Enter your reactants and products as chemical formulas to verify if an equation is balanced. Get an instant atom-by-atom count for each element, see which side has a mismatch, and learn the correct coefficients — all based on the law of conservation of mass.
✓ Verified: Law of Conservation of Mass — IUPAC / Chemistry textbook standard — April 2026
Enter each compound with its coefficient and formula (e.g. 2H2O, Fe2O3). Use capital letters for elements, subscript numbers after each element symbol. Try the presets below to see examples.
Quick Presets
Reactants (Left Side) — separate compounds with +
Please enter at least one reactant (e.g. H2 or 2H2).
Products (Right Side) — separate compounds with +
Please enter at least one product (e.g. H2O or 2H2O).
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Equation Entered
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Atom Count Verification
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Sources & Methodology
🛡️Atom counting based on the law of conservation of mass (Lavoisier, 1789). Formula parsing follows IUPAC chemical nomenclature standards.
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IUPAC Recommendations for Chemical Nomenclature
International Union of Pure and Applied Chemistry standards for chemical formula notation. iupac.org
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Atkins' Physical Chemistry, 12th Edition
Standard university chemistry textbook covering stoichiometry, balancing equations, and the law of conservation of mass in detail.
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Khan Academy: Balancing Chemical Equations
Free educational resource covering the systematic approach to balancing chemical equations used in high school and university chemistry. khanacademy.org
How This Works: Each compound input is parsed to extract the coefficient (number before formula) and the formula. The formula is then broken down element by element using the pattern: Capital letter + optional lowercase + optional subscript number. Atoms on both sides are summed per element. If all elements match, the equation is balanced.
For each compound: atoms(element) = coefficient × subscript
Example: 2H2O → coefficient=2, H=2, O=1 Total H on this side = 2×2 = 4 | Total O = 2×1 = 2 Check: H₂ + O₂ → H₂O is not balanced (2H+2O vs 2H+1O). Correct: 2H₂ + O₂ → 2H₂O (4H+2O each side) ✅
How to Balance Chemical Equations: A Complete Guide
Balancing chemical equations is one of the most fundamental skills in chemistry. It is the direct application of the law of conservation of mass: in a chemical reaction, matter is neither created nor destroyed. Every atom that enters a reaction as a reactant must appear in a product. The number of atoms of each element must be identical on both sides of the arrow.
The Five-Step Balancing Method
The systematic approach to balancing most equations by inspection follows a consistent five-step process. First, write out the correct formulas for all reactants and products — never guess formulas, as using an incorrect formula makes balancing meaningless. Second, count the atoms of each element on both sides of the equation. Third, identify which elements are unequal. Fourth, adjust coefficients (the numbers in front of formulas) to equalize atom counts — never change subscripts. Fifth, verify that all atoms balance and that the coefficients share no common factor.
Reaction Type
Example
Balancing Strategy
Combination
2H₂ + O₂ → 2H₂O
Balance H, then O
Decomposition
2H₂O₂ → 2H₂O + O₂
Balance O last
Single Replacement
Zn + 2HCl → ZnCl₂ + H₂
Balance metals first
Double Replacement
NaCl + AgNO₃ → AgCl + NaNO₃
Often balanced as-is (1:1)
Combustion
CH₄ + 2O₂ → CO₂ + 2H₂O
C first, H second, O last
Redox
2Fe + 3Cl₂ → 2FeCl₃
Balance charges + atoms
Why You Can Never Change Subscripts
Subscripts are part of the chemical formula — they define the identity of the compound. Changing H₂O to H₂O₂ changes water into hydrogen peroxide. These are completely different substances with different properties. Only coefficients can be changed. Coefficients multiply the entire formula — so 2H₂O means two water molecules (4 hydrogen atoms and 2 oxygen atoms total).
