A water softener removes dissolved calcium and magnesium from the whole supply by trading those minerals for sodium on a bed of resin beads, then flushing the loaded beads clean with a strong salt solution. That is the entire job. It stops scale forming in pipes, heaters and fixtures, and it lets soap lather instead of curdling, but it is not a purifier and it does not make unsafe water safe.
The ion exchange, in plain terms
Hardness is dissolved calcium and magnesium, according to the USGS, which classes water as soft at 0 to 60 mg/L as calcium carbonate, moderately hard at 61 to 120, hard at 121 to 180 and very hard above 180 mg/L. Inside a softener sits a tank packed with small plastic resin beads, each holding sodium ions on its surface. Hard water flows down through the bed and the calcium and magnesium, which the resin grips more tightly than it grips sodium, stick to the beads while sodium is released into the water leaving the tank. Nothing is filtered in the ordinary sense: no particle is trapped, no chemical is destroyed. Minerals are simply traded, one kind for another, until the bed runs out of sodium to trade.
Why it has a second tank
The second tank, the shorter one beside the resin, holds salt and water. When the resin has taken on all the hardness it can hold, the control valve draws that concentrated brine slowly through the bed. Sheer strength of the salt solution reverses the trade: sodium floods back onto the beads and the accumulated calcium and magnesium wash to the drain. The valve then rinses the bed, refills the brine tank and returns the softener to service. Older timer valves do this on a fixed schedule whether the house used the capacity or not. Metered valves count gallons and regenerate only when the bed is actually spent, which is why they use less salt and less water for the same result.
What changes in the house
The differences show up in the places hardness was quietly damaging. Scale stops building inside the water heater, so the burner or element stays in contact with water rather than with an insulating crust. Shower doors, kettles and faucet aerators stop crusting over. Soap and detergent lather with far less product because there is no calcium left to react with, which is also why softened water feels slippery: that is soap rinsing off cleanly rather than leaving film on skin. Laundry comes out softer. What does not change is taste, chlorine smell, iron staining beyond a modest amount, bacteria or anything dissolved that is not hardness.
What it deliberately does not do
A softener is a single-purpose appliance and vendors sell it that way. Look at the softener range on a manufacturer site and every unit is rated by grain capacity and service flow, not by contaminant removal, because ion exchange resin is aimed at hardness. It will not remove chlorine taste, sediment, hydrogen sulfide smell, nitrates, lead or bacteria. Heavy iron in the oxidized, rust-colored form actively fouls the resin rather than being removed by it. Houses with those problems put the right stage ahead of or behind the softener: sediment first, carbon for taste and chlorine, an oxidizing filter for iron and sulfur, ultraviolet for bacteria. If you are sizing the whole train, our water softener installation guide covers the order the stages go in.
Questions people ask about what does a water softener do
Does a water softener make water safe to drink?
No. It removes hardness minerals only. Bacteria, nitrates, lead and chemical contaminants pass straight through, so a softener is never a substitute for testing and for the treatment that a test actually calls for.
Does softened water add salt to what I drink?
Ion exchange releases sodium in proportion to the hardness it removes, so harder incoming water means more sodium leaving the unit. Households that would rather avoid it usually leave the kitchen cold tap unsoftened or add a reverse osmosis unit there.
Why does softened water feel slippery?
Without calcium to react with, soap rinses off completely instead of leaving a fine curd on your skin. The slippery feel is clean skin, not leftover soap, and most people stop noticing it within a week or two.
Do I still need a filter if I have a softener?
Usually yes, if your complaint is anything other than scale. Taste, odor, chlorine, sediment and iron all need their own stage, and a softener sits alongside those rather than replacing them.