Tap water, while highly regulated and safe for consumption in many regions, is not considered “pure water” in the scientific meaning of the term. The water that flows from a faucet is a complex mixture containing far more than just hydrogen and oxygen molecules. Tap water must contain various intentional additives and natural minerals to be safe and palatable. It may also pick up trace amounts of unintended contaminants along the way, placing it firmly in the category of a solution, rather than a pure chemical compound.
Defining “Pure Water” (H₂O)
The scientific definition of “pure water” is a substance consisting solely of H₂O molecules. This chemically pure form is completely devoid of any dissolved minerals, gases, salts, or organic matter. True pure water, such as deionized or distilled water, is colorless, odorless, tasteless, and does not conduct electricity.
Water is rarely found in this state because its properties as a universal solvent mean it readily dissolves materials it contacts. This quality allows water to support life by transporting nutrients. However, it also means that any water intended for drinking, even after extensive treatment, will never meet the absolute standard of chemical purity.
Essential Dissolved Solids and Intentional Additives
Tap water contains a measurable level of total dissolved solids (TDS), which are substances that remain in the water after treatment. These include naturally occurring, beneficial minerals like calcium, magnesium, and potassium, picked up as water flows over rocks and through soil. The presence of these inorganic salts is what gives drinking water its characteristic flavor; water with very low TDS can taste flat.
Water treatment facilities also intentionally add chemicals for public health reasons. Chlorine or chloramine compounds are added as disinfectants to kill or inactivate microorganisms and prevent the growth of pathogens in the distribution system. Fluoride is another common additive, introduced to promote dental health. These necessary components ensure safety but prevent tap water from being chemically pure.
Sources of Potential Impurities and Contaminants
The journey from a source—like a river or groundwater—to the tap introduces various potential impurities, which are distinctly different from the intentional additives. Environmental contaminants are often present in the source water itself, originating from agricultural runoff containing pesticides and fertilizers, or industrial discharge. Naturally occurring elements, such as arsenic or radon, can also leach into the water from rocks and soil.
Water may still encounter contaminants as it moves through the infrastructure after treatment. Older distribution systems can lead to the leaching of metals like lead and copper from pipes and plumbing fixtures, especially when the water is corrosive. Emerging concerns include microplastics and trace amounts of pharmaceuticals, which are not always fully removed by conventional treatment methods. Biological contaminants are largely eliminated through the rigorous treatment process.
The Regulatory Framework for Tap Water Safety
Water safety is maintained through a comprehensive regulatory framework overseen by agencies like the U.S. Environmental Protection Agency (EPA). The EPA establishes National Primary Drinking Water Regulations, which are legally enforceable standards for over 90 contaminants, including microorganisms, inorganic chemicals, and organic chemicals. These standards set Maximum Contaminant Levels (MCLs) to limit the amount of harmful substances allowed in public drinking water.
Water treatment facilities employ a multi-step process to transform source water into safe tap water. This typically involves coagulation to aggregate small particles, filtration to remove suspended solids, and disinfection to eliminate pathogens. Consistent and frequent testing is mandatory under the Safe Drinking Water Act, ensuring that water quality meets or exceeds federal standards before delivery.