The primary classification of Earth’s water is based on the concentration of dissolved salts, dividing it into two types: saltwater and freshwater. This distinction influences nearly every aspect of biology, climate, and human civilization. The salt content, or salinity, determines the water’s usability for drinking, agriculture, and industry.
Saltwater: Composition and Distribution
Saltwater is defined by its high salinity, resulting from the presence of dissolved ions, primarily sodium and chloride. The six most abundant ions—chloride, sodium, sulfate, magnesium, calcium, and potassium—make up about 99% of all sea salts by weight. The average salinity of the open ocean is approximately 35 parts per thousand (ppt).
This reservoir of water is found predominantly in the oceans and seas, covering over 70% of the Earth’s surface. Saltwater makes up about 97% of the total water on the planet. Regional variations exist; high evaporation and low precipitation can push surface salinity higher, such as in the Red Sea, while large river inflows can lower it, as seen in the Baltic Sea.
Freshwater: Sources and Human Use
Freshwater is characterized by its low concentration of dissolved salts, typically less than 1 ppt, making it suitable for human consumption and most life on land. Although water is abundant on Earth, only about 3% of it is freshwater, and a significant portion of that is inaccessible. Most freshwater is locked away in ice caps and glaciers, or stored deep underground as groundwater.
The most readily accessible freshwater, found in lakes, rivers, and the atmosphere, accounts for less than 1% of all freshwater. This small fraction sustains life and supports human activities. Worldwide, agriculture is the largest consumer, accounting for roughly 70% of all freshwater withdrawals, followed by industry and domestic use.
Freshwater is continuously collected, naturally purified, and distributed through the hydrologic cycle. Global demand is increasing. Groundwater is an increasingly relied-upon resource, supplying about half of the water withdrawn for domestic purposes, but its sustainability is a growing concern, as roughly half of the world’s population experiences severe water scarcity for at least part of the year.
Hard Water Versus Soft Water
A secondary classification important for domestic use is based on the concentration of dissolved minerals, specifically calcium and magnesium ions. Water with a high concentration of these minerals is called hard water. As rainwater falls and moves through soil and bedrock, it picks up these minerals, increasing its hardness.
Hard water is not a health hazard, but it causes household issues due to the minerals forming limescale deposits. This buildup can clog pipes, reduce water pressure, and shorten the lifespan of water-using appliances. Furthermore, it reacts with soap, making it difficult to form a lather and leaving behind soap scum and residue.
Conversely, soft water contains low concentrations of these minerals, either naturally or because it has been treated. Soft water lathers easily with soap and prevents the scale buildup associated with hard water. However, water softened through a common process often sees a slight increase in sodium content, which may be a consideration for individuals on a sodium-restricted diet.