The Basics

• 
  
  When a salt is added to water, it dissolves into its component molecules until as many salt ions as the water can hold are floating around the hydrogen and oxygen molecules. When this happens, the solution is "saturated." As more salt is dissolved, sodium and chlorine ions bump into each other and re-combine into crystals of salt. This event is called "precipitation" because the solid that is formed falls to the bottom of the water. Salts are "hydrophilic," meaning they are attracted to water. This attraction facilitates a more familiar type of precipitation; raindrops form around minute salt crystals in clouds, giving rain its slightly salty taste.
  
  

Electrolytes

• 
  
  Salt doesn't conduct electricity particularly well in its solid state, but in a molten (liquid) state, salt forms a solution that can conduct electricity quite well. Actual conductivity depends on which salt is used, but these liquids, called electrolytic solutions, are convenient carriers for compounds needed by the human organism. Electrical impulses generated by the brain produce free oxygen, which is used by the blood. The salt molecules are separated into electrolytes (sodium, potassium or calcium) and chlorides in solution: the chlorine is vented through the blood and kidneys into the urine and the electrolytes are distributed throughout the neuro-muscular system in a process that athletes know as re-hydration.
  
  

Colligative Properties

• 
  
  Any solution differs from its original composition because the addition of molecules, even when they don't form new compounds, changes the molecular weight of the liquid and affects its properties. Salt water is denser than fresh water and freezes more slowly. As the water freezes, though, the salt migrates to the surrounding liquid, making it heavier, more saturated, further lowering its freezing point. At the other end of the matter spectrum, the denser salt water requires more heat to transform to a gas but again leaves most of the salt behind as it evaporates, making a denser liquid. As the concentration of salt increases in the liquid, it requires more heat to evaporate until finally the water is gone, leaving a layer of salt in a red-hot pan. Cooks wait to add salt to a kettle until the water is actively boiling to let heat build up in the water so the salt doesn't stop the boil. These qualities, called "colligative properties," help scientists determine the molecular weight of salt water. If the mass of the liquid, the atmospheric pressure, and the liquid's boiling point can be established, scientists can subtract the molecular weight of water to find out which salts are present.