Final answer:
The answer is D) the mass has not changed. The mass of the material inside the jar has not changed because the law of conservation of mass dictates that mass is conserved in a closed system, despite the production of gas bubbles.
Step-by-step explanation:
When the student mixes 2.0 grams of calcium metal with water and a gas is produced, this indicates a chemical reaction is taking place. Specifically, calcium reacts with water to form calcium hydroxide and hydrogen gas:
Ca (s) + 2H2O (l) → Ca(OH)2 (aq) + H2 (g)
This reaction is an example of a single replacement reaction where the metal takes the place of another element in a compound, producing a new compound and a gas. According to the law of conservation of mass, the mass of the reactants before the reaction must equal the mass of the products after the reaction. Keeping the lid on the bottle means that no gas escapes and the mass within the system remains constant. Therefore, the answer is D) the mass has not changed.