Pure water, in chemistry, refers to a substance fundamentally different from the tap or bottled water consumed daily. The chemical definition is rigid, demanding the virtual absence of any substance other than the water molecule itself. This high standard of purity is necessary for sensitive scientific applications, where trace contaminants can invalidate experimental results. Achieving this level of purity is a complex, multi-stage process that contrasts sharply with standard drinking water treatment.
The Chemical Definition of Pure Water
The theoretical ideal of pure water is a compound consisting solely of hydrogen and oxygen atoms in a precise two-to-one ratio (H₂O). At the molecular level, the liquid must be free from dissolved gases, mineral ions, organic matter, and particulates. This conceptual standard is the baseline against which all laboratory water is measured.
This substance possesses fixed physical properties under standard atmospheric pressure. It boils at 100°C (212°F), freezes at 0°C (32°F), and reaches its maximum density of 1 gram per cubic centimeter at 4°C. Pure water also maintains a neutral pH of 7.0 at 25°C, achieved through the balanced self-ionization into equal parts of hydrogen (H⁺) and hydroxide (OH⁻) ions.
Common Impurities Found in Water
Real-world water sources, even after basic filtration, contain contaminants that prevent them from reaching chemical purity. These impurities can be broadly grouped into categories based on their chemical nature. Dissolved inorganic solids, primarily mineral salts like calcium, magnesium, and sodium chloride, are the most common contaminants.
These charged ions act as electrolytes, enabling the water to conduct electricity. They can interfere with sensitive chemical reactions or cause scaling in equipment. Dissolved gases, particularly carbon dioxide (CO₂) absorbed from the atmosphere, are also significant impurities. When CO₂ dissolves, it forms carbonic acid, which lowers the water’s pH below 7.0.
Organic compounds, often measured as Total Organic Carbon (TOC), include residue from industrial processes, natural decay, and cleaning agents. These molecules can leach from storage containers and interfere with analytical techniques like High-Performance Liquid Chromatography (HPLC). Particulates, such as fine colloids and suspended sediment, and biological contaminants like bacteria, viruses, and endotoxins, must be excluded. These contaminants can ruin cell cultures and sensitive molecular biology experiments.
Quantifying Purity Using Resistivity
The primary method for quantifying water purity is by measuring its electrical resistivity, expressed in megohm-centimeters (MΩ·cm). This relies on the principle that pure water is a poor conductor of electricity, as there are virtually no free ions to carry a current. The presence of even a minute concentration of dissolved salts or acids will reduce this resistance.
Resistivity is the reciprocal of conductivity. For ultrapure water, resistivity is the preferred metric because it operates in a high-value range sensitive to trace ions. The benchmark for “Type I” or “Ultrapure” water, required for the most demanding laboratory work, is 18.2 MΩ·cm. This value is measured at a standardized temperature of 25°C.
Maintaining the 18.2 MΩ·cm standard signifies that the water contains only the H⁺ and OH⁻ ions from its own self-ionization. For perspective, contamination of only 0.1 part per billion of sodium chloride can lower the resistivity value. Monitoring resistivity in real-time serves as an accurate, non-destructive indicator of water’s ionic quality.
Techniques for Creating Pure Water
Creating chemically pure water is achieved through a multi-stage purification system that systematically targets different categories of impurities. The process begins with Reverse Osmosis (RO), which forces feed water through a semi-permeable membrane at high pressure. The RO membrane removes 95% to 99% of dissolved inorganic solids, large organic molecules, and particulates.
After initial filtration, the water is routed through Deionization (DI) resin beds, where ion exchange occurs. Cation and anion resins exchange contaminant ions for H⁺ and OH⁻ ions, which combine to form pure water. This process is efficient at “polishing” the water to remove residual charged impurities left after the RO stage.
To address non-ionic impurities like trace organic compounds, the water stream is exposed to ultraviolet (UV) light, which oxidizes and breaks down these molecules. A final stage involves passing the water through a sub-micron filter or an ultrafilter to remove remaining particulates and biological contaminants. This combination of methods ensures the water meets the 18.2 MΩ·cm resistivity standard required for sensitive scientific research.