Why is it hard, and costly to remove salt from water?
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It’s not hard, it’s conceptually quite simple. Take salt water, boil it, condense the vapor. Any elementary school student who learns about the water cycle can understand it.
It’s *costly* because you need to heat water to its boiling point, and it takes a lot of energy to do that. That is just a physical property of water, or any substance, called its specific heat; in water’s case, you need 4184 joules of energy to make 1 kg of water 1 degree C hotter. The cost of doing so is dependent on the market price of energy, usually it works out that finding new sources of freshwater is cheaper than trying to desalinate seawater.
Reverse osmosis is another method which basically involves “filtering out” the salt from the water (to put it in five-year-old terms). It’s usually cheaper than boiling water to desalinate it, but still expensive.
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