Chemistry 2e · Essential Ideas

Phases and Classification of Matter

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On this page 9 sections

In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

Matter can be described in two complementary ways: by its (solid, liquid, or gas) and by its classification (pure substance versus ). The phase tells you how particles are arranged and how much they move: in a solid they vibrate in fixed positions, in a liquid they slide past one another, and in a gas they fly freely through the available space. Classification tells you what kind of matter it is: a pure substance is made of only one kind of component (an such as iron or a such as water), while a mixture contains two or more components physically combined (salt water, air, granite). Together, these descriptions let chemists identify materials, predict behavior, and choose separation methods.

Why this matters

Every material you encounter — a medicine, a beverage, a metal implant, the air you breathe — is a phase and a classification. A pharmacist must know whether a drug is a solution () or a suspension (heterogeneous) to advise patients on shaking the bottle before use. Water treatment plants rely on and to purify mixtures. Cooking is largely phase change: melting butter, boiling water, condensing steam. Understanding phases and classification also builds the decision tree you will use all semester: pure substance or mixture? element or compound? homogeneous or heterogeneous? Getting these labels right is the foundation for stoichiometry, solutions, and states-of-matter topics later in the book.

The college version

Core Concepts

The three common phases

In a solid, particles are packed closely and vibrate about fixed positions, giving a definite shape and volume (an ice cube keeps its shape). In a liquid, particles are close but slide past one another, so the volume is definite but the shape follows the container. In a gas, particles are far apart and moving rapidly, so gases fill the entire container. A fourth state, plasma, consists of ionized (electrically charged) particles and makes up stars, lightning, and neon signs; it is uncommon on Earth's surface but most abundant in the universe.

Phase changes and energy

Phase changes are physical: composition does not change, only the arrangement and motion of particles. Melting (solid → liquid), vaporization (liquid → gas), and sublimation (solid → gas, as with dry ice) absorb energy to break particle attractions. Freezing (liquid → solid), condensation (gas → liquid), and deposition (gas → solid, as with frost) release energy. During a phase change of a pure substance at constant pressure, temperature stays constant — the added or removed energy breaks or forms attractions rather than warming the sample.

Elements and compounds

A pure substance has a fixed composition and cannot be separated into different substances by physical means. An element is a pure substance made of only one kind of atom (Fe, O₂, C); about 118 elements are known. A compound is a pure substance of two or more elements chemically combined in a fixed ratio (H₂O always has two H atoms per O atom). Compounds have properties very different from their constituent elements: sodium (a reactive metal) and chlorine (a toxic gas) combine to form table salt, NaCl.

Mixtures

A mixture is a physical combination of two or more substances in which each component keeps its own identity and properties. A homogeneous mixture (also called a solution) has a uniform composition throughout — salt water, air, and brass are homogeneous. A has visibly different regions — granite, oil and water, and a salad are heterogeneous. Mixtures are separated by physical methods because their components are only mixed, not bonded.

Separating mixtures

The best separation method depends on the physical property that differs between components. Filtration separates an insoluble solid from a liquid (sand from water) using a barrier that lets liquid pass. Distillation separates liquids by boiling point (water from salt water: water boils off, leaving salt). Evaporation or crystallization recovers a dissolved solid by removing the solvent. Chromatography separates components by how strongly they stick to a stationary phase versus a moving solvent, which is how ink pigments are resolved. All these methods work because mixtures are physical combinations — no chemical change is needed.

How It Works / Step-by-Step Process

Observe the sample: does it hold its shape (solid), take the container's shape but keep volume (liquid), or fill the container (gas)?
Ask: can it be separated into different substances by physical means? If no, it is a pure substance.
For a pure substance, decide element (one kind of atom) or compound (fixed ratio of elements).
For a mixture, decide homogeneous (uniform) or heterogeneous (visible regions).
Choose a separation method based on the differing physical property (particle size, boiling point, solubility).

Common Confusions

Do Not Confuse	With	Difference
Element	Compound	An element is one kind of atom (Fe); a compound is a fixed combination of elements (Fe₂O₃).
Compound	Mixture	A compound has a fixed ratio and new properties, and needs a chemical change to separate; a mixture keeps component properties and separates physically.
Homogeneous mixture	Heterogeneous mixture	Homogeneous is uniform throughout (air); heterogeneous has visible regions (granite).
Boiling	Evaporation	Boiling occurs throughout the liquid at its boiling point; evaporation occurs at the surface at any temperature below the boiling point.
Melting	Dissolving	Melting is a phase change of one substance (ice → water); dissolving mixes a solute into a solvent, making a mixture (sugar in water).
"Pure" in everyday language	Chemically pure	"Pure" juice is still a mixture; chemically pure means a single substance (element or compound).

