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Name: Ron
Status: educator
Grade: 6-8
Location: IL
Country: USA
Date: Spring 2010


Question:
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?



Replies:
Ron,

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.

Warren Young


Hi Ron,

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)
Canisius College


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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