The primary substance that causes water to be salty is sodium chloride, commonly known as table salt. Sodium chloride is a compound made up of sodium and chlorine atoms, and it is the most abundant salt found in seawater. The concentration of sodium chloride in seawater is approximately 3.5% by weight, which is why seawater tastes salty. This high concentration of sodium chloride is due to the evaporation of water over geological times, leaving behind the salt.

The presence of sodium chloride in water is a result of natural processes. Rainwater, which falls from the atmosphere, contains dissolved gases and particles, including sodium chloride. When rainwater reaches the ground, it can dissolve minerals from rocks and soil, including sodium chloride. This dissolved salt then enters rivers and streams, eventually making its way to the ocean. Over time, the evaporation of water from the ocean surface leaves behind the salt, increasing the salinity of the water.

According to the National Oceanic and Atmospheric Administration (NOAA), the average salinity of seawater is about 35 parts per thousand (ppt), which is equivalent to 3.5% by weight. This salinity is primarily due to the presence of sodium chloride, although other salts, such as magnesium sulfate and potassium chloride, also contribute to the overall salinity of seawater.

The salinity of freshwater bodies, such as lakes and rivers, is generally much lower than that of seawater. This is because freshwater sources are replenished by rainfall and groundwater, which have lower concentrations of dissolved salts. However, in some cases, such as in the Great Salt Lake in the United States, natural processes can lead to the accumulation of salts, resulting in high salinity levels.

In summary, sodium chloride is the primary substance that causes water to be salty. Its presence in seawater is a result of natural processes, including the evaporation of water and the dissolution of salts from rocks and soil. The salinity of freshwater bodies is generally lower, but can be elevated in certain environments due to natural processes.