If the mass of a reaction mixture increases, what is likely happening?

If the mass of a reaction mixture increases, it typically indicates that a gas that was previously not part of the mixture has been captured within the vessel. In chemical reactions, especially those involving gases, it is common for the total mass to change based on the states of matter of the reactants and products. When a gas reactant is captured, it is added to the total mass of the mixture.

In this context, if a gas is converted into a solid, as suggested by another option, you would expect a decrease in mass if any gas is lost to the environment during the reaction. Similarly, if all products were gases, the mass would not increase, as no additional mass would be added to the system. If no change was occurring in the vessel, then there would be no reason for the mass to increase at all. Therefore, the scenario where a gas reactant is effectively contained and contributes to the overall mass accurately describes the situation where the mass of the reaction mixture increases.