Chemical substances are forms of matter that have a specific chemical composition and distinct properties. They can be elements, like oxygen or gold, which consist of only one type of atom, or compounds, such as water or sodium chloride, which are made up of two or more different elements chemically bonded together. These substances can exist in various states, including solids, liquids, and gases, and they undergo chemical reactions to form new substances. Understanding chemical substances is essential in fields like chemistry, pharmacy, and materials science, as they play a crucial role in everything from medicine to manufacturing.