Matter is classified by its composition and homogeneity. Elements are pure substances made of one atom type, compounds are pure substances made of two or more elements chemically bonded, and mixtures contain multiple substances that are not chemically bonded. Understanding this classification is essential for predicting how substances will behave chemically.
Start by observing everyday materials and categorizing them. Use phase diagrams to show how matter can exist in multiple forms. Distinguish between physical and chemical changes.
Everything you can touch, weigh, or measure in a chemistry lab is matter — anything that has mass and occupies space. The first step in chemistry is learning to classify matter by how it is composed, because composition determines behavior. The classification system has two main branches: pure substances and mixtures, with each branch subdividing further.
A pure substance has a fixed, definite composition. There are two types. An element is a pure substance that cannot be broken down into simpler substances by chemical means — it consists of only one type of atom. Gold (Au), oxygen (O₂), and diamond (pure carbon) are all elements. A compound is a pure substance made of two or more elements chemically bonded in a fixed ratio. Water (H₂O) is always two hydrogen atoms for every one oxygen atom. Table salt (NaCl) is always one sodium for every one chloride. You can break compounds into their elements by chemical reactions, but you cannot do it by physical means like filtering or boiling.
Mixtures contain two or more substances that are physically combined but not chemically bonded. The components retain their individual identities and can be separated by physical methods. Mixtures come in two varieties. A homogeneous mixture (also called a solution) has a uniform composition throughout — every sample you take looks the same. Saltwater, air, and brass (a copper-zinc alloy) are homogeneous mixtures. A heterogeneous mixture has visibly different regions or phases — a handful of trail mix, oil floating on water, or sand in a bucket. The distinction between homogeneous and heterogeneous often depends on the scale of observation: milk looks uniform to the naked eye but is heterogeneous under a microscope (fat droplets suspended in water).
The classification system also helps you distinguish between physical properties and chemical properties. Physical properties — density, boiling point, color, solubility — can be observed without changing the substance's identity. Chemical properties describe how a substance reacts or transforms into other substances — flammability, reactivity with acid, or tendency to oxidize. Similarly, physical changes (melting ice, dissolving sugar) alter form but not composition, while chemical changes (burning wood, rusting iron) produce new substances with different compositions. This framework — classifying matter and distinguishing physical from chemical behavior — is the foundation you will build on as you study atomic structure, bonding, and reactions throughout general chemistry.
This is a foundational topic with no prerequisites.
No prerequisites — this is a starting point.