Why does hot water sometimes freeze faster than cold in my ice trays Everyday Mystery

I want to gently push back on the framing rather than the observation. When people have tried to pin this down carefully with probes and a defined endpoint, the results scatter badly, and a lot hangs on what you count as frozen.

A crust on the surface is not the same as a frozen block. Hot water often crusts earlier because the surface sheds heat fast while the interior is still warm, but total time to a solid tray can go the other way. If you repeat this, decide your endpoint first, and I would use something like a skewer will not push through the centre.

None of that makes what you saw wrong. It means the interesting question is which endpoint and which freezer, not whether water is breaking thermodynamics.

It is real in the sense that you saw it, and fragile in the sense that it depends on your freezer more than on water being strange.

The mechanism that usually dominates at home is contact. Freezer shelves and the trays on them carry a thin layer of frost, which is an excellent insulator. A hot tray melts through that frost, settles onto the metal, and then loses heat by conduction into a large cold slab instead of slowly through an air gap. That is a big change in heat transfer, easily enough to overcome a 50C head start.

Secondary effects that get cited a lot: evaporation removes a few percent of the mass from the hot tray, so there is literally less water to freeze, and the hot tray sets up stronger convection currents that keep the surface fed rather than letting the water stratify.

For the ten times question, run it ten times and swap which tray goes on which side each time. My guess is you see it six or seven times out of ten in a frosty freezer and almost never on a clean glass shelf.

There is a third thing worth knowing: cold clean water can supercool. It will sit below 0C as a liquid until something nucleates it, then freeze suddenly. So the tray that looks like it is losing is sometimes not behind at all, it is waiting for an excuse.

That also explains why the effect feels random. Whether a tray nucleates at -1C or -5C depends on dissolved gas, dust, and scratches in the tray.

Practical note from the ice side: if what you want is fast or clear ice, do not chase this. Use a shallow tray for speed, and for clarity freeze slowly from the top with the sides insulated so the dissolved gas gets pushed downward. Boiled and cooled water helps a little with clarity and nothing with speed.

It is the Mpemba effect and physics still cannot explain it at all, it is one of those genuinely open mysteries.