Water is defined by its molecular structure, \(\text{H}_2\text{O}\). However, the presence of dissolved substances separates the scientific ideal from the water we encounter daily. Understanding the salt concentration, or salinity, of water is necessary for assessing its quality and utility in chemistry, health, and industry. Salinity varies widely, from nearly zero in highly processed water to high levels in oceans and briny lakes.
Defining Pure Water and the Theoretical Baseline
Pure water, in chemistry, is an abstract ideal composed exclusively of \(\text{H}_2\text{O}\) molecules. This means pure water contains no dissolved ions, gases, or solid particulates. Consequently, the salt concentration, or salinity, of truly pure water is zero.
This theoretical baseline results in extremely low electrical conductivity. Dissolved salts are ionic compounds that break apart in water, forming charged ions that allow electrical current to pass through. Since pure water lacks these free ions, its conductivity is negligible, which serves as a measure of its purity. While scientists can produce water remarkably close to this ideal, any water exposed to the atmosphere or containers inevitably dissolves trace amounts of substances.
Understanding Salinity Measurements
Quantifying dissolved solids, commonly referred to as salts, requires standardized measurement techniques. The primary metric used is Total Dissolved Solids (TDS), which represents the combined weight of all dissolved inorganic and organic substances. TDS is commonly expressed in parts per million (ppm) or milligrams per liter (\(\text{mg}/\text{L}\)), which are functionally equivalent for water.
Salinity is also measured using parts per thousand (ppt) or Practical Salinity Units (PSU), particularly in oceanography. One thousand ppm equals one ppt or approximately one PSU. While TDS can be determined directly by evaporating the water and weighing the residue, a faster method involves measuring the water’s electrical conductivity. Conductivity meters use a conversion factor to estimate the TDS level, as dissolved salts are the main conductors in water.
Comparing Salinity in Real World Water Sources
The theoretical zero concentration of pure water contrasts sharply with the vast range of salinity found in nature. Seawater, the most common saline source, typically exhibits a high concentration of approximately 35,000 ppm. This measurement includes sodium chloride (table salt) and other ions like magnesium, calcium, sulfate, and potassium.
Municipal tap water is considered freshwater, often falling between 50 and 500 ppm TDS. The Environmental Protection Agency recommends that drinking water not exceed 500 \(\text{mg}/\text{L}\) to ensure palatability and minimize scale buildup. Bottled spring water varies significantly; low-mineral waters may have a TDS under 50 \(\text{mg}/\text{L}\), while certain mineral waters can exceed 2,000 \(\text{mg}/\text{L}\).
Commercially produced deionized or distilled water is the closest practical approximation of pure water. Distilled water is created by boiling water into steam and condensing it, which removes nearly all dissolved solids. This process results in a TDS reading of less than 10 ppm, often below 0.5 ppm. Water purified by reverse osmosis systems typically achieves a TDS below 25 ppm.