How would you demonstrate the conservation of mass in a reaction in a closed system?

How would you demonstrate the conservation of mass in a reaction in a closed system?

• A. Weigh the products only after the reaction.
• B. Weigh the reactants and products; mass remains the same before and after in a closed container.
• C. Mass increases due to gas production.
• D. Mass decreases because some mass is lost as energy.

Answer: (B)

In a closed system, mass is conserved: the total mass before a reaction is the same as the total mass after, because atoms aren’t created or destroyed, they’re just rearranged. To demonstrate this, weigh the entire closed setup with the reactants inside before the reaction starts. Let the reaction take place without letting any material escape. Then weigh the entire setup again after the reaction. If nothing has left or entered the system, the total mass found before and after will be the same. This is the best approach because it directly tests the idea that the whole system’s mass stays constant, even though substances inside may change form. The other ideas don’t reliably demonstrate conservation: weighing only the products ignores the initial mass; claiming mass increases because gas is produced would only be true if some of the gas escaped, which isn’t the case in a closed system; and saying mass decreases because some mass is lost as energy would require results that aren’t observed in ordinary chemical reactions at this level (energy changes don’t significantly change the total mass).

In a closed system, mass is conserved: the total mass before a reaction is the same as the total mass after, because atoms aren’t created or destroyed, they’re just rearranged.

To demonstrate this, weigh the entire closed setup with the reactants inside before the reaction starts. Let the reaction take place without letting any material escape. Then weigh the entire setup again after the reaction. If nothing has left or entered the system, the total mass found before and after will be the same.

This is the best approach because it directly tests the idea that the whole system’s mass stays constant, even though substances inside may change form. The other ideas don’t reliably demonstrate conservation: weighing only the products ignores the initial mass; claiming mass increases because gas is produced would only be true if some of the gas escaped, which isn’t the case in a closed system; and saying mass decreases because some mass is lost as energy would require results that aren’t observed in ordinary chemical reactions at this level (energy changes don’t significantly change the total mass).