Refrigerators do not use water as a coolant because the freezing and boiling points are too high. Freon, ammonia and other common coolants are gases at room temperature and have very low freezing points.
Refrigerators use a process of expanding and compressing a coolant, also called a refrigerant or working fluid, to collect heat inside the refrigerator and release it outside. The coolant is a gas during part of this process and liquid the rest of the time.
If the coolant freezes the refrigerator would stop working, so the freezing point must be below the coldest temperature the refrigerator can reach. Also, since the coolant must vaporize outside the refrigerator, its boiling point needs to be lower than room temperature. Water does not meet either of these criteria.
Ammonia was once the most common coolant, but these days it is rarely used due to concerns over toxicity. Freon became the standard coolant until it was banned over concerns that it was contributing to ozone depletion.
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