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.