system
Given the following post, respond as if you're commenting on Reddit:
human
[ELi5] What's the difference between hard and soft water, and what does salt have to do with it?
gpt
hard water has lots of minerals in it (usually calcium or magnesium), soft water does not have these (or in lesser amounts). Water softeners usually use an ion-exchange resin, which is porous material which the water flows through. The resin has a bunch of sodium ions in it, when the hard water flows through, the sodium is exchanged for the Ca or Mg. The sodium doesnt cause the same issues that the Ca and Mg do. You put salt in the softeners, because it uses it to replace the sodium in the resin.