Types of Reactions & Net Ionic Equations

You’ll be able to

Distinguish physical changes from chemical changes
Classify a reaction as synthesis, decomposition, combustion, acid–base, precipitation, or redox
Translate a molecular equation into complete-ionic and net ionic form using solubility rules

Physical vs chemical change

A physical change alters form but not identity — melting ice, dissolving salt, or crushing chalk still leave you with the same substance. A chemical change rearranges atoms into new substances, and you can spot it by its telltale signs: a color change, a gas released, a precipitate forming, or heat/light given off. Boiling water is physical (still H₂O); electrolyzing it into H₂ and O₂ is chemical.

Six reaction types to recognize on sight

Synthesis: A + B → AB (e.g. 2H₂ + O₂ → 2H₂O). Decomposition: AB → A + B (CaCO₃ → CaO + CO₂). Combustion: a fuel + O₂ → CO₂ + H₂O. Acid–base neutralization: acid + base → salt + water (HCl + NaOH → NaCl + H₂O). Precipitation: two solutions swap partners and an insoluble solid drops out. Oxidation–reduction (redox): electrons transfer between species. A single reaction can wear two labels at once — combustion is also a redox.

From molecular to net ionic

A molecular equation shows compounds intact. A complete-ionic equation splits every strong electrolyte (soluble ionic compounds, strong acids/bases) into its free aqueous ions, while solids, gases, and pure liquids stay whole. Ions that appear unchanged on both sides are spectator ions — cancel them. What survives is the net ionic equation, the real chemistry. Use solubility rules to decide what is (aq) and what is a solid: nitrates and group-1 salts are always soluble; most chlorides are soluble except Ag⁺, Pb²⁺, Hg₂²⁺; most sulfates are soluble except Ba²⁺, Ca²⁺, Pb²⁺.

Building a net ionic equation

molecular → split strong electrolytes into ions → cancel spectators → net ionic

Only strong electrolytes split. Precipitates (s), gases (g), water, and weak acids stay written as whole formulas.

Worked example

Write the net ionic equation for mixing aqueous silver nitrate and aqueous sodium chloride.

1.Molecular equation: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq).
2.Check solubility: AgCl is one of the chloride exceptions, so it precipitates as a solid; NaNO₃ (group-1 and nitrate) stays dissolved.
3.Complete-ionic: Ag⁺(aq) + NO₃⁻(aq) + Na⁺(aq) + Cl⁻(aq) → AgCl(s) + Na⁺(aq) + NO₃⁻(aq).
4.Cancel the spectators that appear unchanged on both sides: Na⁺ and NO₃⁻.

Answer: Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

Tip

A net ionic equation must balance both atoms and charge. In Ag⁺ + Cl⁻ → AgCl(s) the charges are (+1) + (−1) = 0 on the left and 0 on the right. If your charges do not match, you dropped or mis-split an ion.

Checkpoint

Aqueous BaCl₂ is mixed with aqueous Na₂SO₄, forming a precipitate. What is the correct net ionic equation?

Checkpoint

Classify the reaction 2 KClO₃ → 2 KCl + 3 O₂.

On the exam

AP free-response almost always asks for the net ionic equation, and it must be balanced for mass and charge — spectators earn no credit. Memorize the core solubility rules so you can decide instantly which species split and which stay solid.

Answer the 2 checkpoints as you read.