The oxides of Group I and II metals (Alkali metals and Alkali earth metals) are called base anhydrides. They are obtained by removing water from the corresponding hydroxide salt. If water is added to a base anhydride, a corresponding hydroxide base can be re-formed. A base anhydride is neither an Arrhenius base, nor a Brønsted–Lowry Base, since it does not accept protons and do not increase the hydroxide ion concentration of water. However, a base anhydride is a Lewis Base, since it will share an electron pair with some Lewis acids, most notably acidic oxides.
- Calcium Oxide is the anhydride of Calcium hydroxide
- Sodium oxide is the anhydride of Sodium hydroxide
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