The classification of water often causes confusion because the term “water” describes both a single, pure chemical and the common liquid found everywhere. People frequently encounter water that appears clear but contains many dissolved components, leading to questions about its true chemical nature. This article establishes the difference between pure substances and mixtures to define the chemical identity of water.
Defining Pure Substances and Mixtures
All matter is fundamentally classified into two main categories: pure substances and mixtures. A pure substance has a fixed, uniform composition and a definite set of characteristic physical properties throughout the sample. Pure substances include elements, like oxygen gas, or compounds, which form when two or more different elements chemically bond together. The components of a compound can only be separated by a chemical reaction, not physical means.
A mixture is a physical combination of two or more pure substances where each component retains its individual chemical identity. Mixtures do not have a fixed composition, meaning the ratio of components can vary, and they can typically be separated using physical processes such as evaporation or filtration. A solution is a specific type of homogeneous mixture, meaning the components are uniformly distributed at a molecular level, such as salt dissolved in water. In a solution, the substance present in the largest amount is called the solvent, and the substances dissolved within it are called solutes.
The Classification of Pure Water
Chemically pure water, represented by the formula \(\text{H}_2\text{O}\), is definitively classified as a compound and therefore a pure substance. Each molecule consists of two hydrogen atoms covalently bonded to one oxygen atom, establishing a fixed 2:1 atomic ratio. This fixed chemical composition is the defining trait of a compound, setting it apart from a variable mixture.
The process required to break \(\text{H}_2\text{O}\) into its elemental components, hydrogen and oxygen, must be a chemical one, such as electrolysis, which uses electrical energy to break the chemical bonds. This is another characteristic that confirms its status as a compound, as mixtures can be separated by simple physical methods. Furthermore, a pure compound like water possesses fixed physical properties, such as a precise boiling point of \(100^\circ\text{C}\) and a freezing point of \(0^\circ\text{C}\) at standard pressure.
Understanding Real-World Water
While pure \(\text{H}_2\text{O}\) is a pure substance, nearly all water encountered outside of a controlled laboratory setting is actually a solution, a type of homogeneous mixture. This includes common tap water, bottled mineral water, and vast bodies of water like the ocean. In these sources, the \(\text{H}_2\text{O}\) compound acts as the solvent, with various other substances dissolved throughout the liquid.
Tap water contains additives like chlorine or chloramine for disinfection and fluoride to promote dental health. It also picks up dissolved minerals, such as calcium and magnesium ions, from the soil and pipes, which is why it is often referred to as “hard water.” The presence of these dissolved ions is why tap water conducts electricity, whereas chemically pure water is a very weak electrical conductor. Seawater is a more concentrated example, where the \(\text{H}_2\text{O}\) solvent has a high concentration of various salts, primarily sodium chloride, along with dissolved gases.