Alkali
The word "alkali" is derived from Arabic "al qalīy" (or "alkali"), meaning "the calcined ashes" (see calcination), referring to the original source of alkaline substances. A water-extract of burned plant ashes, called potash and composed mostly of potassium carbonate, was mildly basic. After heating this substance with calcium hydroxide ("slaked lime"), a far more strongly basic substance known as "caustic potash" (potassium hydroxide) was produced. Caustic potash was traditionally used in conjunction with animal fats to produce soft soaps, one of the caustic processes that rendered soaps from fats in the process of saponification, one known since antiquity. Plant potash lent the name to the element potassium, which was first derived from caustic potash, and also gave potassium its chemical symbol K (from the German name Kalium), which ultimately derived from alkali.
Acidic oxide
Although it is difficult to classify these oxides as acids, the property is manifest in reactions with bases. For example, carbon dioxide reacts with alkali.
For this reason, alkali is kept in stoppered vessels to inhibit reaction with atmospheric carbon dioxide. In geochemistry complex silicates are often written as though they are the products of an acid-base reaction. For example, the chemical formula of the mineral olivine can be written either as (Mg,Fe)SiO or as (MgO,FeO)SiO. This mineral is said to be ultramafic, meaning that it has a very high nominal content of the bases magnesium oxide and iron oxide and hence, a low content of the acid silicon dioxide.Aluminium oxide (AlO) is an amphoteric oxide; it can act as a base or acid. For example with base different aluminate salts will be formed: