Does the reaction between

CO2(g) + Mg(OH)2(aq)

produce MgCO3(s) + H2O(l)

or Mg(HCO3)2 ???

Normally, Mg(CO3)2 but I suspect an excess of CO2 will produce the bicarbonate. For example,
CO2(g) + Ca(OH)2(aq) ==> CaCO3(s) + H2O(l) and that is often done by exhaling into a flask will lime water in it. BUT, if we continue to exhale into the liquid, eventually the cloudiness caused by the ppting CaCO3 will clear up and the solution will again be colorless. This is because the CaCO3 has been turned into Ca(HCO3)2. The bicarbonate is solube whereas the carbonate is not. I hope this is not more than you ever wanted to know about the reaction.


The original question is:
Carbon dioxide is bubbled through a saturated solution of magnesium hydroxide. Give the equation plus observations.

With this extra info, is your response above still true?

What did you observe? If you observed a ppt formed, then MgCO3 is the correct answer. If you observed that a ppt formed, then disappeared (dissolved), you would need to write both equations. Everything I said is true but it just depends upon how much CO2 is bubbled through it.

To determine the correct reaction and product formed, the observations are crucial. If a precipitate (ppt) is formed and remains even after continuous bubbling of CO2, then the reaction is:

CO2(g) + Mg(OH)2(aq) → MgCO3(s) + H2O(l)

This reaction results in the formation of Magnesium Carbonate (MgCO3) as a solid precipitate and water as a liquid product.

However, if a precipitate initially forms and then disappears (dissolves) upon further bubbling of CO2, then an additional reaction occurs. This reaction involves the conversion of the precipitated Magnesium Carbonate to Magnesium Bicarbonate (Mg(HCO3)2), which is soluble in water:

MgCO3(s) + CO2(g) + H2O(l) → Mg(HCO3)2(aq)

The presence of a disappearing precipitate indicates the conversion of Magnesium Carbonate to the soluble Magnesium Bicarbonate due to the excess CO2.

Therefore, based on the observations, it is important to discern if a precipitate is formed and remains (indicating MgCO3) or if the precipitate disappears upon continuous bubbling of CO2 (indicating Mg(HCO3)2).