Substance refers to a material or matter that has a definite composition and properties. It can exist in various states, such as solid, liquid, or gas. Examples of substances include water, gold, and oxygen. Each substance has unique characteristics that distinguish it from others, such as density, boiling point, and reactivity. In chemistry, substances can be classified as pure or impure. A pure substance consists of only one type of particle, like sodium chloride (table salt), while an impure substance contains a mixture of different particles, such as air or soil. Understanding substances is essential for studying their behavior and interactions in various scientific fields.