What is a chemical besides water then once frozen will stay frozen or very cold for a long period of time?
Answer:
Anything with a large latent enthalpy of transition in the desired temperature range will do it. Dry Ice is common. Liquid ammonia would be nice if not for the inhalation and flammability hazards, hence closed systems called "refrigerators." Urea dissolving in water.
If you have good insulating walls you need only balance a small heat leak, hence portable Peltier coolers run off batteries or solar cells.
Carbon dioxide
There is also 'dry ice'; it is frozen carbon dioxide. Dry ice exposed to normal air gives off carbon dioxide gas that is odorless and colorless. The gas is so cold that when it mixes with air it cools the water vapour in the air to fog, which looks like a thick white smoke.
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