| MadSci Network: Other |
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I buy Ice-Pops for my son. When I put them in the freezer for several
hours, most have frozen but some remain in a liquid state. When you
"Shock" them(flick them with your finger), they immediately form ice
crystals. Why do they remain liquid and why do they only form ice crystals
after receiving some type of trauma?
An excellent question! We see this a lot in chemistry (there's a question in our archives about beer freezing when opened in Antarctica, but it's not the same process), and it has to do with a sudden disturbance in a "metastable" state, that is, a state that's like a pencil balanced on its point: perfectly stable unless disturbed. When trying to crystallize something from a solution, you normally concentrate the solution and eventually crystals will start to form. But sometimes you boil off some liquid, then let the solution cool, and nothing happens! Yet you know that crystals should be forming. The solution is super-saturated and a small shock is usually enough to start crystallization; the "pencil" has been nudged, and it "falls to the table" -- a more stable state. In the case of your Ice-Pops, you have a solution which has been brought below its freezing point but which hasn't frozen yet; it is supercooled. Again, a slight shock is enough to nudge the "pencil" into falling into the more favorable state.
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