Water is fundamental to life, but its various forms often cause confusion about its chemical classification. Most people encounter water in states like tap water or bottled mineral water, but chemically, these forms are not equivalent. Determining if a water sample is a pure substance or a mixture requires defining these two categories of matter. The distinction depends entirely on the consistency of the water’s composition at a molecular level.
Defining Pure Substances and Mixtures
In chemistry, matter is categorized into pure substances and mixtures based on composition. A pure substance has a uniform and definite composition throughout, consisting of only one type of particle, such as an element or a compound. The compound water (\(\text{H}_2\text{O}\)), with its fixed ratio of two hydrogen atoms to one oxygen atom, is an example of a pure substance when isolated. A pure substance maintains constant properties, such as a predictable boiling point.
A mixture, in contrast, is a physical combination of two or more substances that are not chemically bonded. The components retain their individual chemical properties and can often be separated using physical methods. Unlike a pure substance, a mixture has a variable composition; for example, the amount of salt dissolved in water can vary widely. Because the components are not chemically united, the overall properties of the mixture, such as its boiling point, are not fixed.
The Chemical Nature of Distilled Water
Distilled water is chemically classified as a pure substance, specifically the compound \(\text{H}_2\text{O}\). This is achieved through distillation, a physical separation technique that removes impurities. The process involves heating water until it turns into steam, leaving behind non-volatile contaminants like dissolved salts and minerals. The steam is then collected and cooled, allowing it to condense back into liquid water.
The resulting liquid is nearly pure \(\text{H}_2\text{O}\) because most dissolved solids, heavy metals, and microorganisms do not vaporize during boiling. Since the final product consists almost exclusively of water molecules, its chemical composition is uniform and constant. Distillation effectively separates the water compound from a mixture, confirming the purified product’s status as a pure substance with consistent properties.
Distilled Water Versus Other Common Waters
Comparing distilled water to everyday sources like tap, spring, and mineral water clarifies the distinction between a pure substance and a mixture. These common waters are classified as mixtures because they contain various dissolved substances in addition to \(\text{H}_2\text{O}\) molecules.
Tap water, for instance, is treated with chemicals like chlorine and contains trace minerals, with concentrations varying by location. Spring and mineral waters are also mixtures, containing dissolved solids such as calcium, magnesium, and potassium. These minerals and salts are physically mixed with the water molecules, not chemically bonded. Since the composition of these waters is variable and contains multiple substances, they are all categorized as aqueous mixtures.