Wood is not a pure substance. It is a complex natural material composed of various chemical components. Its intricate structure and varied composition classify it as a mixture.
What Defines a Pure Substance
In chemistry, a pure substance has a constant composition and uniform properties throughout a given sample. Pure substances are categorized into two types: elements and compounds. Elements consist of only one type of atom, such as gold or oxygen. Compounds are formed when two or more different elements are chemically bonded together in a fixed ratio, like water (H₂O) or sodium chloride (table salt).
A mixture is a material made up of two or more substances combined physically, not chemically. Each substance retains its individual chemical identity and properties. Mixtures can be separated into their components using physical methods, unlike compounds which require chemical reactions to break them down. There are two primary types of mixtures: homogeneous and heterogeneous.
Homogeneous mixtures possess a uniform composition and appearance throughout, meaning their components are evenly distributed and indistinguishable. Examples include saltwater or air, where the dissolved particles are spread uniformly. A heterogeneous mixture has a non-uniform composition, with visibly distinct components or regions that have different properties.
The Chemical Composition of Wood
Wood is a complex natural material, primarily composed of three major organic polymers: cellulose, hemicellulose, and lignin. These components are intricately arranged within its cellular structure. Cellulose, a long-chain polymer of glucose molecules, provides primary structural strength and rigidity, forming strong fibers. It typically makes up about 40-50% of wood’s dry weight.
Hemicellulose is a branched polymer made of various sugar units, including xylose, mannose, and glucose. It acts as a binding agent, helping to connect and support the cellulose fibers. Hemicellulose content generally ranges from 15-25% in wood. Lignin is a complex, three-dimensional polymer that acts as a natural glue, impregnating the cellulose and hemicellulose to provide stiffness and resistance to decay. Lignin accounts for approximately 15-30% of wood, varying between softwood and hardwood species.
Beyond these main polymers, wood also contains water, which can constitute a significant portion of its weight. Various “extractives” are also present, which are non-structural organic compounds such as resins, fats, waxes, terpenes, and phenolic compounds. These extractives contribute to properties like color, smell, and natural resistance to insects and fungi. The proportions of all these components can vary significantly depending on the tree species, its age, and even the specific part of the tree.
Why Wood is Not a Pure Substance
Wood is not considered a pure substance because it fails to meet the criteria of having a definite and constant chemical composition. It is composed of multiple different chemical substances, including cellulose, hemicellulose, lignin, water, and various extractives. These components are not chemically bonded together in fixed, uniform ratios throughout the material. Instead, they are physically intermingled, with their proportions varying based on factors like tree species, age, and even location within the same tree.
This variability and the physical combination of distinct chemical entities mean wood is classified as a mixture. More specifically, wood is a heterogeneous mixture. Its heterogeneous nature is evident because its composition is not uniform throughout. For example, different sections of a piece of wood can have varying amounts of water, sap, or density due to the arrangement of its cells and fibers.
Even at a microscopic level, the distinct structures of cellulose fibers, lignin, and hemicellulose are observable, further confirming its non-uniformity. The presence of wood grain and the differing responses to forces in various directions also serve as physical evidence of its heterogeneous composition. Wood is accurately defined as a complex heterogeneous mixture.