temporary hardness of water (1 Viewer)

[mojako]

A question I saw says: "Temporary hardness of water can be caused by:"
and the answer is "calcium and magnesium hydrogen carbonate" (multiple choice)

Is it the carbonate or the hydrogen salt that makes the hardness temporary?
(or perhaps both?)

 

[tennille]

what are the other options for the multiple choice?

 

[mojako]

what are the other options for the multiple choice?

(a) compounds that cannot be removed by boiling
(b) salts such as calccium and magnesium sulfate
(c) calcium and magnesium hydrogen carbonate
(d) none of the above

well for this Q it must be (c), but there might be a Q where u need to choose between calcium carbonate and calcium hydrogen sulfate

 

[tennille]

oh, ok. When you determine water hardness, you express it in terms of CaC03. So it must be C. If they'd ask you the latter question, it'll probably be calcium carbonate. If you have Conquering Chemistry, it mentions CaC03. But I don't know where the hydrogen comes from. I'm assuming it is from its dissolving in H20.

 

[mojako]

hmm.. nope.
you express hardness as the mass of CaCo3 just for a standard (and you add or hypothetically add the CO3 yourself). Maybe its like a tradition and I dont know why they first started doing this.
what it measures is the concentration of Ca2+ and Mg2+ ions collectively, with Mg2+ regarded as Ca2+ in the calculation.

Temporary hardness is water hardness which can be made to disappear by prolonged boiling.

Hmm.. looking at the solubility table I think it's the carbonate, because carbonate compound is insoluble and so the Ca2+ or Mg2+ will be tied up in molecules which can't react with soap.
Maybe heating converts hydrogen carbonate salt to carbonate salt...?

 

[mojako]

yes heating converts calcium hydrogen carbonate to CaCO3.
so its a bit of both factors. I think what happens is this:
calcium hydrogen carbonate (I write it coz Im not sure what the formula is... Ca(HCO3)2?) is soluble in water, and when you put in soap the Ca2+ attaches to it.
when heated, its converted to insoluble CaCO3 so when you add soap it wont react.

if you had CaCO3 from the start, then the water is not hard at all.