Epsom Salt and Washing Soda
Date: Spring 2010
After the precipitate MgCO3 is formed from the reaction of
Epsom Salts and Washing Soda (calcium carbonate), what happens to
the Sodium and Sulfate? Do they combine?
I assume you started with solutions of Epsom salts and washing soda.
We classify both of these compounds as ionic, meaning that they break
into (dissociate) ions when dissolved water. We call their kind of reaction
a double replacement reaction because the two positively charged ions
(cations) (the Mg2+ and Ca2+) change negatively ions (anions) (SO4 2-
and CO3 2-). You can write the reaction in words: Magnesium sulfate plus
calcium carbonate produce (or yield) magnesium carbonate plus calcium sulfate.
MgSO4 (aq) + CaCO3 (aq) = MgCO3 (s) + Ca 2+ (aq) + SO4 2- (aq)
where (aq) = aqueous or in water solution and (s) = solid or precipitate
We know that the MgCO3 is a solid by looking it up in a table of solubilities;
and in the same table we see that CaSO4 is soluble, meaning that the Ca 2+
and SO4 2- is soluble (or dissolved) in water.
Look up a table of solubilities. In such tables, you will find that
-for reasons we do not need to get into right now- for example, all
binary inorganic compounds with the nitrate anion are soluble in
water, sulfates are soluble except for those with Ag, Pb, Ba, Sr and
Ca cations, etc.
So in the reaction of magnesium sulfate and sodium carbonate, and
using the solubility rules, we can write (a double displacement reaction):
MgSO4(aq) + Na2CO3(aq) --> MgCO3(s) + Na2SO4(aq)
Note that from the solubility rules, we know that only the magnesium
carbonate is insoluble in water. The rest of the compounds are water soluble.
Now, knowing that binary inorganic compounds that are soluble in
water also tend to form strong electrolytes - that is, they ionize
completely in water, then we can write a complete ionic equation
(where all compounds that are exist as ions in solution are shown to
be ions instead of compounds) as:
Mg(2+) + SO4(2-) + 2Na(+) + CO3(2-) --> MgCO3(s) +
2Na(+) + SO4(2-)
From here, we note that the sulfate and sodium ions exist in
exactly the same form prior to and after the reaction (left and
right sides of the equation). Hence, we call them "spectator ions".
The net ionic equation (showing only what actually reacted) is:
Mg(2+) + CO3(2-) --> MgCO3(s)
This means that we could have replaced the sulfate with a nitrate or
the sodium with potassium and the reaction would still be what is
shown in the net ionic equation AND any spectator ions remain
unchanged in this reaction.
Thus, in answer to your question as to what happened to the sodium
and sulfate ions, well, nothing. They dissolved, and remain
dissolved. They did not combine with anything. They did not change
their form once they were dissolved in water and the net reaction proceeded.
Greg (Roberto Gregorius)
Sodium sulfate is quite soluble in water. It can precipitate out at
very high concentration. Magnesium carbonate, which is less
soluble, precipitates first.
Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
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Update: June 2012