How to Balance Chemical Equations Step by Step
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Balancing chemical equations becomes straightforward when you remember one rule: atoms are rearranged in a reaction, not created or destroyed. A balanced equation therefore has the same number of atoms of each element on both sides. You achieve that balance by changing coefficients, never by changing the formulas of the substances.
Counting atoms
Start with the correct formulas for all reactants and products. Consider H2 + O2 -> H2O. The left side has two hydrogen atoms and two oxygen atoms, while the right side has two hydrogen atoms and one oxygen atom. The oxygen count does not match.
Place a coefficient of 2 before H2O. Now the right side contains four hydrogen atoms and two oxygen atoms. To match the hydrogen count, place a coefficient of 2 before H2. The balanced equation is 2H2 + O2 -> 2H2O.
This example shows the basic method for balancing chemical equations. Coefficients multiply the entire formula that follows them. A coefficient of 2 before H2O means two water units, so it counts as four hydrogen atoms and two oxygen atoms in total.
Do not change subscripts
Subscripts are part of a substance's chemical formula. Changing H2O to H2O2 would not simply give you more oxygen atoms. It would change water into hydrogen peroxide, a different chemical substance. Balancing must preserve the identities of the reactants and products.
For larger equations, count each element systematically. It often helps to balance elements that appear in only one reactant and one product first, then leave hydrogen and oxygen until later if they occur in several compounds. There is no single order that works best for every equation, but careful counting always works.
After adding a coefficient, recount every affected element. One change can fix one atom count while altering several others. The most reliable way to learn how to balance chemical equations is to keep a small table or written count instead of trying to hold everything in your head.
A balancing routine
Use this sequence for most school-level equations:
- •Write or check the correct chemical formulas first.
- •Count atoms of each element on both sides.
- •Add coefficients to balance one element at a time.
- •Recount after every change.
- •Reduce coefficients to the smallest whole-number ratio when possible.
Some equations become easier if you treat an unchanged polyatomic ion as a unit. If the same ion appears on both sides of the equation without breaking apart, counting the whole group can reduce the number of separate atom counts you need to track. This is a convenience, not a new balancing rule. The final equation must still conserve every individual atom. It is another reason balancing improves with organisation: choose a counting method that reduces clutter, then verify the complete atom totals before you finish.
For complex reactions, a final independent recount is worth the extra minute. Check each element from the beginning as if you had not written the coefficients yourself. That fresh pass often catches a small mismatch.
Two worked combustion examples
Burning propane shows the usual routine. Start with C3H8 + O2 -> CO2 + H2O. Carbon appears in only one reactant and one product, so balance it first: three carbon atoms need 3CO2. Hydrogen comes next: eight hydrogen atoms need 4H2O. Now count oxygen on the right. There are six oxygen atoms in 3CO2 and four in 4H2O, making ten in total, so the left needs 5O2. The balanced equation is C3H8 + 5O2 -> 3CO2 + 4H2O. Leaving oxygen until last works well because it appears in several substances.
Ethane creates a common snag. Start with C2H6 + O2 -> CO2 + H2O. Balancing carbon and hydrogen gives C2H6 + O2 -> 2CO2 + 3H2O, which contains seven oxygen atoms on the right. That would need three and a half oxygen molecules, so temporarily write 3.5O2. Because equations should use whole numbers, multiply every coefficient by two to get 2C2H6 + 7O2 -> 4CO2 + 6H2O. Recount to check: four carbons, twelve hydrogens and fourteen oxygens appear on each side.
Balancing charge, and an algebraic shortcut
In equations involving ions, counting atoms is not enough, because the total electric charge must also be the same on both sides. Consider copper metal reacting with silver ions: Cu + Ag+ -> Cu2+ + Ag. Each element appears once on each side, so the atoms look balanced, but the charges do not. The left side carries a total charge of plus one and the right side plus two. Placing a coefficient of 2 before both silver species gives Cu + 2Ag+ -> Cu2+ + 2Ag, and now both atoms and charges match, with plus two on each side.
For awkward equations, an algebraic method can replace trial and error. Write unknown coefficients in front of each substance, such as a Fe + b O2 -> c Fe2O3, and form one equation for each element. Iron gives a = 2c, and oxygen gives 2b = 3c. Choose a convenient value for one unknown, here c = 2, which gives a = 4 and b = 3. The balanced equation is 4Fe + 3O2 -> 2Fe2O3. The same approach works for much larger reactions, and it is essentially how computer programs balance equations, by solving a set of simultaneous equations.
Weighing the evidence for balanced equations
The rule that atoms are conserved was established by careful weighing. In the eighteenth century many chemists believed that burning released a substance called phlogiston, yet some metals got heavier when heated in air, which was hard to explain. Antoine Lavoisier, working in France in the 1770s and 1780s, heated metals such as tin inside sealed glass vessels and weighed everything before and after. The total mass did not change. When he opened a vessel, air rushed in, showing that the metal had combined with part of the air, which he identified as oxygen. His results made conservation of mass a foundation of chemistry.
A balanced equation lets you repeat Lavoisier's check on paper. Take the propane equation from earlier, C3H8 + 5O2 -> 3CO2 + 4H2O, and use approximate relative masses of 12 for carbon, 1 for hydrogen and 16 for oxygen. One propane molecule has a mass of 44 units and five oxygen molecules have 160, a total of 204. On the other side, three carbon dioxide molecules have 132 and four water molecules have 72, again a total of 204. If an equation is unbalanced, this mass check fails, so it doubles as a second way to test your coefficients.
Limiting reactants and what an equation does not tell you
A balanced equation gives the ratio in which substances react, which lets you predict quantities. Take 2H2 + O2 -> 2H2O, and suppose you have 10 grams of hydrogen and 64 grams of oxygen. Using relative masses of 2 for H2 and 32 for O2, that is 5 moles of hydrogen and 2 moles of oxygen. The equation says 5 moles of hydrogen would need 2.5 moles of oxygen, but only 2 are available, so oxygen runs out first. It is the limiting reactant. The 2 moles of oxygen make 4 moles of water, 72 grams, leaving 2 grams of hydrogen unused, 74 grams in total.
It is easy to read more into an equation than it says. 2H2 + O2 -> 2H2O does not mean that two hydrogen molecules and one oxygen molecule collide at once. The real reaction runs through a chain of short-lived fragments such as single hydrogen atoms and hydroxyl radicals, many small steps that add up to the overall equation. A balanced equation also says nothing about speed, or even whether a reaction happens at all. You can write a perfectly balanced equation for copper reacting with dilute hydrochloric acid to give copper chloride and hydrogen, yet copper sits below hydrogen in the reactivity series and nothing happens.
The takeaway
Balancing chemical equations is a matter of conserving atoms. Keep every chemical formula unchanged, adjust only the coefficients, and count each element until both sides match. Work slowly enough to recount after each change. Once you treat the equation as an atom inventory rather than a guessing game, balancing becomes much more reliable.