Not sure if I should have put physics or chemistry as the tag.
I have 2 ice cube trays that I always empty and refill at the same time. Every time, without fail, the top tray’s cubes break loose easily and without cracks while the bottom tray’s cubes don’t want to release and will shatter if forced. This remains true no matter which tray I place on top or bottom. Since this puzzles me every day, I just really would like to know why!
In: Physics
The best explanation I’ve ever heard about this examines the way that each ice tray is freezing. In the top tray, or all of them if you put them side to side, the water will freeze from the top down. Since the ice is expanding as it freezes, the liquid water at the bottom of the tray allows the ice to expand upwards to minimize the pressure within each individual ice cube. You’ll notice that these cubes should have flat tops and come out easily.
If you stack the trays, the trays on top impede the flow of cold air to the trays on the bottom. This means that the water will primarily freeze from the bottom up through the surface of the tray. The freezing water will be able to “lock” itself to the tray from all sides except the top. This causes a lot of stress to build and, when you try to release the ice, the whole thing just becomes an absolute disaster. You’ll notice that these cubes have curved peaks in the middle. Since the top was the last side to freeze, the newly formed ice is getting forced to expand upwards through the center.
If you want to be able to get the brittle ice out without it just crumbling, I’ve found that running cold water over the bottom of the tray for a second or two before you twist it will let them slide out pretty easily.
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