Balancing a Chemical Equation by Counting Atoms

Hey there — I'm Atlas, and today we are going to crack one of the most important rules in chemistry: atoms are never created or destroyed in a chemical reaction. Scientists call this the Law of Conservation of Mass. When iron rusts, when wood burns, or when baking soda fizzes with vinegar, the same atoms that started the reaction are still there at the end — they have just rearranged into new substances. A chemical equation is our way of writing down that rearrangement. The substances on the LEFT of the arrow are the reactants (what you start with). The substances on the RIGHT are the products (what you end up with). Here is the challenge: a chemical formula is fixed. H₂O always means exactly 2 hydrogen atoms bonded to 1 oxygen atom. You cannot change the little subscript numbers — that would create a completely different substance. So how do we make sure the atom count balances? We place big numbers called COEFFICIENTS in front of entire formulas. A coefficient multiplies every atom in that formula. Let's try it with hydrogen gas reacting with oxygen gas to form water: H₂ + O₂ → H₂O ← unbalanced! Left side: 2 H, 2 O. Right side: 2 H, 1 O. Oxygen is not balanced. I cannot touch the subscripts, so I put a coefficient of 2 in front of H₂O: H₂ + O₂ → 2 H₂O Now the right side has 4 H and 2 O. Hydrogen is now off — I need 4 H on the left, so I put a 2 in front of H₂: 2 H₂ + O₂ → 2 H₂O Left: 4 H, 2 O. Right: 4 H, 2 O. Balanced! The trick is to tally each element separately and adjust one coefficient at a time until every count matches.