what is happening when an ice cube cracks instantly, when dropped into a warm drink?
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When something is hot, that just means its molecules are vibrating. The more they vibrate, the hotter they feel. Now if something is uniformly hot (but not hot enough to melt or worse), then all the molecules are moving as freely as each other, so they’re more flexible.
Conversely, the less a molecule moves, the colder it feels. The water molecules in ice aren’t moving very much, this is ok because although they’re less free to move about, they’re all just as stationary, so there’s not much strain. The molecules in ice form a lattice, aka, crystal.
The problem is when SOME of the molecules get a lot hotter than other molecules, such as in the case of dropping ice in a warm drink. Suddenly, you have molecules that want to dance and molecules that don’t.
The result is that the hot molecules rip away from the cold molecules. This is what causes the cracking.
This is the same reason why rapidly cooling hot glass can cause it to crack.
It’s thermal expansion. Coming out of your freezer, ice cubes should be around −18°C but the surface will quickly warm to 0°C in contact with water, expanding as it does. Now the surface is larger than the centre. Ice is brittle so it easily breaks in this situation.
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