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These compounds may be classified into three types : (a) basic oxides: (b) acidic oxides: and (c) amphoteric oxides, depending on their behavior towards water, acids and alkalies.

(a)    Basic Oxides.
They are formed by the combination of oxygen with highly electropositive metals. For example, alkali and alkaline earth metals form basic oxides. They are soluble in water and react vigorously to give alkaline solutions. They react with acids to form salts. They are ionic in nature with high melting and boiling points. A few of their examples include Li2O and MgO.

Na2O + H2O  →   2NaOH
MgO + 2HCI  →  MgCI2 + H2O

(b)    Acidic Oxides.
They are formed by the combination of oxygen with non-metals. They react with alkalis to form salts. They dissolve in water to give acidic solutions. These compounds are covalent in nature and are usually gaseous or volatile liquids or soluds. Some examples include B2O3, CO2, SiO2,SO2,SO3,N2O3 and P2O5.

N2O5 + H2O  →   2HNO3
CO2 + H2O    →    H2CO3

Basic and acidic oxides often combine to give salts

CO2    +    2KOH  →    K2CO3 + H2O
(acid)         (base)         (salt)

SiO2   +  Na2O    →     Na2SiO3
(acid)      (base)

(c)    Amphoteric Oxides.
are formed by weakly electropositive elements such as Be, Ai, Ga, Sn etc. They show basic as well as acidic behavior depending on reaction conditions, i.e., they react with bases as acidic oxides and with acids as basic oxides. Amphoteric oxides are mainly ionic in nature and have high melting and boiling points.

For example,

Acidic behaviour      AI2O3  +  2NaOH   →      2NaAIO2 + H2O
                                    (acid)         (base)              Sod.metaaluminate
Basic behaviour       AI2O  +  6 HCI      →      2AICI3 + 3H2O
                                     (base)         (acid)

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