how strange it is that ice floats in water. After all, if the water behaved like the rest of the everyday substances around us, the ice cubes would sink like stones to the bottom of the glass because its density would be greater than that of liquid water … But the Image of a piece of ice sinking into a glass of water is still strange.

Now, what if I tell you that there really are certain types of ice that are denser than water? Moreover, would you believe me if I told you that there are forms of ice that would be warm to the touch, if we could touch them?

The different faces of ice

The properties of ice change depending on the pressure and the temperature to which it is subjected, two parameters that determine how its water molecules are ordered. Some combinations of pressure and temperature produce more compact structures in which atoms are close together and others generate geometries in which atoms are more separated. These variations in the distribution of water molecules manifest themselves in the form of many different “types” of ice that would be similar to the naked eye, but have different attributes (such as their density or their melting and freezing temperature).

The combinations of temperature and pressure that give rise to some of the 17 known ice forms are represented in the following phase diagram of the water:

For example, the ice we use to cool our drinks is called “ice Ih” and we all have their properties more or less seen. It floats in water because its density (0.92 g / cm³) is lower than that of liquid water (1 g / cm³), it is formed at atmospheric pressure when the temperature drops below 0ºC and begins to melt when it rises above of this limit.

Now, let’s imagine that we put that ice cube in a pressure chamber and start compressing it while keeping it at 0 ° C. If we follow the phase diagram, we see that the ice will retain its shape while the pressure increases until it reaches 98 atmospheres (10 MPa). From that moment, the water will melt and remain in a liquid state while the pressure continues to increase. At 6,200 atmospheres (632 MPa), the water will freeze again converted to «ice VI», a form of ice that is characterized in that its molecules are distributed following a more compact tetragonal pattern resulting in a density of 1.31 g / cm³. Therefore, “ice VI” would sink in our glass of water … If it were stable under normal conditions, of course.

If the pressure increases further, the water molecules will be restructured again at 19,700 atmospheres (2 GPa) and will adopt an even more compact structure called “ice VIII” that has a density of 1.66 g / cm³. Once 49,300 atmospheres (5 GPa) have been exceeded, «ice VIII» becomes «ice VII». This version of the ice has a slightly lower density (1.65 g / cm³), but it has another very curious feature: if we stopped increasing the pressure acting on it and heated it, it would not start to melt until it exceeded 225 ° C.

Now, do these versions of ice that behave so strangely occur in nature or only exist in laboratories?

Where to find exotic ice

Luckily for us, the pressure and temperature conditions that are needed for these types of exotic ice to form do not occur on the earth’s surface. Even so, the presence of “ice VII” inclusions within some diamonds suggests that these conditions take place under our feet.

In this case, these are tiny drops of water that were trapped inside these crystals as they formed tens of kilometers deep, subjected to immense pressures. When the geological processes drove those diamonds to the surface, their temperature dropped and the tiny drops of pressurized water inside were solidified, turning into “ice VII.” That way, these exotic ice masses are so tiny that they cannot be seen with the naked eye and their presence was deduced from the diffraction patterns of the light they produce (like this).

Entering a bit more speculative terrain, it is possible that there are large amounts of exotic ice inside other worlds, such as Gliese 436b, a planet with a size similar to that of Neptune that is so close to its star that its atmosphere of hydrogen exceeds 400 ° C. These high temperatures cause the atmosphere to expand and the gas from its high layers constantly escape into space, forming a tail similar to that of a comet behind the planet.

The existence of that large hydrogen cloud that envelops the planet could indicate that there is a deep global ocean under its atmosphere. As the water evaporates, the vapor rises to the upper layers due to its lower density and the ultraviolet radiation of the star separates the oxygen atoms of its molecules from those of hydrogen, facilitating the escape of the latter into space.

But, if this hypothesis were correct, it is most likely that that global ocean of Gliese 486b is not in a liquid state, like the terrestrial oceans, but solid. The reason? Although the water in these oceans would be very hot, the planet’s intense gravitational field would be compressing it with enough force to make it one of the exotic forms of ice that can withstand very high temperatures without melting.

The truth is that using one of these pieces of dense exotic ice to heat a drink, instead of cooling it, would be an experience, but, luckily or unfortunately, we do not find these forms of ice in our day to day because they are only stable in a very specific range of temperatures and pressures (which, in addition, are not too benevolent with human life). Therefore, it seems that for now we will have to settle for our old acquaintance: the cold and light “ice Ih”.


  • Although Europe (Jupiter’s satellite) is sometimes used as an example of our solar system where “ice VII” might exist, a 1984 study estimated that the pressure at the bottom of the hypothetical oceans that could be under its Frozen crust would be 10 times lower than necessary to produce this type of exotic ice.



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