Eli explains

The same idea, in plain words

Explain it like I’m 10

Matter is everything around you, and it comes in three main phases: solid (like an ice cube — it holds its shape), liquid (like water — it takes the shape of its cup), and gas (like steam — it spreads out to fill the room). Pure substances are like a box of only red LEGO bricks (elements) or bricks snapped together in one fixed design (compounds). Mixtures are like a bucket of mixed LEGO pieces — you can sort them out without breaking anything, and each piece stays itself.

Worked examples

A student finds a shiny, gray cube labeled "mystery metal." Its mass is 13.5 g and its volume, measured by water displacement, is 5.00 cm³. Compute the density:

$ρ = m V = 13.5 g 5.00 cm 3 = 2.70 g/cm 3$

This matches the known density of aluminum (about 2.70 g/cm³), far below iron (7.87 g/cm³) or copper (8.96 g/cm³), so the cube is most likely aluminum. Note the unit: density is mass per volume, so g/cm³ (equivalent to g/mL) is the standard lab unit.

Express aluminum's density in the SI unit kg/m³. Use the conversion factors 1 kg = 1000 g and 1 m = 100 cm (so 1 m³ = 10⁶ cm³):

$2.70 g cm 3 \times 1 kg 1000 g \times (100 cm 1 m) 3 = 2.70 \times 103 kg/m 3$

The g cancels and the cm³ cancels with (cm)³, leaving kg/m³. So 2.70 g/cm³ = 2700 kg/m³ — the same density in different units, useful for comparing with tables that use SI.

Key takeaways

Solid: fixed shape and volume; liquid: fixed volume, container's shape; gas: fills the container.
Phase changes are physical: melting, vaporization, and sublimation absorb energy; freezing, condensation, and deposition release energy.
An element is one kind of atom; a compound is two or more elements combined in a fixed ratio; both are pure substances.
Mixtures keep each component's identity and are separated by physical methods.
Homogeneous mixture = uniform (solution); heterogeneous = visibly different regions.
Density, ρ= m/V, is a powerful identifier of pure substances.
Classification checklist: pure substance? → element or compound? Mixture? → homogeneous or heterogeneous?

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

Arrange solids, liquids, and gases in order of increasing particle motion and increasing distance between particles.
Show answer
Solid (particles vibrate in place, closest together) → liquid (particles slide past each other) → gas (particles far apart, fastest motion).
Which phase changes absorb energy? Which release energy?
Show answer
Absorb energy: melting, vaporization, sublimation. Release energy: freezing, condensation, deposition.
Classify each as element, compound, homogeneous mixture, or heterogeneous mixture: distilled water, oxygen gas, milk, granite, salt water.
Show answer
Distilled water: compound. Oxygen gas: element (O₂). Milk: heterogeneous mixture (cream separates from the liquid; it is an emulsion). Granite: heterogeneous mixture. Salt water: homogeneous mixture (solution).
How would you separate (a) sand from water, (b) salt from salt water, (c) two liquids with different boiling points?
Show answer
(a) Filtration; (b) evaporation/crystallization or distillation; (c) distillation.
A 10.0 cm³ sample of a metal has a mass of 89.6 g. What is its , and is the metal likely copper (8.96 g/cm³), iron (7.87 g/cm³), or gold (19.3 g/cm³)?
Show answer
ρ= 89.6 g / 10.0 cm³ = 8.96 g/cm³, matching copper.
Is dissolving sugar in water a physical or chemical change? Justify your answer.
Show answer
Physical. The sugar molecules remain sugar molecules; the composition is unchanged, and evaporation of the water recovers the original sugar.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

phase: The physical state of matter: solid, liquid, gas, or plasma.
element: A pure substance made of only one kind of atom.
compound: A pure substance of two or more elements in a fixed ratio.
mixture: Two or more substances physically combined, each keeping its identity.
homogeneous mixture: A mixture with uniform composition throughout (a solution).
heterogeneous mixture: A mixture with visibly different regions.
filtration: Separating an insoluble solid from a liquid through a barrier.
distillation: Separating liquids (or dissolved solids) by boiling point.
density: Mass per unit volume, ρ= m/V.

Sources & references

openstax.org — Chemistry 2e

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