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Physical and chemical changes

Snapping a stick, boiling water, and burning wood look like three kinds of change, but only burning makes new substances. The one rule that cuts through every trick question: did the composition change?

§1

The one rule.

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The core rule: a chemical change alters composition — new substances with new properties form. A physical change alters form, size, or state, but the substance stays the same.

Melting, boiling, dissolving, and grinding are physical: the molecules survive. Burning, rusting, and reacting are chemical: atoms recombine into different substances.

The catch is that dramatic effects — bubbling, color, heat — accompany both kinds of change, so they cannot be the test. Only a change in the identity of the substance decides it.

UNIT 4 TOPIC 4.4 • PHYSICAL AND CHEMICAL CHANGES CHANGE CLASSIFIER CORE RULE Chemical change: bonds break or form (new substances). Physical change: only spacing / arrangement / attractions. MELTING ICE PHYSICAL H₂O molecules stay H₂O. IMF pattern changes. ordered (ice) liquid PRECIPITATION CHEMICAL Dissolved ions join into a new white solid compound. AgCl(s) forms AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq) SALT DISSOLVING CONTEXT DEPENDENT Ionic bonds break; ion–dipole forms. lattice dissolved Na⁺ Cl⁻ Cl⁻ Na⁺ Na⁺ Cl⁻ Cl⁻ Na⁺ Same ions before & after. COMBUSTION CHEMICAL Fuel + O₂ become CO₂ and H₂O; bonds rearrange. CH₄ + 2O₂ ↓ CO₂ + 2H₂O CLAIM + EVIDENCE (CED ANCHOR) Macroscopic evidence supports a claim; the particle model explains it. Ask: Are chemical bonds changing, or only spacing and attractions? Note: salt dissolving is defensible either way with clear reasoning. AP Chemistry · Unit 4 · Chemical Reactions
Fig. 4.4.1 The core rule: a chemical change alters composition (new substances form); a physical change does not. Melting ice keeps H₂O molecules as H₂O — physical. Bubbles, color, or drama alone do not decide it; the change in substance identity does.
§2

Classifying a change.

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Ignore the drama; ask about composition.

  1. Ask: did the substance's identity change? New substance with new properties → chemical. Same substance in a new form → physical.
  2. Discount surface signs. Bubbles, color changes, and heat happen in both kinds of change; they are not the test.
  3. Check reversibility as a clue (not proof). Many physical changes reverse easily (freezing/melting); chemical changes usually need another reaction to reverse.
  4. Name the new substances if any. If you can identify a genuinely new substance formed, it is chemical.
§3

The pieces you'll meet.

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Composition is the deciding factor.

chemical
Chemical change
Composition changes; new substances form.
physical
Physical change
Form or state changes; substance identity unchanged.
composition
Composition
What the substance is made of — the real test.
state change
State change
Melting, boiling, freezing — physical.
dissolving
Dissolving
Physical: particles disperse, substance unchanged.
signs
Surface signs
Bubbles, color, heat — occur in both, so not decisive.
§4

Worked example: three changes to classify.

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Boiling water. Liquid water becomes steam, still H₂O. Same substance, new state → physical.

Dissolving salt. Salt disperses into ions in water but remains salt (recoverable by evaporation). Same substance → physical.

Burning wood. Wood combines with oxygen to form ash, carbon dioxide, and water vapor — new substances with new properties. Composition changed → chemical.

Lesson. All three can look dramatic, but only burning changes what the substances are.

§5

Mistakes that cost real points.

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Pitfall · 01

"If it bubbles or fizzes, it's a chemical change."

Bubbling is not the test. Boiling water bubbles, and a dissolving carbonated drink fizzes, with no new substance formed. Only a change in composition makes a change chemical.

Fix. Ignore bubbles as a verdict; ask whether the substance's identity changed.

Pitfall · 02

"Boiling is a chemical change because the water 'disappears.'"

Boiling is a physical change: liquid water becomes water vapor, still H₂O. It does not disappear or become a new substance; it changes state. Condense the vapor and you get the same water back.

Fix. Classify boiling (and all state changes) as physical: the substance is unchanged, only its state differs.

Pitfall · 03

"A dramatic, energetic change must be chemical."

Drama — vigorous bubbling, heat, color — accompanies some physical changes too (like a violent dissolving). Energy or spectacle is not the criterion; a change in composition is.

Fix. Judge by composition, not intensity. Even a calm change can be chemical, and a dramatic one can be physical.

§6

Skill Check.

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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.

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