Chemistry · Interactive
Balancing Chemical Equations
An equation is balanced when each element has the same number of atoms on both sides. Drag the coefficients of H₂ + O₂ → H₂O below and watch the atom counts match up. Free to use, and exportable into your slides.
Drag the coefficient sliders until both sides balance. Open fullscreen ↗
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What does balancing a chemical equation mean?
Balancing a chemical equation means choosing coefficients so that each element has the same number of atoms on the reactant side and the product side. This has to be true because atoms are never created or destroyed in a reaction, only rearranged, which is the law of conservation of mass. Drag the coefficient sliders in the tool above to change how many of each molecule react, and watch the atom counts update.
Coefficients versus subscripts
A coefficient is the big number written in front of a formula, and it multiplies the whole molecule. A subscript is the small number inside a formula, like the 2 in H₂O, and it tells you how many atoms of that element are in one molecule. When you balance an equation you may only change the coefficients, never the subscripts, because changing a subscript would change the substance itself.
A worked example: hydrogen and oxygen
Consider hydrogen reacting with oxygen to form water. Written unbalanced as H₂ + O₂ → H₂O, the oxygen does not match: two oxygen atoms on the left but only one on the right. Putting a 2 in front of water gives H₂ + O₂ → 2H₂O, and then a 2 in front of hydrogen fixes the hydrogen count. The balanced equation is 2H₂ + O₂ → 2H₂O, with 4 hydrogen and 2 oxygen on each side.
A simple strategy for balancing
Start by counting the atoms of each element on both sides. Balance one element at a time by adjusting a coefficient, usually leaving hydrogen and oxygen until last. After each change, recount, because a new coefficient can affect elements you already balanced. When every element matches and the coefficients are the smallest whole numbers that work, the equation is balanced.
Frequently asked questions
What does it mean to balance a chemical equation?
Balancing means choosing coefficients so that each element has an equal number of atoms on both sides of the equation, satisfying the law of conservation of mass.
What is the difference between a coefficient and a subscript?
A coefficient is the number in front of a formula and multiplies the whole molecule. A subscript is the small number inside a formula that counts atoms of one element. You balance equations by changing coefficients only, never subscripts.
How do you balance H₂ + O₂ → H₂O?
Put a 2 in front of water and a 2 in front of hydrogen to get 2H₂ + O₂ → 2H₂O. That gives 4 hydrogen and 2 oxygen atoms on each side, so the equation is balanced.
Why can you not change subscripts when balancing?
Subscripts define the identity of a substance. Changing the 2 in H₂O to make H₂O₂, for example, turns water into hydrogen peroxide, a different chemical. Only coefficients may be changed.
What is the law of conservation of mass?
It states that matter is neither created nor destroyed in a chemical reaction. The total mass and the number of atoms of each element stay the same, which is exactly why equations must be balanced.
How do I use this balancing tool?
Drag the coefficient sliders for H₂, O₂, and H₂O. The molecules and the atom counts for each element update instantly, and a badge tells you when both sides match. You can also export the balanced diagram.