Unit 4 Chemical Reactions

AP Chemistry Physical vs Chemical Changes

The bond-based test the AP course uses, examples sorted with reasons, particle diagrams, and the dissolving-salt case that can go either way, in four worked examples.

Earlier science classes sort changes by what you can see: bubbles, color, heat. AP Chemistry asks a sharper question about the particles: are chemical bonds breaking or forming? That question settles almost every case, and it also explains the one famous case it does not settle. This page uses it on everyday examples and on the particulate diagrams the exam loves.

The AP Test: Are Chemical Bonds Breaking or Forming?

Chemical change: chemical bonds break and/or form, so the particles at the end are different substances from the ones at the start. A balanced equation rearranges atoms into new combinations.
Physical change: only the interactions between particles change. Phase changes and dissolving a molecular solid such as sugar leave every molecule intact.
Borderline: dissolving an ionic solid breaks ionic bonds but makes no new substance, so it can be argued either way (Example 3).

Gas bubbles, light, a temperature change, a precipitate, or a color change are useful clues that a chemical change may have happened, but none of them is proof on its own. Boiling water bubbles, and dissolving some salts makes the water noticeably colder, yet both are physical. Go back to the bonds when the clues disagree.

What Physical and Chemical Changes Look Like at the Particle Level

Fig. 1 · Physical, chemical, and borderline changes as particle diagrams

In the physical change, the same six H₂O molecules simply move apart. In the chemical change, the atoms regroup: two H₂ and one O₂ become two H₂O, with four hydrogen and two oxygen atoms on both sides, as any balanced equation requires. In the borderline case, the ionic lattice comes apart and water molecules turn their oxygen ends toward Na⁺ and their hydrogen ends toward Cl⁻.

Physical vs Chemical Change Examples With Reasons

Change	Type	Reason
Ice melting	Physical	H₂O molecules stay intact; hydrogen bonds between them loosen
Water boiling	Physical	Molecules separate into a gas; no bonds inside them break
Sugar dissolving in water	Physical	Sucrose molecules stay whole and mix with water
Iodine subliming	Physical	I₂ molecules go straight from solid to gas
Iron rusting	Chemical	Iron and oxygen form a new compound, an iron oxide
Methane burning	Chemical	CH₄ and O₂ become CO₂ and H₂O
Baking soda and vinegar fizzing	Chemical	An acid-base reaction makes new species, including CO₂ gas
Silver tarnishing	Chemical	Silver reacts with sulfur compounds to form black Ag₂S
Cooking an egg	Chemical	Bonds within the proteins break and new ones form
Electrolysis of water	Chemical	H₂O breaks down into H₂ and O₂
Table salt dissolving in water	Borderline	Ionic bonds break, but no new substance forms

Worked Example 1: Melting Ice Versus Electrolysis of Water

Both processes start with water. Classify each and explain at the particle level.

Melting ice: physical. The O–H covalent bonds inside each H₂O molecule are untouched; only the hydrogen bonds between molecules loosen enough for the molecules to slide past one another.
Electrolysis, 2H₂O → 2H₂ + O₂: chemical. The O–H bonds break and new H–H and O=O bonds form, producing two new substances.

The starting material is the same, so the classification depends entirely on which kind of bond changes: bonds between molecules for a physical change, bonds within them for a chemical one. Those between-molecule attractions are the intermolecular forces, and the energy that goes into weakening them shows up on a heating curve.

Worked Example 2: Why Bubbles Do Not Prove a Chemical Change

Water boiling and magnesium dropped into hydrochloric acid both produce bubbles. Classify each.

Boiling water: physical. The bubbles are H₂O vapor, the same substance in a different phase.
Mg in HCl, Mg + 2HCl → MgCl₂ + H₂: chemical. The bubbles are hydrogen gas, a new substance, and magnesium atoms become Mg²⁺ ions.

Identify what the bubbles are made of before deciding. The second reaction is also a redox reaction, the kind sorted in types of chemical reactions.

Worked Example 3: Arguing Both Sides of Dissolving Salt

A free-response question asks: “Is dissolving NaCl in water a physical or a chemical change? Justify your answer.” Write a justification for each side.

