Matter can undergo many observable changes in the lab. In AP Chemistry, the key distinction is whether a process changes chemical composition (identity) or only changes physical properties like phase, particle arrangement, or mixing.

Core idea: properties vs composition

Chemical composition (what a substance is)

A sample’s composition stays the same if the same particles are present in the same chemical ratios, even if the sample’s appearance changes. Many measured properties (color intensity, density, conductivity, temperature, volume) can change without any change in composition.

Physical properties (what a substance looks like or how it behaves)

Physical properties describe a substance without changing what it is. Common property changes include:

• Phase (solid, liquid, gas)

• Temperature (warming/cooling)

• Shape and size (cutting, grinding)

• Distribution in space (mixing, separating mixtures physically)

Physical change: composition unchanged

A physical change rearranges particles or changes their spacing/energy, but does not create new chemical species.

Heating curve for water illustrating how temperature changes as heat is added, with flat plateaus at phase transitions. During these plateaus, added energy goes into overcoming intermolecular forces (latent heat) rather than increasing temperature, so the substance’s chemical identity remains unchanged. Source

At the particle level:

• In a phase change, the particles are the same before and after; only intermolecular attractions and particle spacing/ordering change.

Phase diagram of water showing which phase (solid, liquid, or gas) is thermodynamically stable at different temperatures and pressures. The boundary curves represent equilibrium between phases, and crossing a boundary corresponds to a phase change without changing chemical composition (still H2O). Source

• In mixing, different substances become interspersed, but each substance retains its own identity.

Common physical changes you should recognise in AP Chemistry include:

• Melting/freezing (e.g., ice to liquid water)

• Boiling/condensing

• Sublimation/deposition

• Mixing two substances without reaction (e.g., combining sand and iron filings)

• Grinding/crushing a solid (smaller pieces, same composition)

Important nuance: a dramatic appearance change can still be physical. For example, crushing a crystal changes surface area and light scattering, but not composition.

Chemical change (contrast): composition changed

A chemical change occurs when composition changes—meaning at least some particles are transformed into different particles with different bonding and/or different ratios of atoms. In AP Chemistry terms, chemical change implies:

• New substances are formed (new identities)

• Chemical bonds are broken and/or formed

• Reactant formulas differ from product formulas

Do not rely on “easy” rules like “irreversible = chemical”: some physical changes are hard to reverse under typical conditions, and some chemical changes can be driven backward.

How to decide quickly (AP-style reasoning)

Use identity-first thinking

Ask: after the process, do you still have the same substances you started with?

• If yes, it’s a physical change (composition unchanged).

• If no, it’s a chemical change (composition changed).

Look at what would be written in formulas

Physical changes can often be represented by changing only the state label while keeping the formula the same (for example, water solid to water liquid). Chemical changes require different formulas for products than for reactants.

Mixing vs reacting

“Mixing” is explicitly part of the physical-change idea in the syllabus statement:

• Physical mixing: substances coexist; composition of each substance remains unchanged.

• Chemical reaction during mixing: composition changes (new species appear), so it is not merely a physical change.

When evaluating a description, separate:

• What is being done (heating, cooling, stirring, grinding) from

• What is being produced (same substances vs new substances)