Saline water is water that contains a significant concentration of dissolved salts (mainly NaCl) and is commonly known as salt water. The salt concentration is usually expressed in parts per thousand (permille, ‰) or parts per million (ppm). The United States Geological Survey classifies saline water in three salinity categories. Salt concentration in slightly saline water is around 1,000 to 3,000 ppm (0.1–0.3%), in moderately saline water 3,000 to 10,000 ppm (0.3–1%) and in highly saline water 10,000 to 35,000 ppm (1–3.5%). Seawater has a salinity of roughly 35,000 ppm, equivalent to 35 grams of salt per one liter (or kilogram) of water. The saturation level is dependent on the temperature of the water. At 20 °C one milliliter of water can dissolve about 0.357 grams of salt; a concentration of 26.3%. At boiling (100 °C) the amount that can be dissolved in one milliliter of water increases to about 0.391 grams or 28.1% saline solution.