2019-01-10 00:00:00

At which temperature does water freeze?

Surprise!

We’ve all been taught that fresh water freezes at 32 degrees F, 0°C, 273.15 K. However, it is possible to keep water liquid at lower temperatures! Scientists have cooled water down to -42 °C in labs.

What is supercooling?

Supercooling is a state where liquids do not solidify even below their normal freezing point!

A good example of this phenomenon is found everyday in meteorology: clouds in high altitude are an accumulation of supercooled droplets of water below their freezing point. How is that possible?

In order to freeze water, you need to give it a small quantity of energy (ie: vibration) or have a core that triggers crystallization. The absence of seed crystals in high altitude prevents the tiny droplets of water from forming ice despite the low temperatures. This also explains why you can keep water liquid inside a bottle at -10°C in your freezer : it does not interact with outside energy. If you did that experience, you could have shaked the bottle and you would have seen ice forming itself. You might also have observed supercooling rain. It transforms itself instantly in ice when touching tree branches or the ground!

When was it discovered?

Supercooling was discovered long ago in 1724 by Daniel Gabriel Fahrenheit, but even today, the phenomenon remains a subject for intense discussions.

In fact, over the last 60 years the existence of supercooling liquids has led to speculations that their internal structure was not compatible with crystallization. Hypotheses explain that a large proportion of atoms in liquids arrange in five-fold coordinated clusters, which cannot form crystals. However, in order to form a crystal, matter needs a structure that can be repeated periodically, filling the entire space. This is not possible with the five-fold coordinated clusters.

In a 2-dimensional analogous situation, a delimited plane cannot be filled with pentagons only, while other shapes like triangles, hexagons and triangles can fill it exactly. Therefore, the pentagons prevent the filling of that plane, and it happens just the same with crystallization.

Conclusion

In everyday life, liquids structures formed below their freezing point are sometimes incompatible with crystallization. These liquids need a small quantity of energy or a core that triggers crystallization in order to freeze. That explains why we can keep water liquid below its frezing point!