3. Balancing Chemical Equations

Because atoms are conserved, a chemical equation must be balanced — there has to be the same number of each kind of atom on both sides of the arrow.

You balance by adjusting the coefficients — the big numbers in front of each formula. You must never change the subscripts (the small numbers inside a formula), because that would change the substance itself.

Take the formation of water: 2 H₂ + O₂ → 2 H₂O. Count the atoms:

• Hydrogen: left = 2 × 2 = 4; right = 2 × 2 = 4. ✓
• Oxygen: left = 2; right = 2 × 1 = 2. ✓

Both sides match, so the equation is balanced — and it obeys conservation of mass.

⚠️ Common misconception: to balance, change the coefficients, not the subscripts. Changing H₂O to H₂O₂ makes a different substance (hydrogen peroxide), not balanced water.

💡 Tip: count each element on both sides; adjust the coefficients until they match.