What is the conservation of atoms in a chemical reaction?

• A. The principle that energy is conserved only.
• B. The principle that substances are turned into energy.
• C. The principle that mass increases during reaction.
• D. The principle that atoms are neither created nor destroyed in a chemical reaction.

Answer: (D)

Atoms are neither created nor destroyed in a chemical reaction; they are just rearranged to form new substances. This means the total number of atoms of each element on the left must equal the total on the right. That’s why chemical equations are balanced: to show that the counts of each element stay the same before and after the reaction.

For example, forming water from hydrogen and oxygen can be written as 2H2 + O2 → 2H2O. There are four hydrogen atoms and two oxygen atoms on the left, and the right side also has four hydrogen atoms and two oxygen atoms. The atoms aren’t appearing or disappearing; they’re reassembled into a new substance.

Energy changes can happen in a reaction, but they don’t affect the number of atoms. The other statements describe energy changes or mass behavior that isn’t about atoms being conserved in the chemical process.