Classification and Properties of Matter

What Counts as Matter

Matter is anything that has mass (a measure of the amount of material) and volume (the space it occupies). Almost every Regents chemistry question is built on a sample of matter, so you must classify it correctly before you can reason about it.

Matter exists in three common phases: solid (definite shape and volume), liquid (definite volume, takes the shape of its container), and gas (no definite shape or volume). On the exam, phase symbols appear in formulas as (s), (l), (g), and (aq), where (aq) means aqueous — dissolved in water.

Why classification matters for the exam

Structures and Properties of Matter is the single largest blueprint band (30–40% of the test). Misclassifying a mixture as a compound, or a chemical change as a physical change, is one of the most common ways students lose easy 1-credit multiple-choice points.

The Classification Tree

Every sample of matter is either a pure substance or a mixture.

• Pure substance: a sample with a single, fixed composition. It is either an element or a compound.
• Mixture: two or more substances physically combined; the components keep their own properties and can be separated by physical means.

Pure substances split further:

• Element: the simplest form of matter; cannot be broken down by chemical means (for example, oxygen, gold, sodium). Elements are listed on the Periodic Table.
• Compound: two or more elements chemically bonded in a fixed whole-number ratio (for example, H₂O, NaCl, CO₂). A compound can be separated into its elements only by a chemical change.

Mixtures split by uniformity:

• Homogeneous mixture (solution): uniform throughout; the same in every sample (saltwater, air, brass).
• Heterogeneous mixture: non-uniform; you can see or sample distinct regions (sand in water, oil and vinegar, granite).

Laws That Govern Composition

Two conservation ideas appear constantly:

• Law of Definite Proportions: a given compound always contains the same elements in the same mass ratio. Water is always about 11% hydrogen and 89% oxygen by mass, whether it comes from a lake or a lab.
• Law of Conservation of Mass: in any chemical or physical change, mass is neither created nor destroyed. The total mass of reactants equals the total mass of products. This underlies every balanced equation you will see later.

Physical vs. Chemical

Properties and changes come in two flavors, and the exam loves to test the difference.

• A physical property can be observed without changing the substance's identity: color, density, hardness, melting point, boiling point, solubility, conductivity.
• A chemical property describes how a substance reacts to form new substances: flammability, reactivity with acid, ability to rust.
• A physical change alters form but not identity: melting ice, dissolving sugar, cutting metal, boiling water. The particles are still the same substance.
• A chemical change produces one or more new substances with new properties: burning, rusting, digesting, electrolysis of water.

Evidence of a chemical change

Look for these clues in a cluster stimulus: a color change, gas bubbles forming (not from boiling), a precipitate (solid forming in solution), light or heat released, or a new odor. None of these alone is proof, but together they signal a chemical reaction.

Using the 2025 Reference Tables

Do not memorize property values — the test supplies them. The 2025 Reference Tables for Physical Science: Chemistry give:

• The Properties of Selected Elements table (the section the legacy edition labeled Table S): atomic number, mass, melting/boiling points, density, and electronegativity.
• The Mathematical Relationships and Mathematical and Computational Models sections (the section the legacy edition labeled Table T): useful formulas, including density = mass ÷ volume.

Worked example: A 20.0 g metal sample has a volume of 2.5 cm³. Density = 20.0 g ÷ 2.5 cm³ = 8.0 g/cm³. Compared against the Properties of Selected Elements table (legacy Table S), that value matches copper, identifying the metal.

Separating mixtures

Because mixtures are held together only physically, they are separated by exploiting differences in physical properties. Know these techniques and the property each one uses:

• Filtration — separates an insoluble solid from a liquid by particle size (sand from water).
• Evaporation — drives off a liquid to leave a dissolved solid behind (salt from saltwater).
• Distillation — separates liquids by differences in boiling point (alcohol from water).
• Chromatography — separates dissolved substances by how strongly they cling to a surface (ink dyes).

None of these is a chemical change. Any separation that uses only a physical property confirms you started with a mixture, not a compound.

Extensive vs. Intensive Properties

Properties also split by whether they depend on how much matter is present.

• Extensive properties depend on amount: mass, volume, and total energy. Cut a sample in half and these halve.
• Intensive properties do not depend on amount: density, temperature, melting point, boiling point, and color. They are the same for a drop or a bucket of the same substance, which is why they are used to identify a substance.

This is why density is so useful on the exam: it is intensive, so a measured density can be matched against the Properties of Selected Elements table (legacy Table S) to name an unknown sample regardless of how big the sample is.

Common Regents traps

• Calling dissolving a chemical change — it is physical; the solute can be recovered by evaporation.
• Thinking density changes when you cut a sample in half — density is intensive (independent of amount); mass and volume are extensive.
• Forgetting units: density must carry g/cm³ or g/mL, never just a bare number.
• Assuming a separation technique like filtration or distillation proves a chemical change — it never does; it confirms a mixture.