Substance (chemistry)

A chemical substance is a unique form of matter. It possesses a constant chemical composition and specific characteristic properties. These properties allow scientists to identify and distinguish one substance from another.

Substances can exist in several different physical states, also known as phases. These phases include solids, liquids, gases, and plasma. A substance transitions between these phases when temperature or pressure changes. Even when the phase changes, the chemical composition remains exactly the same. For example, liquid water and steam are both the same substance.

There are two primary categories of substances: elements and chemical compounds. An element is a substance made of only one kind of atom. It cannot be broken down into something else by a chemical reaction.

Elements can be classified into different groups based on their traits. Metals, such as gold or iron, often have a characteristic lustre. They also conduct heat and electricity well and are malleable. Non-metals, like oxygen or nitrogen, lack these metallic properties. Some elements, called metalloids, show characteristics of both groups.

A chemical compound is a substance formed when two or more elements combine through a chemical reaction. All compounds are substances, but not all substances are compounds. Compounds can consist of atoms bonded in molecules or in a crystalline lattice.

Chemical compounds are further organized by their internal structures and compositions. Organic compounds are those based primarily on carbon and hydrogen atoms. Inorganic compounds include all other types of chemical compounds. Some specialized substances, called organometallic compounds, contain bonds between carbon and a metal. Compounds can also be classified by how their atoms are held together. Covalent compounds involve atoms that share electrons. Ionic compounds, often called salts, consist of oppositely charged ions. Some complex substances, known as coordination complexes, use a dative bond to hold a metal center and a ligand together.

Understanding the history of these definitions is essential to modern chemistry. In the late eighteenth century, Joseph Proust conducted important research on pure compounds. He studied substances like basic copper carbonate to understand their makeup. Proust deduced that all samples of a compound have the same proportions of elements by mass. This discovery is known as the law of constant composition.

Later, researchers encountered the complex problem of isomerism. Isomers are substances that have the same chemical composition but different atomic arrangements. Friedrich August Kekulé provided clarity by describing the structure of benzene. The discovery of stereoisomerism further explained how three-dimensional arrangements create distinct substances.

Chemistry uses many specific numbers to describe the world of substances. As of 2025, there are 118 known elements in the periodic table. Approximately 80 of these elements are considered stable. Stable elements do not change through radioactive decay. Pure water is a perfect example of a substance with a strict ratio. Every molecule of water contains exactly two hydrogen atoms and one oxygen atom. This 2:1 atomic ratio is a characteristic property of H2O. Other substances like refined sugar have much larger and more complex molecular formulas.

It is important to distinguish a pure substance from a chemical mixture. A mixture contains two or more substances that are combined without reacting. Mixtures do not have a fixed composition like pure substances do.

You can often separate mixtures using mechanical processes like distillation or evaporation. For example, you could use a magnet to separate iron from sulfur. In a mixture, the components retain their own identities. In contrast, a chemical reaction creates a new substance with entirely new properties. Even things that look uniform can be mixtures. Cranberry glass appears to be one color, but it is actually a mixture containing gold particles.

Different scientific fields apply the term "substance" in unique ways. In geology, inorganic solid substances with uniform composition are called minerals. When minerals combine into aggregates, they are defined as rocks. In the field of law, the term can include mixtures with a defined manufacturing process. For example, charcoal is a complex mixture, but it can be identified by how it is made. Polymer chemistry also uses unique definitions for long chains of molecules. Even though polymers like polyethylene are mixtures of different molecular masses, they are defined by their specific chemical precursors.