Fun Freezer Facts

I placed an open glass jar of cold h2o, 1+ inch from the rim, into my freezer. 3+ diameter wide. Glass cracks neatly around the BOTTOM RIM of the jar when water turns to ice. Tried this twice, same results. I read on internet, that to test if freezer is working properly, freeze a jar of water, place a coin on frozen ice, see if coin moves down; indicating malfunctioning freezer if coin sinks. Why?
- Jan Talaro (age 62)
98115

Here's why the bottom cracked, I think. The ice will initially form at the top, as usual because ice floats. The top will then form a rigid ice seal, fixing the volume available to the water + ice. Now as more water freezes, expanding in the process, it squeezes the remaining water, raising the pressure. By the time only a little water is left, it's very compressed, and it's at the bottom. So that's where the jar breaks. 

If the freezer is very cold, the ice is frozen very solid so the coin will just sit on top. If it's not far below 0°C, the freezing point, then there's a little liquid-ish layer on its surface. The coin sinks through that. A little more liquid-ish layer forms where the coin touches the ice. Gradually the coin will sink. In effect, you just have a slow-reading but very sensitive thermometer, where the rate at which the coin sinks depends very strongly on the ice temperature.

Mike W.

(published on 12/15/2016)