Special Cases: Polyatomic Ions and Parentheses
When a polyatomic ion like SO₄²⁻ or NH₄⁺ appears unchanged on both sides of a reaction, treat it as a unit rather than counting individual atoms — it's faster and less error-prone. When parentheses appear in a formula such as Ca(OH)₂, the subscript outside the parentheses multiplies all atoms inside: Ca(OH)₂ contains 1 Ca, 2 O, and 2 H.
💡 Common Balancing Mistakes: (1) Changing subscripts instead of coefficients. (2) Forgetting to multiply all atoms in a formula by its coefficient. (3) Leaving out state symbols (s), (l), (g), (aq) — these don't affect balancing but matter for thermodynamics. (4) Not simplifying coefficients to the lowest whole-number ratio.
Balancing Combustion Reactions Step by Step
Combustion of hydrocarbons is the most common type you'll encounter. The general reaction is: CₓHᵧ + O₂ → CO₂ + H₂O. The balancing strategy is always: (1) Balance carbon by setting the CO₂ coefficient equal to the number of carbon atoms in the fuel. (2) Balance hydrogen by setting the H₂O coefficient to half the number of hydrogen atoms in the fuel. (3) Count total oxygen needed on the right side, then set the O₂ coefficient accordingly. (4) If O₂ coefficient is fractional, multiply all coefficients by 2.
Frequently Asked Questions
To balance a chemical equation: 1) Write correct formulas for all reactants and products. 2) Count atoms of each element on both sides. 3) Add coefficients (whole numbers before formulas) to equalize atom counts. 4) Never change subscripts — only coefficients. 5) Verify each element balances and coefficients are in simplest whole-number ratio.
The law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction. The total mass of reactants equals the total mass of products. This means the number of atoms of each element must be identical on both sides of a balanced chemical equation.
Coefficients are whole numbers placed in front of chemical formulas to indicate how many molecules or formula units of each substance are in the reaction. In 2H₂ + O₂ → 2H₂O, the coefficients are 2, 1, and 2. A coefficient multiplies all atoms in the formula it precedes.
Subscripts are small numbers within a chemical formula indicating atoms per molecule (H₂O has 2 H and 1 O). Coefficients are numbers in front of the formula multiplying all atoms in it. When balancing equations, you can only change coefficients. Changing subscripts would change the identity of the chemical compound entirely.
For combustion of a hydrocarbon (CxHy + O₂ → CO₂ + H₂O): 1) Balance carbon first by matching C atoms to CO₂. 2) Balance hydrogen by matching H atoms to H₂O. 3) Balance oxygen last by adjusting the O₂ coefficient. If this gives a fractional coefficient, multiply everything by 2. Example: CH₄ + 2O₂ → CO₂ + 2H₂O.
Chemical equations must be balanced to obey the law of conservation of mass. Balanced equations are also essential for stoichiometry — determining molar ratios, theoretical yield, limiting reagents, and percent yield. An unbalanced equation cannot be used for any quantitative chemistry calculation.
Balancing by inspection: 1) Start with elements appearing in only one compound per side. 2) Balance metals first, then nonmetals, then hydrogen, and finally oxygen. 3) Adjust one coefficient at a time and recount. 4) Verify all atoms balance. 5) Divide all coefficients by any common factors to get the simplest ratio.
An unbalanced equation violates the law of conservation of mass — it implies atoms are created or destroyed. In practice, unbalanced equations cannot be used for stoichiometry or yield calculations. Every valid chemical equation must have equal atom counts for each element on both sides.
No. Never change subscripts when balancing chemical equations. Subscripts define the chemical formula — changing H₂O to H₂O₂ changes water into hydrogen peroxide. Only coefficients (numbers in front of formulas) can be changed when balancing.
A half-reaction represents either the oxidation or reduction part of a redox reaction. Balancing redox equations requires the half-reaction method: balance each half-reaction separately for both atoms and charge, then combine them so that electrons cancel. This is more complex than simple equation balancing and requires accounting for charge as well as atoms.