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Monday, November 21, 2016

Lab: The Supercooling of Water

This lab is actuall very simple, but it isn great way to learn about how entropy and enthalpy come together in thermodynamics. I have written an article as theory for this lab and as an introduction to these terms, so for the sake of time I will just build off of that.

The supercooling of water is something that occurs when you have water that is still in liquid form under the freezing point, 32°F or 0°C.

This occurs because in order for the reaction to occur, the gibbs free energy of the system must be negative.

The first time I attempted this lab, I failed because the water I used wasn't pure enough, so it froze before I wanted it to. Less pure water has a smaller change in entropy because it has a higher entropy to begin with. The change in entropy generally must be quite great for a reaction to occur, and in this case must be greater than the change in enthalpy times the temperature (~2nd 71K).
Grpah representing the freezing point and the nucleization point of water at the point with high entropy relative to the time of the reaction.

When the lab finally does occur, it happens because the gibbs free energy is not yet negative, because the change in entropy is not great enough. In order to push it over the edge, the bottle must be smashed down, increasing the entropy of the liquid enough to make the gibbs free energy negative, thus allowing the reaction to occur. In this case, the reaction is a physical change, but these thermodynamic laws still apply. The reaction is the freezing of the water in the bottle in front of your eyes, beginning from the hearth of greatest entropy where you hit it.

The change itself is very exciting when you finally get it to occur. Just remember to make sure that your water is pure, and keep in mind that the change in enthalpy doesnt have to be as high if the entropy of the system is great enough, and the more water, the higher the entropy.

I will leave a video of the reaction below, so that you know what to look for. Good luck!

Vinay Konuru

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