Common salt, also referred to as table salt or by its chemical formula NaCl, is an ionic compound of sodium and chloride ions. All life depends on its chemical properties to survive. Human have used it in food for thousands of years, for food preservation and seasoning. Salt's ability to preserve food was a founding contributor to the development of civilization. It helped eliminate dependence on seasonal availability of food, and made it possible to transport food over large distances. However, salt was often difficult to obtain, so it was a highly valued trade item, and used as currency by some societies. Many salt roads, such as the Via Salaria in Italy, had been established by the Bronze Age.