Question

a piece of magnesium metal is placed in a jar filled with hydrochloric acid. a lid is tightly fitted onto the jar so no gas can escape. the magnesium dissolves in the acid and bubbles rise to the surface of the acid. what has happened to the mass of the jar?
increases
decreases
does not change
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question 2
a piece of magnesium metal is placed in a jar filled with hydrochloric acid. before the reaction, the mass of the materials is 59 grams. the magnesium dissolves in the acid and bubbles rise to the surface of the acid. after the reaction, the mass of the materials is 54 grams. what happened to the 5 grams of material?
5 grams of material disappeared
the measurements must be wrong
some of the mass was lost as gas escaped
a reaction happened so the materials got lighter

Explanation:

Question 1

Step1: Recall Law of Conservation of Mass

The Law of Conservation of Mass states that in a closed system, the total mass of reactants equals the total mass of products. Here, the jar is a closed system (lid is tight, no gas escapes).

Step2: Analyze the Reaction

Magnesium (Mg) reacts with hydrochloric acid (HCl) to form magnesium chloride and hydrogen gas ($\ce{Mg + 2HCl -> MgCl_{2} + H_{2} \uparrow}$). Since the system is closed, the mass of the jar (containing reactants and products) remains constant as no matter is added or removed.

Step1: Analyze the System

In this case, the problem doesn't state the jar is closed (unlike Question 1). When magnesium reacts with HCl, hydrogen gas is produced. If the system is open (no tight lid to prevent gas escape), the gas can leave.

Step2: Relate to Mass Change

The initial mass is 59 g, final is 54 g. The 5 g difference is because the hydrogen gas produced escaped from the jar, so mass was lost as gas. Other options are incorrect: matter can't disappear (violates conservation of mass), measurements aren't necessarily wrong, and reactions don't make materials lighter (mass is conserved in a closed system, but here it's open).

Answer:

does not change

Question 2