The difference between ferric and ferrous iron is that ferric iron is in a plus-3 oxidation state, while ferrous iron is in a plus-2 oxidation state. This means that ferric iron needs to share three electrons with an oxygen molecule to make the ion neutral, while ferrous iron only needs two more electrons. With the addition of oxygen, the ferrous ion can easily become a ferric ion.
Iron is one of the most abundant elements on Earth, and is also fairly reactive with acids. When exposed to the air, iron will rust — becoming iron (III) oxide. Ferrous iron is soluble in water, no mater the pH level. Because of this, water containing ferrous iron is usually clear. Water containing ferric iron, however, will often have a reddish tint or cloudy appearance. Though iron is important to health in humans, ferrous iron, when dissolved in water supplies, can cause it to have an unpleasant, bitter taste.
Ferric ions, when combined with certain compounds, can create a color known as Prussian blue. This is sometimes added to laundry detergent or used as a dye to make blueprints. When added to a solution, ferrous ions usually give the solution a yellow or brownish color.