Removing the Mg+2 and Ca+2 ions from hard water (1 Viewer)

[emilios]

Some texts are telling me we add hydroxide to hard water to precipitate out the magnesium as magnesium hydroxide, which would work since that salt is insoluble. It also says to add carbonate ions so as to precipitate out the calcium ions, as calcium carbonate is insoluble in water.

However, wouldn't just adding carbonate ions suffice, as both magnesium carbonate AND calcium carbonate are insoluble in water? What's with adding the hydroxide?

Thanks

 

[QZP]

MgCO3 is "slightly soluble" (although not much. Yes you could be fine with using carbonate as you said but the aim is to soften it as much as possible) hence we precipitate with hydroxide to Mg(OH)2 which is very insoluble. If we're talking about water purification in general instead of focusing on softening water, then the use of a hydroxide also precipitates iron ions (added bonus).

 

[emilios]

MgCO3 is "slightly soluble" (although not much. Yes you could be fine with using carbonate as you said but the aim is to soften it as much as possible) hence we precipitate with hydroxide to Mg(OH)2 which is very insoluble. If we're talking about water purification in general instead of focusing on softening water, then the use of a hydroxide also precipitates iron ions (added bonus).

i was thinking more along the lines of brine purification in the solvay process, but i didn't want to post in the Industrial Chem section cos nobody sees that haha. anyway thanks for the help, i guess specifying the addition of two different ions won't hurt, i'll just mention both hydroxide and carbonate ions being added

 

[QZP]

i was thinking more along the lines of brine purification in the solvay process, but i didn't want to post in the Industrial Chem section cos nobody sees that haha. anyway thanks for the help, i guess specifying the addition of two different ions won't hurt, i'll just mention both hydroxide and carbonate ions being added

Yeah I thought so hahaha The same points would still apply:
Pro's of hydroxide vs. carbonate: MORE Mg2+ precipitated, and other impurities (iron ions) precipitated

 

[emilios]

Yeah I thought so hahaha The same points would still apply:
Pro's of hydroxide vs. carbonate: MORE Mg2+ precipitated, and other impurities (iron ions) precipitated

hey one more question:

how fussy are they with the states? Because in the 'formation of sodium hydrogen carbonate' step, it's carried out at temps of 0C so that the majority of the NaHCO3 precipitates. Is it ok to write (aq/s) or do we have to write one: e.g. just write (s) or (aq)

 

[QZP]

If NaHCO3 precipitates how can it be in (aq) My teacher always emphasised having the correct states for this dot point and she was a previous HSC marker so I assume they are "fussy".

BTW this is the full equation: NaCl(aq)+CO_2(g)+H_2 O(l)+NH_3(aq)→NaHCO_3(s)+NH_4 Cl(aq)

 

[someth1ng]

I would say that it doesn't matter if you do (s) or (aq) for NaHCO3 because they would both exist. It's produced as (aq) before it becomes (s) but I'd say both are fine.