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Introduction to Reactions

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A chemical reaction rearranges atoms into new substances, while a physical change (melting, dissolving) leaves the substance's identity intact. Because atoms are conserved, a reaction is written as a balanced equation: the same number of each atom on both sides.

UNIT 4 TOPIC 4.1 • INTRODUCTION TO REACTIONS REACTION SPOTTER PHYSICAL CHANGE Composition stays the same. EXAMPLE H₂O(s) → H₂O(l) solid H₂O(s) liquid H₂O(l) Same particles before and after; only arrangement or spacing changes — no new substance. PARTICLE TEST Count the particle types — they are identical on both sides. CHEMICAL CHANGE New substances form. EXAMPLE 2H₂ + O₂ → 2H₂O reactants products Balanced: H 4 = 4 · O 2 = 2 EVIDENCE OF CHANGE heat or light released gas forms precipitate forms color change Particle model: atoms rearrange into brand-new combinations. DECISION RULE Classify by whether the composition changes — not by appearance alone. PHYSICAL Substance identity stays the same. Same particles. CHEMICAL Atoms rearrange into different substances. EVIDENCE TO WATCH Heat / light, gas, a precipitate, or a color change signal a reaction. CED ANCHOR A visible change is not enough. Classify by whether composition changes: physical = same particles, chemical = new substances. AP Chemistry · Unit 4 · Chemical Reactions
A reaction makes new substances; a physical change does not. Melting ice keeps H₂O as H₂O (physical), while a chemical reaction rearranges atoms into new substances — and the balanced equation must conserve every atom.
Reaction Spotter · Open the sandbox →

The traps here are about what counts as a reaction and what a balanced equation must satisfy: bubbles or dissolving do not by themselves prove a chemical change, and an equation is not valid until every atom is balanced. Conservation of atoms is the anchor.

The work

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Lesson
Introduction to Reactions ›

A reaction forms new substances and its equation must conserve atoms. The lesson separates chemical change from physical change and balances equations, then closes with a ten-scenario check.

Skill check · 10 scenarios
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Diagnostic
10-item topic check ›

Ten items spanning the Topic 4.1 misconceptions: bubbles taken as proof of a chemical change, dissolving misread, unbalanced equations accepted, and dissolving imagined to destroy molecules.

Not started · 10 items · ~15 min
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Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.

Take the diagnostic to identify your misconceptions
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