Summary
Highlights
Matter is defined as anything that has mass and occupies space. It exists in three primary phases: solid, liquid, and gas. A solid has fixed shape and volume with touching, immobile particles. A liquid has a fixed volume but no fixed shape, molding to its container with touching, fluid particles. A gas has no fixed volume or shape, completely filling its container with far apart, freely moving particles.
Chemical changes are central to chemistry. A physical change alters the form or appearance of a substance but not its chemical composition (e.g., ice melting into water). A chemical change involves breaking and forming chemical bonds to create entirely new substances with different chemical compositions (e.g., hydrogen and oxygen combining to form water).
A pure substance cannot be separated into other materials by physical processes. Elements are pure substances that cannot be broken down further by chemical or physical means (e.g., oxygen, hydrogen). Compounds are pure substances made of two or more elements chemically combined in a fixed ratio (e.g., water, H2O).
A mixture is composed of two or more pure substances that are physically combined and can be separated by physical processes (e.g., boiling salt water). Homogeneous mixtures have evenly distributed substances, appearing uniform throughout (e.g., sugar dissolved in water). Heterogeneous mixtures have unevenly distributed substances, with different sections appearing distinct (e.g., oil and water).
Matter can be broadly classified into pure substances (elements, which are one type of atom, and compounds, which are one type of molecule made of different atoms) and mixtures (multiple types of molecules, arranged either homogeneously or heterogeneously).