This document defines key chemistry concepts including matter, elements, compounds, mixtures, physical and chemical changes. It explains that matter is anything that has mass and volume, while elements are pure substances made of only one type of particle. Compounds are pure substances made of two or more elements. Mixtures contain two or more pure substances that are either blended together (homogeneous) or remain separate (heterogeneous). Physical changes alter the form or state of a substance without changing its chemical makeup, while chemical changes create a new substance.