In a chemical reaction equation, a number that appears before a chemical formula is called a coefficient. Coefficients are used to balance the equation, ensuring that the same number of atoms of each element appears on both the reactant and product sides. This practice follows the law of conservation of mass, which states that mass cannot be created or destroyed in a chemical reaction.

The number that appears to the right of an element symbol within a formula is a subscript. Subscripts indicate the number of atoms of that element in the compound and are fixed; they cannot be altered when balancing an equation.

Balancing a Simple Reaction: Hydrogen and Oxygen to Form Water

Unbalanced equation: H₂ + O₂ → H₂O

Step 1 – Adjust the product side: H₂ + O₂ → 2H₂O

Step 2 – Balance hydrogen atoms: 2H₂ + O₂ → 2H₂O

Result: Four hydrogen atoms and two oxygen atoms on both sides, so the equation is balanced.

General Steps for Balancing Chemical Equations

1. Identify the Most Complex Compound

Choose an element that appears in a single compound on one side of the equation. Add a coefficient to that compound to equalize the number of atoms of that element on both sides.

2. Balance Polyatomic Ions as Whole Units

Keep ions such as NO₃^- or CO₃^2- intact. For example:

Unbalanced: Ca + HNO₃ → H₂ + Ca(NO₃)₂

Balanced: Ca + 2HNO₃ → H₂ + Ca(NO₃)₂

3. Balance Remaining Elements

After handling the complex species, adjust coefficients for any remaining elements. For combustion of heptane:

Unbalanced: C₇H₁₆ + O₂ → CO₂ + H₂O

Balanced: C₇H₁₆ + 11O₂ → 7CO₂ + 8H₂O

4. Verify Your Work

Count each element on both sides to confirm equality. For complex equations, consider creating a table to track atom counts.

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