A class is given a demonstration of solution chemistry and solubility equilibria. A 6.57-g sample of NiSO46H2O (molar mass 262.84) is dissolved in enough water to make 50.00 mL of a green solution, and 7.15 g of Na2CO310H2O (molar mass 286.14) is dissolved in enough water to make 50.00 mL 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 6 M 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 6 M NH3. The precipitate dissolves, and a blue solution forms.

What is the gas that evolves? Why? What reaction happens with the HCL? Which ion is responsible for the color of the solutions? Thank you.