Which factor is most likely to increase the mass in a reaction mixture?

The introduction of gas reactants from the air is likely to increase the mass in a reaction mixture because gases have mass, and when they are added to a system, they contribute to the overall mass of that system. When a reaction occurs in an open container, gases can be taken in from the surroundings and mix with the reactants. This results in a net increase in mass as those gas molecules combine with solid or liquid reactants in the mixture.

In contrast, the presence of solid reactants alone does not inherently change the mass; it remains constant as long as no material is added or removed. Reactions that produce gases as products often result in a loss of mass if those gases escape into the atmosphere. Cooling the reaction mixture may result in condensation of gases, but it does not directly contribute to an increase in total mass. Thus, introducing gas reactants from the air is the factor most likely to lead to a measurable increase in mass during a chemical reaction.