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Does boiling water reduce its hardness, or on the contrary, increase it?

It seems there are ambivalent phenomena at play here:

  • boiling will evaporate pure water, thus increasing the concentration of minerals including calcium, thereby increasing hardness
  • boiling will remove $\ce{CO2}$, making apparently $\ce{CaCO3}$ precipitate, thereby reducing hardness

This is based on rather uncertain sources, as I didn't find any better, and maybe I missed other aspects.

I would appreciate any clarification, but I understand that the answer might depend on the context and type of water considered.

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$$\ce{CaCO3(s) + H2O(l) + CO2(g) <=> Ca(HCO3)2(aq)}$$ is the reaction which shows how hardness gets into water in chalky areas. The reaction is reversible, reversed in boiling as $\ce{CO2 (g)}$ is driven off promoting the back reaction, softening "temporarily hard water" This is the only form of hardness that can be reduced by boiling. Other soluble $\ce{M^2+}$ salts such as calcium sulfate (makes the lovely Burton on Trent beers) need alternative methods to soften them such as ion exchange or "bath salts" (sodium carbonate). Can you give the equation for how bath salts work?

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You may need to think outside chemistry a little bit and pay a little bit attention to your everyday life. This is what happened when you boil water in a container: enter image description here

Most hardness in water, calcium/magnesium, will precipitate onto the hot surface and is removed from water. Now if you can get more something by removing it, is it against the very fundamentals of science?

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Boiling water will NOT reduce hardness of water, whereas heating hard water to the boiling point will reduce solubility of $CaCO_3$ (Edit: and other carbonates and bicarbonates), thus reducing hardness, if filtered before cooling back. Solubility of $CaCO_3$ reduces with increasing $T$ hence boiling will not help, as the temperature is constant.

The $CO_2$ removal would not have much effect unless you have a closed vessel, where you can substantially increase the $P[CO_2]$ by removing small amount of dissolved $CO_2$ in water.

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    $\begingroup$ This is quite wrong. Much of the hardness is caused by hydrocarbonates, which would decompose, so hardness will decrease significantly. As for solubility of carbonates, it is too low to be considered at all. $\endgroup$ – Ivan Neretin Aug 29 '17 at 9:21
  • $\begingroup$ @IvanNeretin The hydro-carbonates will precipitate but it's rate won't increase during the boiling, when temperature is constant. I guess I should have included all carbonates and bicarbonates in first point $\endgroup$ – ABC Aug 29 '17 at 9:24
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    $\begingroup$ That's right, but it will increase during the heating which precedes boiling. $\endgroup$ – Ivan Neretin Aug 29 '17 at 9:27
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    $\begingroup$ Again, that's technically right, but conveys the wrong impression. You can't boil water without heating. When people speak of boiling in relation to water treatment, this implicitly includes heating. $\endgroup$ – Ivan Neretin Aug 29 '17 at 9:32
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    $\begingroup$ Yes, I got that. Still, this wording has a decent chance to be misunderstood. Besides, you are still wrong on more than one point. Solubility of CaCO3 is irrelevant. Filtering is irrelevant. Bicarbonates are removed by decomposition and not because of decrease in their own solubility. $\endgroup$ – Ivan Neretin Aug 29 '17 at 9:43

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