This question has bugged me for years:
If heating things tends to melt them, and cooling tends to solidify… why does cooking cause some foods to go from liquid or semi-solid to solid? Dough, eggs, meatballs? Why? Especially the eggs. They baffle me the most.
Crack an egg into a hot pan, and whoosh, it unmelts!
What’s the Straight Dope on this phenomenon?
Thanks for any help you can give…
My WAG is that the action of cooking removes the water content from the food in question. If you cook anything long enough, it will turn dry and tasteless, provided you are dry-cooking it, versus boiling, per se.
With eggs, that doesn’t fly, though. Egg drop soup is a good example.
We’re looking at two different phenomena here.
For elemental substances and simple compounds (e.g., water), apply heat and and the substance melts (or sublimes, but let’s pass that for the moment).
For complex substances, however, application of heat can induce a chemical change. In essence, the substance doesn’t “unmelt”, it become something else. When egg white (essentially, a protein solution in water) is heated, the protein is denatured by the heat, and becomes insoluble.
(Is denaturing a chemical change? We’re moving from chemistry to metaphysics in a way. I say that it’s a chemical change. We should probably place discussion of that point in GD.)
Table sugar (sucrose) is about at the borderline. Heat it carefully, and you can melt it (that’s where spun sugar comes from). Heat it less carefully, and it undergoes a chemical change; it carmelizes.
In the case of eggs, the egg doesn’t “unmelt” when you apply heat, it coagulates.
coagulate vti
When the protein in an egg is raised to approximately 156 degrees F. it coagulates. Which is why poaching and egg makes it semi-solid, even though you do that in water.
… but you gotta admire the term “unmelt”.
It’s the difference between chemical and physical changes.
Melt ice into water and you still have H2O. The stuff itself hasn’t changed at all, just its state.
On the other hand, adding energy (coooking) some things causes chemical changes to occur. In a chemical change, the molecules move around a bit and you get new compounds, with different characteristics.
This is why you can freeze the water into ice again, but you can’t “uncook” a cake. 
I’m not sure about this, but I think that denaturing is not a chemical change. When you denature something your not actually changing its chemical composition all your doing is breaking bonds. For example is you heat up DNA and denature it you just have to leave it for a while at room temperature and it will renature by itself. I don’t know if proteins will renature by themselves. So maybe denaturing is, or is not, a chemical change depending on what you denature. QAs for eggs I think its more than just denaturing the proteins I think that there is an actual chemical change going on between the proteins, and the water caused by the increase of energy. But this is just speculation.
If there’s bonds being broken (or formed), it’s a chemical change. Denaturing DNA is just an easily-reversible chemical change.