Water is often called the universal solvent because of its ability to dissolve more substances than any other liquid. This property means that pure water, consisting only of H₂O molecules, rarely exists in nature. As water passes through the environment, it dissolves components, leading to a complex chemical composition in nearly every drop we encounter. When people ask if water has “salt” in it, they are usually referring to common table salt (sodium chloride). However, the chemical reality involves a much broader array of dissolved minerals that give water its distinct characteristics and play a role in human health.
Defining “Salt” and Total Dissolved Solids
In chemistry, the term “salt” is defined much more broadly than just sodium chloride (NaCl). A chemical salt is an ionic compound formed by the neutralization reaction between an acid and a base. These compounds consist of positively charged ions (cations) and negatively charged ions (anions). When water dissolves these compounds, the ions separate, which is why water containing salts can conduct electricity. These dissolved components include ions like calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺), chloride (Cl⁻), and bicarbonate (HCO₃⁻).
The total measure of all these dissolved inorganic and organic substances is known as Total Dissolved Solids (TDS). This measurement includes salts, minerals, metals, and certain organic matter remaining after filtration. TDS is typically expressed as parts per million (ppm) or milligrams per liter (mg/L), which are interchangeable units. For instance, a TDS of 100 ppm means that 100 milligrams of dissolved solids are present in every liter of water.
TDS levels are generally measured using a conductivity meter. Higher concentrations of these charged particles result in higher electrical conductivity, which the meter converts into a TDS reading. While the TDS value tells you the amount of dissolved material, it does not identify the specific substances present. However, this number provides a quick indicator of the water’s mineral content and overall purity.
Mineral Content Across Common Water Sources
The concentration of total dissolved solids varies dramatically depending on the water source. This variance dictates the water’s taste, aesthetic qualities, and classification. The United States Environmental Protection Agency (EPA) has set a secondary maximum contaminant level for TDS in drinking water at 500 mg/L. This standard is based on aesthetic factors, not health risk, as water above this level may have a noticeable, often salty or metallic, taste.
Ocean water represents the extreme high end of the TDS spectrum and is classified as saline water. Seawater typically contains over 35,000 ppm of dissolved solids, with sodium chloride being the dominant component. This hyper-saline concentration makes ocean water unsafe for human consumption, as the kidneys cannot process the massive salt load without using more water than is taken in.
Freshwater sources, such as rivers, lakes, and tap water, have TDS levels well below 1,000 ppm, often ranging from 100 to 400 ppm. The specific composition depends heavily on local geology. For example, water flowing through limestone picks up high levels of calcium and magnesium. These minerals are the primary components of “hard water,” which can create scale buildup in pipes and appliances.
Bottled water presents a wide range of TDS levels based on its type. Natural spring water is sourced from underground formations and contains naturally occurring minerals, similar to tap water, with varying TDS levels. “Mineral water” is a regulated term requiring the water to contain at least 250 ppm of naturally dissolved solids.
Purified or distilled water is processed using techniques like reverse osmosis or deionization to remove nearly all dissolved solids. These methods are designed to remove contaminants and result in a TDS level often less than 10 ppm. While this processing strips away natural minerals, it leads to a very neutral or flat taste.
The Biological Role of Water Minerals
The dissolved salts found in water, particularly sodium, potassium, calcium, and magnesium, are known as electrolytes once they enter the body. Electrolytes are minerals that carry an electric charge when dissolved in fluid, and they are essential for numerous biological functions. These charged particles help regulate the flow of water in and out of cells, which is fundamental to maintaining proper hydration and fluid balance throughout the body.
Electrolytes are also necessary for nerve signaling and muscle contraction. Sodium and potassium ions facilitate the electrical impulses that allow nerves to communicate, including signals that control the heartbeat. Calcium and magnesium ions are critical for enabling muscle fibers to contract and relax.
While these minerals are beneficial, the majority of necessary mineral intake comes from a balanced diet. Therefore, consuming water with extremely low TDS, like distilled water, is generally not a health concern for most people. However, water with high levels of calcium and magnesium can contribute beneficially to daily mineral intake.
The danger lies at the extreme ends of the TDS scale. Water with very high TDS actively draws water out of the body’s cells to balance the salt concentration, leading to severe dehydration. Conversely, low-TDS water is safe, but its lack of minerals can contribute to a bland taste, which is why some purified water brands add a small amount of minerals back in for improved palatability.