Thanks

No

Thanks

Freezing is a chemical reaction with an activation energy. That activation energy changes depending on whether or not the substance is forming a new solid (cold water with no ice in it) or forming on the surface of an existing solid (water with an ice cube already in it).

The activation energy of freezing is often different between those two cases. If forming without solid has a higher activation energy, than the substance can be cooled below its boiling point without it turning into a solid. It can be thought of as a jenga tower that is wobbly but doesnt fall. It just needs one push to start the rest of the reaction. If an ice cube is dropped in, it may instantaneously start freezing.

So it takes energy to freeze?

Yep, water-water bonds are broken and ice-water or ice-ice bonds are formed. Forming bonds releases heat/energy and breaking bonds absorbs surrounding free energy/heat, both reactions have an "activation energy" to proceed..

This post was edited by EndlessSky on Dec 2 2018 01:48pm

I didnt know there's actual bonds. Always thought it was just intermolecular forces

So like why do they say shaking the liquid makes it freeze? It feels so backward from the basics

Shaking the liquid would add kinetic energy to the system which could help overcome the activation energy of homogeneous nucleation. The shaking changes the gibbs free energy of the solution.



The intermolecular force applied over a distance (radius of the bond) equates to energy. That energy difference can be described as a bond because it takes that much energy to separate the two molecules.

He's not explaining it very well.

For a freezing to happen there's a "bump" of energy that needs to be overcome. After it overcomes that "bump" it ultimately releases energy, but not until it gets over that bump. For freezing that "bump" is really small, so just jostling the container is enough to overcome it. If you add some solid ice then the "bump" becomes so small that it doesn't even matter because the freezing can just happen on the surface of the already existent ice cube instead of having to form a new ice cube from scratch.

Water has "hydrogen bonds" which are only bonds if you take a looser definition than is traditionally applied. The energy of a hydrogen bond is really strong for an intermolecular force, but weak for a covalent bond, so it's kind of in this grey area where it blurs the line between an intermolecular force and a true bond.



That's not really accurate.

No matter how you look at it, all bonds are "water-water" bonds.

Also it's not multiple activation energies to break and reform bonds. It's one activation energy for the intermediate state between the bonds breaking and the new bonds forming.

This post was edited by Thor123422 on Dec 2 2018 10:32pm