When I eat hot pizza or a melted cheese sandwich, the cheese feels a lot hotter than the crust or bread: in particular, the cheese might scald the roof of my mouth. but the crust will not. Is this
Two reasons:
the cheese has a higher specific heat capacity than the crust;
the cheese has a higher thermal conductivity than the crust.
When you cool a given weight of cheese or crust from the oven temperature to your mouth temperature, the amount of heat it gives up depends on its specific heat. So the cheese, with its high specific heat, gives up more heat than the crust and hence heats your mouth more. The cheese also conducts heat better so it can deliver the heat to your mouth more quickly, and again this makes your mouth hotter.
All of which is fine, but actually it may simply be that the crust cools faster than the cheese while the pizza is sitting on your plate. I can't say if this is the case because I've never done the experiment!
The hot cheese is gooey and thereby makes more complete contact with a substrate (the cheese flows to some extent), leaving fewer voids than crust does with, say, the roof of your mouth. That intimate contact facilitates heat transfer. For the same reason, it also takes longer to dislodge the cheese once you realize it's burning you, exacerbating the damage...
I imagine surface tension is at play here, since the hot cheese tends to spread out on a substrate instead of balling up...
Yes. As John says, the high specific heat capacity of cheese causes it to absorb more heat to raise its temperature than the crust does. If you do an experiment by heating a sample of it in an oven (say to about 300 degrees), you could observe that the crust reaches the temperature more quickly than the cheese. So, cheese takes a longer time to absorb heat to get itself to that temperature.
When you allow the pizza to cool down, the crust cools down. Because it doesn't have to absorb much heat energy to raise its temperature, it doesn't have to lose much heat energy to cool off either. The crust could also transfer enough heat to your fingers to cool it, without transferring enough to burn your fingers.
But, the cheese makes the pizza more effective. It cools very slowly. It also transfers heat more easily — i.e. conducts (as John says). So, whenever "OUCH" comes in your mouth or fingers, the cheese or the sauce has given a lot of heat (probably a burn) to your skin…!
Note also that, in probability, the pizza may have been prepared earlier & merely re-heated in a microwave oven immediately prior to reaching you. Cheese being a more dense solid as compared to the porous dough crust and having a higher Sp.Heat (in concurrence with John's answer up there) heats quicker and retains that heat longer.
(off-topic: i know from personal experience having scalded the inside of my palate to blistered strips the first time i used a microwave oven to heat goodies ... )
