In a closed system, the Law of Conservation of Mass states that mass is:

• A. Created from nothing
• B. Doubled during reactions
• C. Conserved; mass before equals mass after
• D. Varies with temperature

Answer: (C)

In a closed system, the total mass stays the same. Substances can change form or rearrange into different compounds, but the amount of matter doesn't disappear or appear out of thin air. Chemical reactions rearrange atoms, and because atoms are conserved, the sum of the masses of all reactants equals the sum of the masses of all products.

Think of weighing everything before and after a reaction inside a sealed container—the total weight remains the same (aside from tiny measurement errors). Mass isn’t created, destroyed, or altered by temperature in this context; heating may change how it’s distributed or how dense things are, but not the total mass.