What Is a Balanced Chemical Equation?

A chemical equation describes a reaction. The reactants are on the left, the products are on the right, and the arrow between them reads “yields.” A balanced equation has the same number of atoms of each element on both sides.

Take hydrogen burning in oxygen. Written as H2 + O2 → H2O, the left side has two oxygen atoms and the right side has only one. An oxygen atom has gone missing, and atoms are never created or destroyed in a chemical reaction.

Balanced: 2H2 + O2 → 2H2O. Now each side has four hydrogen atoms and two oxygen atoms.

The large numbers in front are coefficients, and they count molecules. The small numbers inside a formula are subscripts, and they are part of the substance itself. To balance an equation, you choose the coefficients and leave the subscripts alone.
Cut-paper illustration of two wooden trays holding the same colored atoms, grouped into different molecules in each tray

Remember

Four rules that hold for every chemical equation.

  • Count every atom: each element needs the same number on both sides, because atoms are neither created nor destroyed.

  • Change only the coefficients, never the subscripts. A new subscript makes a new substance: one more oxygen turns water into hydrogen peroxide.

  • Use the smallest whole numbers, and leave out a coefficient of 1.

  • A balanced equation gives the ratio in which substances react, which is where stoichiometry begins.

Practice: Balance It Yourself

Thirty reactions in three levels. The atom count updates as you type, a hint shows where to start, and the algebra is one more click away. Checking your homework? Switch to Balance your own equation.

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Why Balance? The Law of Conservation of Mass

In the 1770s, the French chemist Antoine Lavoisier heated metals such as tin inside sealed glass vessels and weighed everything before and after. The shiny metal turned into a dull powder, but the sealed vessel weighed exactly what it had at the start. Nothing had been gained or lost; the matter inside had only been rearranged.

Once chemists understood that matter is made of atoms, the Law of Conservation of Mass took the form used today: the atoms present before a reaction are the same atoms present after it. A balanced chemical equation is that law written in symbols.

It is also where the arithmetic of chemistry begins. In stoichiometry, the coefficients of a balanced equation give the ratio in which substances react, so they tell you how many moles, and how many grams, of each substance a reaction uses and makes.
Cut-paper illustration of a candle burning inside a sealed glass bell jar that rests on one pan of a brass balance, in an old laboratory

Balanced Equations at Work

  • Fertilizer from the Air

    N2 + 3H2 → 2NH3. The Haber–Bosch process makes ammonia from the nitrogen in air, and most of that ammonia becomes fertilizer. The coefficients set the feed: three molecules of hydrogen for every molecule of nitrogen.

  • Rocket Propellant

    2H2 + O2 → 2H2O. A rocket that burns liquid hydrogen must carry its own oxygen. Burned exactly as the equation says, that is eight kilograms of oxygen for every kilogram of hydrogen; real engines run with a little extra hydrogen.

  • Rusting Iron

    4Fe + 3O2 → 2Fe2O3. Iron left in damp air slowly combines with oxygen. The oxygen atoms come from the air, so a rusted nail weighs more than the nail did, and the balanced equation accounts for the difference.

FAQ

  • What does it mean to balance a chemical equation?

    It means choosing a coefficient for each substance so that every element has the same number of atoms on both sides of the arrow. The balanced equation then shows the ratio in which the reactants combine and the products form.

  • Why can’t I change the subscripts?

    Subscripts are part of a substance’s formula, so changing one changes the substance itself. H2O is water, but H2O2 is hydrogen peroxide. Only the coefficients in front of each formula may change.

  • Which method is better, inspection or algebra?

    Inspection is usually quicker for short equations: count the atoms, adjust one coefficient, and count again. Algebra is steadier for long equations with many substances, where trial and error can go in circles. Both give the same answer, so you can use one to check the other.

  • Which element should I balance first?

    Start with an element that appears in only one substance on each side, often a metal or carbon. Leave hydrogen and oxygen for last, since they tend to appear in several substances at once.

  • What if a coefficient comes out as a fraction?

    Multiply every coefficient by the same number to clear it. In the algebra method, setting one coefficient to 1 often makes another come out as ½; doubling all of them gives whole numbers. Chemists write balanced equations with the smallest whole numbers.

Keep Going with Our Free Chemistry Course

Both balancing videos are lessons in College Chemistry, alongside the mole, molar mass, stoichiometry and limiting reactants, each with example problems worked in full.