Brewed tea is a complex mixture, not a pure substance, despite its uniform appearance. A pure substance has a constant composition and uniform properties throughout, but tea is the result of many different compounds dissolved into water. Brewing physically combines components of the tea leaves with the solvent (water) without forming new chemical bonds. This combination of multiple substances, which retain their individual chemical identities, defines a mixture.
What Defines a Pure Substance?
In chemistry, matter is broadly classified into pure substances and mixtures. A pure substance is either a single element, such as gold, or a single compound, like water (H₂O). These materials have a fixed chemical composition and predictable, constant physical properties, such as a specific boiling or melting point. For example, water will always boil at 100°C at sea level.
A compound is formed when two or more different elements are chemically bonded together in a fixed ratio and cannot be separated by physical means. In contrast, a mixture contains two or more substances that are only physically combined, meaning no new chemical bonds are created. The individual components in a mixture retain their own properties and can be separated using physical processes like filtration or evaporation. The proportions of substances in a mixture can also vary, unlike a pure compound.
The Diverse Chemical Components of Brewed Tea
Brewed tea is a complex blend of organic and inorganic materials extracted from the Camellia sinensis leaves. Water acts as the solvent, dissolving numerous compounds to create the beverage. Significant components include polyphenols, such as catechins and flavonoids, which make up about 30–40% of the dry weight of the leaves. These compounds are responsible for the tea’s color and astringency.
Another major component is the alkaloid caffeine, a stimulant that contributes to the tea’s bitterness. The caffeine content in a standard 250-milliliter cup can range from 30 to 90 milligrams, depending on the tea type and brewing method. Tea also contains amino acids, such as theanine, which provides a savory flavor. Volatile aromatic compounds and trace minerals are dissolved into the water, contributing to the overall aroma and flavor profile.
Classifying Tea as a Solution
Mixtures are categorized as either heterogeneous or homogeneous. A heterogeneous mixture has components that are not uniformly distributed and are often visually distinguishable. Brewed tea, after the leaves are removed, is classified as a homogeneous mixture.
A homogeneous mixture that is uniform at the molecular level is called a solution. In a solution, the dissolved substances (solutes) are uniformly dispersed throughout the solvent, which is the water. The dissolved particles in tea, such as caffeine and polyphenols, are so small that they do not settle out, and the liquid remains transparent. This uniform dispersion ensures a cup of tea appears clear and consistent, even though it is chemically composed of dozens of different molecules.