| MadSci Network: Chemistry |
This sounds like a quality-control item. The freezing point will depend on the alcohol content; is the half-gallon bottle the same brand and proof as previous bottles that didn't freeze? There's going to be some variation in alcohol content between batches, though not more than a few proof (that is, "80 proof" is probably between 78 and 82 proof, or 39-41% alcohol).
If the stuff "thickens" in the freezer, it sounds like it's pretty close to freezing anyhow, and a small amount of extra water (even having it open for a few hours -- and not all at once, either -- with high humidity, summer in Tennessee) could cause it to tip from slushy to frozen solid. It wasn't a brand-new bottle, was it?
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