Argument for chemical: the ionic bonds holding Na⁺ and Cl⁻ together in the crystal lattice break, and new ion-dipole interactions form between the ions and water molecules.
Argument for physical: no new substance is produced. The ions are the same before and after, and evaporating the water recovers the original NaCl.

The AP course accepts either answer, so the points come from the justification, not the label. Name what breaks and what forms, and tie the conclusion to it.

Worked Example 4: Classifying a Change From a Particle Diagram

Diagram A shows four N₂ molecules packed together in one box and the same four N₂ molecules spread out in the next. Diagram B shows two N₂ and six H₂ molecules before and four NH₃ molecules after. Classify each.

Diagram A: physical, liquid nitrogen boiling. The molecules are unchanged; only their spacing changes.
Diagram B: chemical, N₂ + 3H₂ → 2NH₃. New N–H bonds form. Count to check: 4 N and 12 H atoms before, 4 N and 12 H after.

Counting atoms is the fastest check. If the same molecules appear after the change, it is physical; if the atoms have regrouped into new molecules, with the totals unchanged, it is chemical.

Five Physical and Chemical Change Practice Questions With Answers

Is dry ice (solid CO₂) turning into gas a physical or chemical change?
Show answer
Physical. It is sublimation; the CO₂ molecules stay intact.
A copper wire is heated in air and turns black as copper(II) oxide forms. Classify the change.
Show answer
Chemical. Copper and oxygen form a new compound, CuO, so new bonds form.
Mixing two clear solutions produces a yellow solid, PbI₂. Classify the change.
Show answer
Chemical. Pb²⁺ and I⁻ ions bond into a new ionic solid, a precipitation reaction.
Dissolving ammonium nitrate in water makes the water colder. Does the temperature drop prove a chemical change?
Show answer
No. A temperature change is only a clue. Like NaCl, this is a borderline case of an ionic solid dissolving; no new substance forms.
A particle diagram shows 2 H₂O₂ molecules before and 2 H₂O and 1 O₂ after. Classify the change and check the atoms.
Show answer
Chemical (2H₂O₂ → 2H₂O + O₂). Before: 4 H and 4 O. After: 4 H and 2 + 2 = 4 O.

Common Physical and Chemical Change Mistakes

Treating clues as proof. Bubbles, heat, and color changes can come from physical changes too.
Calling boiling a chemical change because bonds break. Boiling overcomes intermolecular attractions, not the covalent bonds inside molecules.
Saying a state change makes a new substance. Ice, liquid water, and steam are all H₂O.
Giving only a label for dissolving salt. Either answer works only with a justification about what breaks and what forms.
Losing atoms in a particle diagram. Every atom before the change must appear after it.
Calling all dissolving physical. Sugar dissolving is physical, but dissolving an ionic solid is borderline, and some gases react with water.

Classifying changes and reading particle diagrams are quick multiple-choice points in Unit 4. To see how stronger Unit 4 answers would move your overall estimate, try the AP Chem Score Calculator.

Frequently Asked Questions

What is the difference between a physical and a chemical change?

A chemical change breaks and/or forms chemical bonds, producing a new substance. A physical change only changes the interactions between particles, such as during melting, boiling, or dissolving sugar, so the substance stays the same.

Is boiling water a chemical change?

No. Boiling is a physical change. The H₂O molecules stay intact; only the intermolecular attractions between them are overcome as the liquid becomes a gas. The bubbles are water vapor, not a new substance.

Is dissolving salt a physical or chemical change?

It can be argued either way. Ionic bonds between Na⁺ and Cl⁻ break and ion-dipole attractions to water form, which supports calling it chemical, but no new substance is made and evaporating the water recovers the salt, which supports calling it physical. The AP course accepts either classification if the reasoning is explicit.

What are examples of chemical changes?

Iron rusting, methane burning, baking soda reacting with vinegar, silver tarnishing, cooking an egg, and electrolysis of water are all chemical changes, because bonds break and new substances form.

Are signs like bubbles or a color change proof of a chemical change?

No. They are clues, not proof. Boiling water makes bubbles and dissolving some salts changes the temperature, yet both are physical. The test is whether chemical bonds break and new substances form.

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