4.4 Physical and Chemical Changes

Physical vs. chemical change

A physical change alters a substance's form or state without changing its chemical identity — no new substance forms (e.g., melting ice, dissolving salt). A chemical change produces new substances with different properties because bonds break and form (e.g., rusting iron, burning methane). Chemical changes involve rearrangement of atoms, not creation or destruction of them.

Evidence of chemical change

Clues that a chemical reaction occurred include color change, gas bubble formation, precipitate formation, temperature change, or light emission. However, some physical changes (like dissolving) can also show these signs, so the real test is whether new substances with different chemical composition are formed.

Conservation during change

In both physical and chemical changes, mass is conserved and the number/type of atoms stays the same — only how they're arranged or their state changes. In physical changes, intermolecular forces are affected but covalent/ionic bonds within molecules stay intact; in chemical changes, intramolecular bonds break and reform.

Key terms

Physical change

A change in form or state of a substance where no new substance is formed (e.g., melting, dissolving).

Chemical change

A change that produces new substances with different properties due to bond breaking and forming.

Evidence of chemical change

Color change, gas formation, precipitate formation, temperature change, or light emission — though not always conclusive alone.

Conservation of mass

Mass and the number/type of atoms remain constant during both physical and chemical changes.

Intermolecular vs. intramolecular

Physical changes affect forces between molecules (intermolecular); chemical changes break/form bonds within molecules (intramolecular).

Try an AP-style question

Which process breaks covalent bonds within molecules?

1. Dissolving sugar
2. Electrolysis of water into H₂ and O₂
3. Melting ice
4. Boiling water into steam