Dear Matthew Alice: So why, when ice floats (it’s less dense than the surrounding liquid), do piroghi and ravioli sink when frozen and float when thawed? Do other pastas have this same buoyancy? Why? What about other foods? Well, gotta go. The piroghi are floating. — Alfred Stumpfhausen, in the kitchen
Somewhere in the wheezy office fridge, we’ve got a big block of ice questions just waiting for the elves and me to chip away at. As many of you have apparently noticed, ice is weird. While ravioli are tasty and well behaved, frozen water just goes its own way. Yep, ice cubes float because they’re less dense than water. They’re less dense because when water freezes, it expands. It’s not supposed to do that. It’s supposed to shrink and get denser, like the piroghi and ravioli and pretty much everything else. As your ravioli cook, they not only thaw and get less dense, but any little pockets of air in the dough warm and expand and eventually create enough buoyancy to lift the ravioli off the bottom of the pan.
A hot thing is just a thing that has its atoms moving faster. A cold thing’s atoms hardly move at all. It takes more room for a hot molecule to hop around than to sit, so warmed things expand, frozen things don’t. Except water. To form ice crystals, water molecules have to be separated from one another at a greater distance than in liquid form, so when you fill that cube tray all the way up, the finished product will have expanded over the top.