Chemical equations describe how substances transform during a reaction. Balancing them ensures we follow the law of conservation of mass, which says atoms can't be created or destroyed. Every atom that goes into a reaction must come out the other side.

This section covers how chemical equations are written, what each part means, and how to balance them step by step.

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Chemical Equations

Components and Representation

A chemical equation is a shorthand way of describing a chemical reaction. Reactants (the starting substances) go on the left side of an arrow, and products (what forms) go on the right.

The arrow (→) means "yields" or "produces"
Plus signs (+) separate multiple reactants or multiple products
State symbols in parentheses tell you the physical form of each substance:
- (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous (dissolved in water)
Catalysts or special conditions (like heat) are sometimes written above or below the arrow

For example, the equation for hydrogen reacting with oxygen to form water looks like this:

$H_2(g) + O_2(g) \rightarrow H_2O(l)$

This equation shows the correct substances, but it isn't balanced yet. More on that below.

Law of Conservation of Mass and Balanced Equations, Basically, this means that chemical equations must be consistent with the law of conservation of ...

Subscripts and Their Significance

Subscripts are the small numbers in a chemical formula that tell you how many atoms of each element are in one molecule or formula unit. In $H_2O$ , the subscript 2 means there are two hydrogen atoms bonded to one oxygen atom.

A single atom with no subscript is understood to be 1 (for example, the O in $H_2O$ means one oxygen atom)
You must never change subscripts to balance an equation. Changing a subscript changes the substance itself. $H_2O$ is water, but $H_2O_2$ is hydrogen peroxide, a completely different compound.
Coefficients (the big numbers placed in front of a formula) multiply the entire molecule. $2H_2O$ means two molecules of water, which gives you 4 hydrogen atoms and 2 oxygen atoms total.

The distinction between coefficients and subscripts is one of the most common sources of mistakes in balancing. Coefficients change how much of a substance you have. Subscripts define what the substance is.

Balancing Equations

Law of Conservation of Mass and Balanced Equations

The law of conservation of mass says that matter is not created or destroyed in a chemical reaction. The total mass of the reactants must equal the total mass of the products. At the atomic level, every type of atom must appear the same number of times on each side of the equation.

A skeleton equation is an unbalanced equation that shows the correct formulas for reactants and products but doesn't have the right ratios. Your job when balancing is to add coefficients (whole numbers in front of formulas) until every element has equal atoms on both sides.

Balancing Process: Step by Step

Here's a reliable method for balancing equations:

Write the skeleton equation. Make sure all chemical formulas are correct before you start.
Count atoms of each element on both sides. Making a quick table helps a lot here.
Start with the most complex molecule, or an element that appears in only one reactant and one product. This gives you a clear starting point.
Place coefficients in front of formulas to equalize that element on both sides.
Work through the remaining elements one at a time, adjusting coefficients as needed.
Balance polyatomic ions as a unit when they appear unchanged on both sides. For example, if $SO_4^{2-}$ shows up intact in both reactants and products, count it as one piece rather than counting S and O separately.
Save hydrogen and oxygen for last. These elements tend to appear in multiple compounds, so they're easier to balance once everything else is locked in.
Do a final check. Count every atom on each side to confirm they match.

Example: Balance $Fe + O_2 \rightarrow Fe_2O_3$

Start by counting: the left side has 1 Fe and 2 O. The right side has 2 Fe and 3 O.
The oxygen is tricky because $O_2$ gives you atoms in multiples of 2, but $Fe_2O_3$ needs them in multiples of 3. The least common multiple of 2 and 3 is 6. So place a 3 in front of $O_2$ (giving 6 O on the left) and a 2 in front of $Fe_2O_3$ (needing 6 O on the right):

$Fe + 3O_2 \rightarrow 2Fe_2O_3$

Now recheck Fe: the left has 1 Fe, but the right has $2 \times 2 = 4$ Fe. Place a 4 in front of Fe:

$4Fe + 3O_2 \rightarrow 2Fe_2O_3$

Final count: 4 Fe on each side, 6 O on each side. Balanced.

Common Mistakes to Avoid

Changing subscripts instead of coefficients. This is the number one error. It changes the identity of the substance, not the amount.
Forgetting to recount after adjusting. Every time you change a coefficient, it affects all atoms in that formula. Go back and recheck.
Overlooking oxygen atoms hidden in polyatomic ions. If a compound contains $NO_3^-$ or $SO_4^{2-}$ , those oxygen atoms still need to be counted.
Using fractions as your final answer. Fractions can be a useful shortcut during the balancing process, but your final answer should have whole-number coefficients. If you end up with a fraction, multiply all coefficients by the denominator to clear it.
Not reducing coefficients. If all your coefficients share a common factor, divide them down. For instance, $2H_2 + 2Cl_2 \rightarrow 2H_2Cl_2$ should be simplified so every coefficient is divided by 2.

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