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Measurement & matter · Topic 2 of 15

Matter, States & Phase Changes

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Chemistry · matter

Matter, States & Phase Changes

Matter is a pure substance or a mixture; changes are physical or chemical; phase changes toward gas absorb heat, toward solid release it.

01 · Classify matterPure substance = element or compound
02 · Physical or chemicalChemical change = a new substance
03 · Heat in or outToward gas absorbs heat; toward solid releases
01

What it is

Solid: fixed shape + volume · liquid: fixed volume only · gas: neither must know
Melt s→l · freeze l→s · vaporize l→g · condense g→l · sublime s→g · deposit g→s must know
Element: can't be broken down chemically · compound: elements joined in a fixed ratio
Particles: solid vibrate in place · liquid slide past · gas far apart, free
Evaporation absorbs heat, so evaporating sweat cools the skin
Sublimation: dry ice → gas · deposition: frost forming from vapor
02

The rules

Absorb heat: melt, vaporize, sublime · release heat: freeze, condense, deposit must know
Temperature holds steady during a phase change
03

Don't confuse

Pure substance (element or compound) vs mixture (physically combined, separable) must know
Physical change keeps the substance; chemical change makes a new one must know
Homogeneous mixture = uniform (a solution); heterogeneous = visibly uneven
Physical property: color, density, boiling point · chemical: flammability, reactivity
Evaporation: surface only, even below boiling · boiling: throughout, at the boiling point
04

Physical or chemical change?

Commonly confused
Physical changeChemical change
Identity afterwardSame substanceNew substance(s)
ExamplesMelting, dissolving, grindingBurning, rusting, cooking
Phase changesAll of themNone of them
Related propertyColor, density, boiling pointFlammability, reactivity
05

Common mistakes

Trap: dissolving and every phase change are physical, not chemical
06

Also know

Chemical changes: burning, rusting, cooking, digesting, rotting
07

More detail

Show 3 more facts
Plasma: a 4th state, a gas full of charged particles (stars)
Extensive: depends on amount (mass) · intensive: does not (temperature)
Lower outside pressure → lower boiling point (high altitude)
Pearl

Ask one question: is there a new substance afterward? Yes means a chemical change.

Sources · OpenStax Chemistry 2e §1.2 Phases and Classification of Matter · OpenStax Chemistry 2e §1.3 Physical and Chemical Properties · OpenStax Chemistry 2e §10.1 Intermolecular Forces (particles in solids, liquids, gases) · OpenStax Chemistry 2e §10.3 Phase Transitions · LibreTexts Introductory Chemistry §12.4 Evaporation and Condensation · NurseHub — HESI A2 Chemistry review course (unit: States of Matter) · TSTC Library — HESI A2 guide (Chemistry list: solids, liquids and gases; evaporation, vaporization, condensation) · Union Test Prep — HESI Chemistry study guide, page 2 (compounds, solutions and mixtures) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

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