Alkali compounds are a type of base that dissolves in water; many types of bases do not dissolve in water, and so they are not alkali. Any type of base reacts with acid and neutralizes it. Most alkali compounds contain a hydroxyl group, which is a negative ion containing bonded oxygen and hydrogen.
Bases and alkali substances are very common in both the natural environment and in daily use. Most toothpastes contain mild alkali substances, and baking soda is another alkali substance commonly encountered. Ammonia is an exception to the common pattern for alkali substances, as it is a water-soluble base without a hydroxyl group. Copper oxide is a relatively common base that is not water soluble, and thus it is not an alkali.
Any basic substance has a power of hydrogen, or pH, above 7.0, the pH of pure water. Acidic substances have a pH of below 7.0. Basic substances are basic because they are capable of accepting hydrogen ions, and hydrogen ions are what give acids their caustic properties. In the case of many alkali substances, the hydrogen ions bond with hydroxyl ions to create water. The non-hydroxyl portions of the bases often form salts with the remnants of the acids. In the case of ammonia, the hydrogen ion is accepted to form ammonium.