A material can be classified as either an element, compound or mixture. An element is composed of identical atoms bound together. A compound is composed of two or more elements in a certain ratio and can be broken into individual elements by breaking the bonds between them. Mixtures contain two or more elements in non-specific ratios and can be easily separated into individual elements.
Wood is composed of a number of compounds such as lignin, cellulose, water, hemicellulose, etc. The relative composition of wood varies from plant to plant. Unlike a compound, wood does not have a fixed chemical formula. Hence, wood is a mixture. In fact, wood is a heterogeneous mixture, since these constituent compounds are mixed unevenly in a given wood sample.
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