This document discusses the differences between physical and chemical changes. It defines physical changes as changes that affect a substance's physical properties, like state, shape or size, without changing its chemical identity. Chemical changes form new substances with different properties than the reactants. The document also outlines signs that indicate a chemical change occurred, like gas production or color change. It explains that the law of conservation of mass states that mass is never created or destroyed in chemical or physical changes, only rearranged into different substances.