Tap water is a mixture, not a compound. This distinction clarifies the difference between the pure water molecule (\(\text{H}_2\text{O}\)) and the substance flowing from a faucet. While \(\text{H}_2\text{O}\) is a pure substance, tap water contains many other dissolved materials that change its classification.
Compound Versus Mixture
A chemical compound is formed when two or more different elements are chemically bonded together in a fixed, definite ratio. This chemical union results in a new substance whose properties are distinctly different from the original elements. Components of a compound cannot be separated by simple physical means, such as filtration, requiring a chemical reaction to break the bonds.
A mixture involves two or more substances that are physically combined, not chemically bonded. The substances retain their individual chemical identities and properties. A mixture does not have a fixed composition; the ratio of its components can vary widely, such as in saltwater.
Mixtures can often be separated using physical processes like distillation or filtration. They can be either homogeneous, appearing uniform throughout, or heterogeneous. Tap water is considered a homogeneous mixture, or a solution, because the dissolved components are evenly distributed and not easily seen.
The Chemical Identity of Pure Water
The substance recognized as water, represented by the chemical formula \(\text{H}_2\text{O}\), is definitively a compound. Each molecule consists of two hydrogen atoms chemically bonded to one oxygen atom. This is a precise and unvarying two-to-one ratio.
The atoms within the water molecule are joined by covalent bonds, sharing electrons to create a single, stable unit. This strong chemical bond ensures that pure water, such as distilled or deionized water, is a pure substance. \(\text{H}_2\text{O}\) satisfies the definition of a compound because its components cannot be separated by physical processes.
What Makes Tap Water a Mixture
Tap water is a mixture because it is not pure \(\text{H}_2\text{O}\); it is a solution containing various substances dissolved or suspended within the water compound. The exact composition is variable, depending on the source (river, reservoir, or groundwater) and the treatment processes used by the local municipality. This variability in content classifies it as a mixture.
One major category of components is dissolved minerals, which are naturally picked up as water flows over rock and soil. These minerals often include ions of calcium and magnesium, which contribute to water hardness, along with trace amounts of sodium and potassium. The concentrations of these minerals fluctuate based on the local geology.
Municipal water treatment facilities also intentionally add chemicals to ensure public health and safety. Disinfectants like chlorine or chloramine are introduced to eliminate harmful bacteria and pathogens. Many communities also add fluoride compounds to help prevent tooth decay.
Tap water also contains dissolved gases that are physically mixed with the water molecules. Atmospheric gases, primarily oxygen and carbon dioxide, enter the water through natural aeration. The presence of these gases is why a glass of tap water often develops small bubbles as it warms.
The combination of naturally occurring minerals, added treatment chemicals, and dissolved atmospheric gases makes tap water a complex solution. Because the ratios of these components are not fixed—they vary by source and treatment—tap water is accurately classified as a homogeneous mixture. The presence of these physically combined substances prevents it from being a pure compound.