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In 2017, the Global Tree Search published a study that confirmed there are over 60,000 tree species on the planet. Each tree species has variations in its chemical make up, which means some wood types are more useful for certain tasks than others.
Wood is a heterogeneous mixture of compounds and elements. The building blocks of all matter, elements cannot be broken down. Combinations of elements are called compounds. As wood comprises many compounds and elements, it is classified as a mixture, and chemical composition varies in tree species.
This article looks into why wood is classified as a heterogeneous mixture. To help you understand what wood’s made of, I’ll explain why wood is a mixture, look at how wood grows, examine the functions of a tree’s main components then address how different components affect wood properties.
What Makes Wood a Mixture
Wood is a complex material with many variations in its composition and properties. The non-uniform nature of its components makes it a mixture.
The Elements That Make Up a Wood Mixture
Materials classified as a mixture contain combinations of elements and compounds. Wood is a mixture as it is formed of combinations of the elements Carbon, Oxygen, Hydrogen, Nitrogen, and a small percentage of other elements. These elements cannot be reduced to smaller parts through simple processes. Elements can, however, combine with other elements in a chemical reaction. When this occurs, the result is known as a compound.
The Difference Between a Compound and an Element
When elements bond with other elements in a chemical reaction, you get a compound. Let’s look at an example. When the element Hydrogen (H) combines with the element Oxygen (O) in a chemical reaction, the outcome is the compound water (H2O). Similar to elements combining with other elements, compounds appear alongside other compounds. When various compounds are found in clusters, they make mixtures, which are materials like wood.
Three Basic Components of Wood Mixture
The three main components of wood are Cellulose, Hemicellulose, and Lignin. The composition and distribution of these compounds vary according to tree species. This means that Cellulose, Hemicellulose, and Lignin present in different ways and amounts in different wood types. There is no consistent amount of one ingredient or another. In fact, the distribution of compounds within the wood is inconsistent.
The Hallmark Traits of Wood Mixture
When you cut through a piece of wood, some inconsistencies are obvious to the naked eye or to the touch. In the same way, that variation in properties is clear to see, and there is similar diversity on a molecular level.
Unlike Gold, which is made of only one element and is identical in composition no matter the sample, wood has no uniformity. There’s no consistent chemical composition across wood samples. In other words, no one piece of wood is the same as another due to inconsistencies in the distribution of compounds. It’s this quality that means wood is classified as a heterogeneous mixture.
How Wood Mixture Develops
Trees gain mass through a process called photosynthesis. This process changes compounds into plant matter.
What Happens to Compounds During Photosynthesis
A compound such as water occurs after a chemical reaction between Hydrogen and Oxygen, as the H and O atoms bond. This is not a permanent union because, like most compounds, the state of water is susceptible to change. During a chemical reaction, the bonded elements within a compound separate then combine to make different compounds or elements.
An example of this occurs when CO2 and H2O come together during the photosynthesis process. The products of the chemical reaction are Glucose and Oxygen.
Photosynthesis Produces Compounds for Growth
Photosynthesis produces glucose, which trees need to grow. Through a complex process of biosynthesis, glucose contributes to forming cellulose, hemicellulose, or lignin. These are the main three components that make up wood. The components travel to parts of the tree to supply new cells.
Recalling that wood is a mixture, there is no uniformity to how the cells or components are distributed. Where compounds end up within a tree is due to organic and complex processes.
Function of Compounds in Wood Mixture
The compounds within trees play different roles in tree growth at a cellular level.
Cellulose (C5H10O5) Function Within Tree Matter
Like all living organisms, trees are made of microscopic cells. In trees, one of the most common ingredients is cellulose, and the string-like structure of molecules gives cellulose its strength. It so happens that cellulose molecules form strong bonds with other cellulose molecules, also. In turn, the strength of these bonds gives wood its strength.
It is important to highlight that cellulose is not the only part of a cell. There are many molecular arrangements of proteins, carbohydrates, and enzymes within a plant cell that make up tree matter.
Hemicellulose Function Within Tree Matter
More complex in its shape and composition than cellulose, Hemicellulose forms in branch-like structures. Its function is to bring strength to a cell. It also brings together cellulose and pectins that make up the tree cells. These joined components form microfibrils, which make up the tree’s matter.
Lignin Function Within Tree Matter
If it were not for Lignin, trees would be like grass because Lignin binds the microfibrils made up of cellulose and gives wood its strength. It’s a bit like the skeleton in the human body or like concrete poured between the struts of a building. The concrete sets and gives strength to the building. In a similar way, Lignin links between cells. Besides providing strength, Lignin also conducts water around a tree.
Composition Affects the Properties of Wood
Wood has many functions. It is harvested for pulping, fuel, or building materials. Alternatively, some woods are used for decorative purposes. Hardwoods are often used for floors or furniture as their composition makes them highly durable. It’s fair to say that some woods are better for one purpose while other wood types are more appropriate for other uses.
If looked at closely, different types of wood have unique properties. Wood may be more or less porous, flexible or rigid, light or dark in color. Why is this? A consequence of the variations in a mixture becomes apparent when analyzing a cross-section of wood samples.
Chemical Composition: The Reason Why Wood Properties Differ
A study completed in Oaxaca, Mexico, examined five different wood types and their chemical composition. The study discovered a variety in the range of elements and components within each of the tree species. The chemical components found were extractives, ash, lignin, and holocellulose. The range in the percentage of lignin went from 23.1% to 37.2%. The percentage of hemicelluloses ranged from 12.3% to 21.0%.
The study concluded that the chemical composition of the trees in the study meant the trees were good for fuel use and pulp production. Consider the findings on a larger scale: each tree species would reflect this range in composition, meaning each tree possesses different chemical make-up and growth processes that affect wood properties and, consequently, how best those wood types can be used.
As this article shows, the wood mixture is made up of compounds and elements. The wood matter results from chemical reactions that produce a mixture of sugars, proteins, and carbohydrates to assist tree growth. We’ve learned how the wood composition is prone to variation, meaning the wood is classified as a heterogeneous mixture. Finally, the wood composition varies, so some wood is better used for specific purposes than other wood types.