Year 9 Chemical Reactions Properties, Equations, And Energy Changes Practice Test

Balanced chemical equations follow the law of conservation of mass: the same number of atoms of each element must appear on both sides of the equation. That is why this option is the best description—the counts of each type of atom match on the reactant and product sides, so mass and matter are preserved. To balance, you adjust the coefficients in front of formulas (not the subscripts inside a molecule), so you’re just showing more or fewer molecules involved in the reaction. For example, H2 + O2 -> H2O isn’t balanced because there are four hydrogen atoms and two oxygen atoms on the left, but only two hydrogen and one oxygen on the right. Balancing gives 2 H2 + O2 -> 2 H2O, which has the same number of each atom on both sides (four hydrogen and two oxygen). If total mass changed, that would contradict the conservation law, which is why that choice isn’t correct. An equation with only products on one side wouldn’t represent a real balanced reaction, and the idea that an equation cannot be solved isn’t accurate—most reactions can be balanced by adjusting coefficients.