Substances Versus Mixtures

Pure substances are materials that have a constant composition and distinct chemical properties. There are two types of pure substances: elements and compounds. An element is the simplest form of matter and cannot be broken down into simpler substances by chemical means. Examples include individual atoms on the periodic table like iron (Fe), oxygen (O), and carbon (C). A compound is formed when two or more different elements are chemically bonded together in a fixed ratio. These can be broken down into simpler substances. For instance, water (H₂O) is a compound made of hydrogen and oxygen, and table salt (NaCl) is a compound of sodium and chlorine.

A mixture, on the other hand, consists of two or more substances that are physically combined but not chemically bonded. Mixtures are classified as either homogeneous or heterogeneous. A homogeneous mixture has a uniform composition throughout, meaning you can't see the different components. Saltwater is a great example; once the salt is dissolved in water, you can't distinguish between the two. Other examples include air (a mixture of gases) and brass (a mixture of copper and zinc). In contrast, a heterogeneous mixture does not have a uniform composition, and its individual components are often visible. A classic example is sand and water; the sand settles at the bottom and is clearly separate from the water. Other examples include a salad, where you can see all the different vegetables, or a mixture of oil and water.