In geography, the term "solution" describes the process of rock formations, such as limestone, being dissolved in a river setting. The chemical process that occurs is one in which carbon dioxide in the presence of water forms carbonic acid that dissolves limestone.
The limestone, carbon dioxide and water are all said to be in solution at that point when the limestone is dissolved. For a river to dissolve limestone, generally the water must be naturally acidic on the pH scale.
In other physical and life science applications, a solution is a homogeneous molecular mixture of two or more nonreacting substances; for example, sodium chloride, better known as table salt, dissolves in water to form a sodium chloride solution.