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Chemical reactions · Topic 6 of 15

Chemical Equations & Balancing

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Chemistry · equations

Chemical Equations & Balancing

Reactants go left, products right; balance by changing coefficients only, so every atom and all the mass is conserved.

01 · Read the numbersCoefficient counts units; subscript counts atoms
02 · BalancedSame count of each atom on both sides
03 · The ruleChange coefficients, never subscripts
01

What it is

Reactants (start) left of the arrow → products (formed) right must know
Conservation of mass: total mass of products = mass of reactants must know
02

The rules

Coefficient = how many units; subscript = atoms in one unit must know
Balanced = the same number of each kind of atom on both sides must know
Balance with coefficients only — never change a subscript must know
Atoms = coefficient × subscript (2Al(OH)₃: 2 Al, 6 O, 6 H)
Coefficient 1 is not written; use the smallest whole numbers
03

Know by heart

States: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water
+ separates substances; ⇌ = reversible; Δ over the arrow = heat added
04

Balancing by inspection

Write the skeleton equation with correct formulas
Count every kind of atom on each side
Adjust one coefficient at a time, then recount all atoms
End with the smallest whole numbers (clear any fraction by multiplying)
05

Worked examples

Al + O₂ → Al₂O₃ balances as 4Al + 3O₂ → 2Al₂O₃
2Al + 3Cu(NO₃)₂ → 2Al(NO₃)₃ + 3Cu (2 Al, 3 Cu, 6 N, 18 O each side)
10.0 g CaCO₃ → 5.6 g CaO + 4.4 g CO₂ (mass in = mass out)
06

Coefficient or subscript?

Commonly confused
Coefficient (2H₂O)Subscript (H₂O)
Where it sitsIn front of the formulaSmall, lower right of a symbol
What it countsUnits (molecules or moles) of the substanceAtoms of that element in one unit
Change it to balance?Yes — that is how you balanceNever — it changes the substance
Example2H₂O = 4 H and 2 O in allH₂O = 2 H and 1 O per molecule
07

Common mistakes

Write diatomic elements in pairs: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂
08

More detail

Show 1 more fact
Net ionic equation: only the species that change (spectators dropped)
Pearl

Coefficients are the only numbers you may touch; subscripts are part of the substance's name.

Sources · OpenStax Chemistry 2e §4.1 Writing and Balancing Chemical Equations · OpenStax Chemistry 2e §1.2 Phases and Classification of Matter (law of conservation of matter) · OpenStax Chemistry 2e §2.1 Early Ideas in Atomic Theory · Union Test Prep — HESI Chemistry study guide, page 2 (chemical equations, balancing) · Union Test Prep — HESI Chemistry study guide, page 3 (conservation of mass in stoichiometry) · TSTC Library — HESI A2 guide (Chemistry: balance and identify chemical reactions) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

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