Boiling changes water in four distinct ways, and only one of them is the reason people are usually told to do it. Understanding all four is what prevents the common and genuinely dangerous mistake of boiling water to deal with a contaminant that boiling makes worse rather than better.
It inactivates organisms
This is the useful effect. A rolling boil kills bacteria, viruses and the protozoan cysts that resist chlorine, which is why boil water notices exist and why boiling is the standard advice after a main break, a flood or a positive coliform result on a well. It needs no equipment and no consumables. Cloudy water should be filtered through a cloth first, since suspended solids shield organisms from the heat and consume any disinfectant used alongside.
It drives off some volatiles
Chlorine largely evaporates within a few minutes of boiling, which is why boiled and cooled water tastes flatter and less chemical, and it is a legitimate way to prepare water for an aquarium or a pond in a pinch. Volatile organic compounds behave similarly, though a lidded pan simply condenses them back into the water. Hydrogen sulfide, the rotten egg smell, is also driven off. None of these is a treatment method so much as a side effect worth knowing about.
It precipitates some hardness
Heating drives carbonate hardness out of solution, which is the white crust in the kettle and the scale in a water heater. That does slightly reduce the calcium in the water that comes out, but the effect is partial, it only touches carbonate hardness rather than the permanent kind, and it deposits everything it removes onto the appliance you were using. Boiling is emphatically not a way to soften water, and the kettle furring up is the visible cost of the small change it makes.
It concentrates everything else
This is the part that matters most and is least known. Water leaves as steam and dissolved contaminants do not, so nitrate, arsenic, lead, fluoride, heavy metals and total dissolved solids all become more concentrated in what remains in the pan. Boiling water contaminated with nitrate or arsenic and then drinking it delivers more of the contaminant, not less. Those need reverse osmosis, distillation or a specific media. Our softener guide covers the hardness side of the question.
Questions people ask about what does boiling water do
Does boiling make water safe?
Against bacteria, viruses and cysts, yes. Against dissolved chemicals it does the opposite, concentrating them as water evaporates.
Does boiling remove chlorine?
Largely, within a few minutes, which is why boiled water tastes flatter. A lidded pan retains volatile organics, though.
Does boiling soften water?
Only partially and only carbonate hardness, and it deposits what it removes as scale inside the kettle or heater.
Which contaminants does boiling make worse?
Nitrate, arsenic, lead, fluoride, heavy metals and dissolved solids. They stay behind while the water leaves as steam.