Water of crystallization degree of hydration can be critical to the resulting chemical properties. For example, anhydrous RhCl is not soluble in water and is relatively useless in organometallic chemistry whereas RhCl•3HO is versatile. Similarly, hydrated AlCl is a poor Lewis acid and thus inactive as a catalyst for Friedel-Crafts reactions. Samples of AlCl must therefore be protected from atmospheric moisture to preclude the formation of hydrates. Crystals of hydrated copper(II) sulfate consist of [Cu(HO)] centers linked to SO ions. Copper is surrounded by six oxygen atoms, provided by two different sulfate groups and four molecules of water. A fifth water resides elsewhere
Color of water cosmetic, bacteriological and chemical problems easier. The presence of color in water does not necessarily indicate that the water is not drinkable. Color-causing substances such as tannins may be harmless. Color is not removed by typical water filters; however, slow sand filters can remove color, and the use of coagulants may also succeed in trapping the color-causing compounds within the resulting precipitate. Other factors can affect the color seen: Various cultures divide the semantic field of colors differently from the English language usage and some do not distinguish between blue and green in the same way. An example is Welsh