is the most abundant substance on the surface of our planet and is essential for all life. Water in rivers, lakes and the oceans is by the heat of the sun. The water vapour formed rises into the and forms clouds. The clouds cool as they rise further, to produce rain which flows down streams and rivers back to the sea. This is known as the water . Drinking water is treated by passing river water through filter beds made of to remove particles and then chlorine is added to kill .

is an important raw material and has many uses. It is used as a solvent and as a coolant. It is also used in many industrial processes including the manufacture of sulphuric . Many compounds dissolve in water producing solutions while many other compounds react with water to give acids or .

water readily forms a lather with . Hard water does not easily form with . Hard water contains dissolved compounds that react with soap to form scum. Most hard water contains dissolved or compounds. The hard water is formed when water flows over ground or rocks containing calcium or magnesium compounds which dissolve.
Hard water contains dissolved compounds that are good for health. Hard water often provides calcium compounds that help the development of strong and teeth and help to reduce attacks. Using hard water can increase costs because more soap is needed to clean effectively and hard water often leads to deposits (scale) forming in heating systems or kettles. Hard water can be made by the dissolved calcium and magnesium ions. The addition of sodium carbonate solution precipitates out calcium or magnesium carbonate. Ion columns contain hydrogen ions or sodium ions which replace and magnesium ions when hard water passes down the column. Distillation (which involves first evaporation then ) can also be used to make hard water soft, but this method is very expensive due to the amount of fuel required.

Many gases are in water. The solubility as the temperature decreases and as the increases. Carbonated water is produced by dissolving under high pressure. When the pressure is released the gas bubbles out of the . water is used in fizzy drinks. Dissolved oxygen is essential for aquatic life. Hot water from stations may be discharged into rivers or lakes. This discharge reduces the amount of dissolved in the water and this can kill fish and other animals.

Chlorine water is made by dissolving gas in water. Chlorine water is used to materials such as paper or cotton and kill bacteria. The use of artificial fertilisers results in many natural waters being contaminated with dissolved nitrate and ammonium ions. Dissolved nitrate ions can have harmful effects on babies and so the levels of nitrate are carefully monitored. Most compounds are soluble in . Most compounds are insoluble in water. The of a solute in water, or any other solvent, is usually given in grams of solute per grams of water (or solvent) at that temperature. The solubility of most solutes as the temperature increases. A solution is one in which no more solute will dissolve at that temperature. When a hot saturated solution cools some of the solute will from the solution.