A silly question I asked in elementary school never got answered, and although it may still seem silly, I gotta know. No snickering, please!
In first grade, we were taught that solids turn to liquids if heated enough. So my question was “what about a cake?” The batter is a thick liquid, but becomes a solid if heated. If you heat it too much, it becomes a harder solid.
Now I know a cake does not violate physics, but why does it get harder? My guess is the liquid part is boiled away, turned to steam, and the hard part is all that’s left?
Actually that’s not correct. If it were, you could bake a cake by leaving it out on the counter for a couple of days. The reason the cake gets solid is due to the presence of polymers in the batter. (I’m not sure which ingredient is key here, maybe the eggs.) Heating them up past a given temperature allows them to wiggle around and connect with other polymers. Once enough connections form, it solidifies. The degree to which the polymers cross-link greatly affects the consistency (hence the texture of overdone cake) – this is also why you don’t want to stir or knead some batters and doughs too much – too many connections form and it gets hard or crunchy.
I think I mean polymers, but I could be wrong. (This is well outside my area of expertise, in that I don’t cook and I’m bad at biology.) Solidification during cooking was the example given to me by a physicist who worked on cross-linking polymers. His favorite example was Jell-o. Pudding is another good one, I believe.
The key is that melting ice is just a phase change, but baking a cake is a chemical change. Cake batter is not just melted cake (or antimelted, or whatever), it’s an entirely different substance.
And I think that the primary source of the polimers is the flour, not the eggs, since there are eggless bread recipes.
ALso depending on pressure and temp you could have something go from liquid to solid when heated (I think). I seem to remember something like this in theromdynamics.
Please correct me if I’m wrong, but I’ve been under the impression that the phase shifting (Solid-Liquid-Gas) rules are really only meant for the Elements. Sure, they may apply to more complex chemicals, but all Elements have a Solid, Liquid, or Gas state depending upon temperature and pressure. Once the elements start gettin’ jiggy with each other, all bets are off…
Some materials melt when heated, but others react chemically before they get hot enough to melt. You could TRY to melt a piece of cake, but it would turn into charcoal and water vapor first.
Also, some solids evaporate when heated (they never melt at all.) I think carbon is one of these, so even if you tried melting the charcoal left behind from the cake-melting experiment, it would get red hot and then evaporate.
And, IIRC, Helium is predicted to be one of the only elements that will never have a solid phase, even at 0K (even Hydrogen can form a solid in a diamond anvil and under enough pressure). But I may be mistaken…
No, heat the cake in the absence of oxygen. I was talking about attempts to melt some solidified bread/cake.
Try to melt a cake in air and you’re left with a tiny bit of ash plus lots of CO2 and H2O clouds. So do it under nitrogen so you just get charcoal: chemistry happens rather than melting, oxy or no oxy.
astro, great link! This thread got me interested in the problem, but my own Google searches turned up nothing.
Anthracite, you’re right, but only with the caveat of “at ambient pressure”. Because helium is both light and inert, quantum motion prevents freezing even at T=0 K. Helium does have a solid phase (my first published paper was on solid helium), but only under pressure.
HighSoci, eggs coagulate because the heat breaks up the protein structure of the raw egg, and a new structure forms. The first kind of protein happens to be liquid, the new structure is solid.
