Chemical formulae and balancing equations
Chemistry · Key Stage 3 · The School
What a formula tells you
A formula like H₂O shows which atoms are in a substance, and how many of each. The small number after a symbol is a SUBSCRIPT. H₂O means two hydrogen atoms and one oxygen atom, joined. CO₂ means one carbon atom and two oxygen atoms. No subscript means just one atom of that kind.
Why equations need balancing
Atoms are never made or destroyed in a reaction. So a correct equation must show the same number of each atom on both sides. Mg + O₂ → MgO looks fine at first glance. Count the oxygen: two on the left, only one on the right. That equation is wrong as it stands.
How to actually balance one
You balance an equation by placing a BIG number in front of a whole formula. This is a COEFFICIENT, and it multiplies everything inside that formula. So 2H₂O means two whole water molecules: four hydrogen atoms and two oxygen atoms in total. Never touch the small subscript numbers. Only add coefficients in front.
Start with Mg + O₂ → MgO. Count atoms on each side. Left: 1 magnesium, 2 oxygen. Right: 1 magnesium, 1 oxygen. Oxygen does not match. Try 2MgO on the right instead: now magnesium is 2 on the right, but only 1 on the left. So add a 2 in front of magnesium too: 2Mg + O₂ → 2MgO. Recount. Left: 2 magnesium, 2 oxygen. Right: 2 magnesium, 2 oxygen. Balanced. Every number added was a coefficient in front of a formula. Not one subscript was touched.