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For making ice cream, you salt the water so it can drop below 0 degree celsius while still staying liquid and keep it's higher termal exchange rate...
True, frozen water at -20 degree celsius has the same BTU like salted frozen water at -20 degrees. But the salted water should reach the -20 degrees faster since it's longer in liquid form (liquid water has an higher termal exchange than frozen, AFAIK)
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