This was asked in my test and I remembered that all compounds that can remove permanent hardness of water can remove temporary hardness of water also. But my teacher marked it wrong and I cannot find the answer anywhere.

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    $\begingroup$ I guess you are right.. $\endgroup$ – Zenix Apr 15 at 21:32
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    $\begingroup$ You teacher apparently has no clue of basic chemistry. You are right. Sodium carbonate is added in most laundry detergents to soften it. $\endgroup$ – M. Farooq Apr 16 at 0:42
  • $\begingroup$ $$\ce{Ca(HCO3)2_{(aq)} + Na2CO3_{(aq)} -> CaCO3_{(S)} +2NaHCO3}_{(aq)} $$ $$\ce{Ca(Cl)2_{(aq)} + Na2CO3_{(aq)} -> CaCO3_{(S)} +2NaCl}_{(aq)} $$ $\endgroup$ – Adnan AL-Amleh Apr 16 at 1:05

Definition of hardness per a source:

The simple definition of water hardness is the amount of dissolved calcium and magnesium in the water. Hard water is high in dissolved minerals, largely calcium and magnesium. You may have felt the effects of hard water, literally, the last time you washed your hands.

Adding $\ce{Na2CO3}$ introduces the carbonate ions:

$$\ce{Ca++/Mg++ + CO3(2-) -> CaCO3/MgCO3 (s) }$$

which results in a precipitate of insoluble carbonates.

So, yes it is a temporary solution to address water hardness.

[EDIT] I recommend relatedly to hard water treatment with Washing Soda, a very interesting experiment of adding Na2CO3 (aq) to MgSO4(aq) as the entire solution, at times (it depends on respective concentrations), can convert to a wet hydrate creating so-called Magnesium alba. Cited reaction per Wikipedia:

If magnesium chloride (or sulfate) is treated with aqueous sodium carbonate, a precipitate of basic magnesium carbonate—a hydrated complex of magnesium carbonate and magnesium hydroxide—rather than magnesium carbonate itself is formed:

$\ce{5 MgCl2(aq) + 5 Na2CO3(aq) + 5 H2O(l) → Mg(OH)2·3MgCO3·3H2O(s) + Mg(HCO3)2(aq) + 10 NaCl(aq) }$

where the magnesium mix oxide/carbonate is known, but not uniquely in name or precisely in chemical composition, as Magnesium alba. Now, MgSO4 aqueous per a source:

Magnesium sulfate salts can form hydrated compounds with up to seven ... Magnesium sulfate hexahydrate is formed by coordinating 6 water molecules to Mg2+ ...

So, Magnesium sulfate forms $\ce{MgSO4(H2O)6.H2O}$ with six coordinating water molecules (which could be created upon concentration of the aqueous MgSO4 solution, further extracting water from the mix). Further, if Magnesium alba similarly also extracts water, with excess MgSO4, it may be understandable as to why the entire solution can apparently turn into a voluminous wet precipitate, somewhat surprisingly.

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  • $\begingroup$ As my answer was, yet again, baselessly down-voted (Maurice?), I might as well add some interesting chemistry. $\endgroup$ – AJKOER Apr 16 at 13:03
  • $\begingroup$ according to this equation : $$\ce{5 MgCl2(aq) + 5 Na2CO3(aq) + 5 H2O(l) → Mg(OH)2·3MgCO3·3H2O(s) + Mg(HCO3)2(aq) + 10 NaCl(aq) }$$,the water still hard $\endgroup$ – Adnan AL-Amleh Apr 16 at 23:54
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    $\begingroup$ Having prepared Mg(HCO3)2, I can share that over a course of days in a closed container, a precipitate of MgCO3 develops, so the cited reaction may be technically correct, but in time, it should not be a substantial source of ones hard water (more like problematic deposits). $\endgroup$ – AJKOER Apr 17 at 16:35

Yes, you can add sodium carbonate to remove temporary hardness of water. Temporary hardness is caused due to presence of calcium/magnesium bicarbonate. Treating them with sodium carbonate also known as washing soda helps precipitation of insoluble calcium/magnesium carbonate ultimately softening of water. Overall reaction is:

$$\ce{M(HCO3)2 + Na2CO3 -> MCO3 + 2NaHCO3 ~~~~~ M = Ca,Mg}$$

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  • $\begingroup$ The above reaction is not accurate with all magnesium ions in the presence of Na2CO3. See my expanded answer with references. I also recommend a very interesting chemistry experiment. $\endgroup$ – AJKOER Apr 16 at 13:31

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