AP Chemistry 4.4: Physical and Chemical Changes

• A physical change changes the state, arrangement, or physical appearance of matter without changing its chemical identity. A chemical change rearranges atoms and produces substances with different chemical identities.

• The best way to tell the difference is to think about what is happening to the particles rather than relying only on what you can see.

Physical Changes

• Consider melting ice:

  • H₂O(s) → H₂O(l)
  • The particles before the change are H₂O molecules, and the particles after the change are still H₂O molecules.
  • The molecules become more mobile and their arrangement changes, but the O—H covalent bonds inside each water molecule remain intact.

• Common physical changes include melting, freezing, boiling, condensation, sublimation, deposition, cutting, crushing, and many dissolving processes.

• During a phase change, energy changes the way particles interact and move relative to one another. The substance itself does not become a different chemical substance.

Chemical Changes

• Now consider:

  • 2H₂(g) + O₂(g) → 2H₂O(g)

• Before the reaction, the substances are H₂ and O₂. After the reaction, the particles are H₂O molecules.

  • The original H—H and O=O bonding arrangements are replaced by O—H bonds.

• Because the particles have different chemical identities after the change, this is a chemical change.

  • Khan Academy’s current Unit 4 practice makes this distinction at the molecular level by focusing on whether chemical bonds are broken and new ones are formed.

A Better Rule Than “Bonds Break = Chemical”

• Be careful with oversimplified rules.

• A common shortcut says:

  • physical change = no bonds change
  • chemical change = bonds change
  • That can be useful at first, but it is not perfect because some physical processes involve major changes in particle interactions.

• The safer question is:

  • “Did the chemical identity of the particles change?”

• If the same particles remain but their spacing, phase, or arrangement changes, the process is usually physical.

• If atoms become connected differently and new substances appear, the process is chemical.

Dissolving Ionic Compounds

• Dissolving NaCl is a useful example:

  • NaCl(s) → Na⁺(aq) + Cl⁻(aq)

• In the solid, Na⁺ and Cl⁻ are part of an ionic crystal lattice. When the crystal dissolves in water, water molecules pull the ions apart and surround them.

  • The ion-ion attractions in the solid are replaced by ion-dipole attractions between the ions and water molecules.

• Even though the particle interactions change dramatically, the sodium ions are still Na⁺ and the chloride ions are still Cl⁻.

  • For introductory AP purposes, dissolving NaCl is commonly treated as a physical process because the chemical identities of the ions are retained and the salt can be recovered by removing the water.

Evidence of a Chemical Reaction

• Possible signs of a chemical reaction include a precipitate forming, gas production, an unexpected color change, a temperature change, production of light, or an odor change.

• However, these are evidence, not automatic proof.

  • For example, boiling water produces bubbles, but:
  • H₂O(l) → H₂O(g)
  • is a physical change.
  • Similarly, a color change might result from dilution or another physical effect rather than a reaction. Always connect the observation to what happened to the particles.

Common Mistakes

• Do not classify something as chemical only because it “looks different.” Do not assume bubbles always mean a chemical reaction. Do not assume dissolving is always a chemical change. Instead, focus on whether new chemical identities appeared.

Remember This

• Ask:

  • “Are the particles chemically different afterward?”
  • If yes, the process is chemical. If their identity stayed the same and only arrangement/state changed, it is generally physical.