Matter, defined by mass and space, occurs as solids with fixed shapes, liquids that conform to their containers, and gases that fill any available volume. Physical changes, like ice melting to water, adjust the arrangement of molecules without altering their identity, while chemical changes break and form bonds to generate entirely new substances. Pure substances include elements that resist further breakdown and compounds made of different atoms, whereas mixtures consist of multiple pure substances that separate through physical processes. This exploration underlines how differences in structure and transformation reveal the complexity and versatility of matter.