Dissolved in solution automatically mean disassociation?

[instantresults]

Homework Statement

a 6.57g sample of NiSO₄*6H₂O (molar mass 262.84) is dissolved in enough water to make 50.00mL of a green solution and 7.15g of Na₂CO₃*10H₂O (molar mass 286.14) is dissolved in enough water to make 50.00mL of a colorless solution. The two solutions are mixed and a green precipitate forms. The resulting slurry is divided into two equal portions. To one portion is added an excess of 6M HCl, which results in the disappearance of the precipitate and a rapid evolution of a gas. To the second portion is added a few milliliters of 6M NH₃. The precipitate dissolves and a blue solution forms.

Q: according to the info in the passage the gas that evolves is

1) sulfur dioxide
2) sulfur trioxide
3) carbon dioxide
4) carbon monoxide

Homework Equations

The Attempt at a Solution

The solution says that HCl reacts with CO₃^2- to form CO² gas.
I don't understand why they assume that Na₂CO3 dissassociates into its respective ions when dissolved in water (or is the word "dissolved" automatically mean the molecule has separated?). ALso what's to stop carbon monoxide from forming? Thanks for any help

 

[Borek]

Not everything dissociates when dissolved - for example dissolved glucose doesn't. But most salts do.

I can't think of a reaction that would produce carbon monoxide in these conditions. Carbonates reacting with acid always behave the same way.