I know it’s difficult to get drinking water from salt water, but why is it so expensive? — MP, Chicago, IL
The simple answer is entropy—the ever-increasing disorder of the universe. Salt water is far more disordered than the salt and water from which it’s formed, so separating those components doesn’t happen easily. The second law of thermodynamics observes that the entropy of an isolated system cannot decrease—you can’t reduce the disorder of the salty water without paying for it elsewhere. In effect, you have to export the salty water’s disorder somewhere else as you separate it into pure water and pure salt.
In most cases, this exported disorder winds up in the energy used to desalinating sea water. You start with nicely ordered energy—perhaps electricity or gasoline—and you end up with junk energy such as waste heat. While some desalination techniques such as reverse osmosis can operate near the efficiency limits imposed by thermodynamics, they can’t avoid those limits. If you want to desalinate water, you must consume ordered resources and those resources usually cost money (an exception is sunlight). The desalinating equipment is also expensive. Until water becomes scarce enough or energy cheap enough, desalinated water will remain uncommon in the United States.