[hya_been]

I have a question that asks about a reaction between limewater and excess carbon dioxide. It asks for a balanced chemical equation.  

I know that the product is Calcium Hydrogen Carbonate, but what I don't understand is how you get the correct formula.  

I guessed that the formula was CaH2CO3, but it turned out to be Ca(HCO3)2.  How do you determine this formula? Why is H in the brackets/ How do you know it's in the brackets?

[mike]

When carbon dioxide is bubbled through water it forms carbonic acid:

CO₂ + H₂O ---> H₂CO₃

Hydroxide then neutralises the carbonic acid:

H₂CO₃ + 2OH^- ---> CO₃^2- + 4H₂O

Carbonate is then able to form insoluble precipitate with Ca2+

Ca²⁺ + CO₃^2- ---> CaCO_3(s)

note that H

When excess CO2 is added, nore carbonic acid is produced and the OH- is consumed so that excess carbonic acid reacts with CaCO_3(s) to form the soluble bicarbonate salt:

CaCO_3(s) + H₂CO₃ ---> Ca²⁺ + 2HCO₃^-