The term “pure water” in science refers to a substance composed solely of water molecules (\(\text{H}_2\text{O}\)). In nature, this state of purity is never achieved because water is a nearly universal solvent, immediately dissolving minerals, gases, and organic materials. The water we encounter daily is a solution containing a mixture of various dissolved substances, not a pure compound. Scientific purity must be manufactured to create a controlled medium necessary for accurate laboratory experiments and industrial processes.
Defining Scientific Purity
Chemically pure water is defined by the absence of any substance other than H2O molecules, including dissolved solids, gases, or contaminants. This ideal state is measured by the water’s electrical conductivity, which rises significantly when impurities are present. Pure water provides maximum resistance to an electric current because there are no charged particles to carry the flow. The theoretical maximum resistivity for pure water is 18.2 MΩ-cm (megohm-centimeters) at 25°C. The degree of purity is quantified by how close the water comes to this theoretical maximum.
Common Types of Purified Water
Two common types of water purification are distillation and deionization, which target different kinds of impurities. Distilled water is produced by boiling water into steam and condensing it back into a liquid, leaving behind non-volatile contaminants like minerals, salts, and heavy metals. While effective at removing inorganic solids, distillation may not remove volatile organic compounds (VOCs) that vaporize along with the water.
Deionized (DI) water is created by passing water through ion-exchange resins that chemically remove charged mineral ions such as calcium, sodium, and chloride. The resins exchange these undesirable ions for hydrogen (H+) and hydroxide (OH-) ions, which combine to form new water molecules. While DI water is exceptional at removing ionic impurities, it does not reliably remove non-ionic contaminants like bacteria, viruses, or uncharged organic molecules.
The Highest Grades of Purity
For the most sensitive scientific applications, water purity is regulated by strict international standards that go beyond simple distillation or deionization. This water is often referred to as Ultrapure Water (UPW) or reagent-grade water. These standards categorize water by specific limits on resistivity, total organic carbon (TOC), and the presence of microorganisms.
For example, ASTM International’s D1193 standard defines four types of reagent water, with Type I being the most pure. Type I water must achieve a resistivity of at least 18 MΩ-cm and contain a maximum of 50 ppb (parts per billion) of total organic carbon. Achieving these grades typically requires a combination of multiple purification steps, including reverse osmosis, deionization, and ultraviolet light exposure.
Why Pure Water is Not Drinking Water
While chemically pure water is necessary for laboratory precision, it is not suitable for long-term human consumption. Drinking water naturally contains trace minerals like calcium and magnesium, which are essential electrolytes for biological function. Purified water lacks these vital components.
The absence of dissolved ions makes pure water chemically aggressive, causing it to actively seek to dissolve substances it contacts, including trace minerals from the body. For human physiology, the mild mineral content in ordinary drinking water is a necessary part of a healthy diet, helping to maintain electrolyte balance.