Balancing Chemical Equations — Conservation of Atoms and Charge

One-sentence answer: Balancing adjusts coefficients — never subscripts — so each element and net charge is the same on both sides.

Objectives

• State that a balanced equation conserves atoms and charge (conservation of mass in closed system).
• Balance by inspection without changing subscripts.
• Verify with an atom-inventory (reactants vs products) table.

Prerequisites

• Chemical formulas and the meaning of subscripts vs coefficients
• Topic: Chemical Reactions and Equations

Why balance matters (plain language)

Macroscopic: you weigh reactants and products — mass is conserved. Particle level: atoms rearrange; bonds break and form but atoms are neither created nor destroyed. Symbolic: coefficients count how many of each molecule react. If subscripts were changed you would be describing a different substance.

Representations

• Macroscopic: Mass measurements before and after; conserved total mass.
• Particle level: Same number of each atom on both sides; charge summed. Diagram alt: four CH₄ molecules plus eight O₂ molecules rearrange to four CO₂ plus eight H₂O — each C, H, O counted.
• Symbolic: Balanced coefficients. Example below uses dimensionally consistent formulas only; no unit conversion hides a missing atom.

The rule

Never change subscripts to balance. Adjust only coefficients. Include phases where relevant (e.g., (g), (l)) and check charge if ions are present.

Worked example — combustion of methane

Balance:

$\ce{CH4(g) + O2(g) -> CO2(g) + H2O(l)}$

Step 1 — inventory atoms:

Element	Reactants	Products (unbalanced)
C	1	1
H	4	2
O	2	3 (2 in CO₂ + 1 in H₂O)

Step 2 — balance H first (appears in fewest compounds): put 2 before $\ce{H2O}$:

$\ce{CH4 + O2 -> CO2 + 2 H2O}$ → H: 4 = 4 ✓, O: 2 vs 4.

Step 3 — balance O: need 2 $\ce{O2}$:

\ce{CH4 + 2 O2 -> CO2 + 2 H2O} \tag{balanced}

Verification table:

Element	Reactants	Products	Balanced
C	1	1	✓
H	4 (CH₄)	4 (2×2 H)	✓
O	4 (2×2)	4 (2 + 2×1)	✓
Charge	0	0	✓

All counts match. The balancer in /reactions/combustion-methane shows the same verification: C 1=1, H 4=4, O 4=4.

A second form with ionic charge, e.g., $\ce{Ag+ + Cl- -> AgCl(s)}$, would add a charge row: +1 + (−1) = 0 on the left, 0 on the right.

Dimensional / unit note

Balancing is a count, not a unit conversion. Mass-to-amount conversion ($n=m/M$) may follow, but the chemical step is conserved entities. Keep temperature in K only when you move to gas laws.

Pause and predict

What would happen if you balanced by changing $\ce{H2O}$ to $\ce{H2O2}$ instead of adding a coefficient? Predict: you’d change the product identity (hydrogen peroxide, different properties and hazard) and fail conservation — the correct fix is a coefficient, not a subscript change.

Common mistakes and misconceptions

• Changing subscripts ($\ce{CH4}$ → $\ce{CH2}$) to fix counts — this invents a different substance.
• Forgetting diatomic elements ($\ce{O2}$, $\ce{N2}$, $\ce{H2}$, halogens) are molecular in the elemental form.
• Ignoring charge — for net ionic equations, sum of charges must balance (see /lessons/net-ionic-equations).
• Dropping phases when they affect the observation (gas evolution, precipitate).

Quick knowledge check

Why is CH₄ + O₂ → CO₂ + H₂O unbalanced?

Answer: Reactants have O₂ (2 O) while products need 4 O (2 in CO₂ + 1 in H₂O). The balanced form is $\ce{CH4 + 2 O2 -> CO2 + 2 H2O}$ — only coefficients changed, counts now match: C 1=1, H 4=4, O 4=4.

Summary

• Balanced equations conserve atoms and charge; change coefficients only.
• Verify with an inventory table (reactants vs products) and check phases/charges.
• Link to practice: /calculators/equation-balancer for verification and /reactions/combustion-methane for the same equation in the curated reaction library.

Sources

• IUPAC Compendium — Gold Book, entries for chemical reaction, stoichiometric coefficient, chemical equation — goldbook.iupac.org (iupac-gold). Accessed 2026-08-21, reviewed 2026-08-23.
• NIST Chemistry WebBook — reaction data conventions (balanced equations, phases) — webbook.nist.gov (nist-2022 / nist-webbook). Reviewed 2026-08-23.

Author and dates

• Author: Chemistry Fundamentals Editorial Team
• Reviewer: Editorial Review Board (chemistry reviewer; placeholder — no fabricated degrees; see Editorial Policy)
• Published: 2026-08-21 — Last reviewed: 2026-08-23 — Review due: 2027-08-23
• Correction? See Corrections Policy — Contact