Oxides are binary compounds of oxygen with another element, e.g., \(CO_2 \), \( SO_2 \), CaO, CO, ZnO, \(H_2O \), etc. These are termed as oxides because here, oxygen is in combination with only one element. Based on their acid-base characteristics oxides are classified as acidic, basic, amphoteric or neutral:

1. Neutral oxide:

   These are oxides which are neutral to litmus. They are neither acidic nor basic. They do not readily react with acids or bases under standard conditions. Examples are Carbon monoxide (CO), water \( H_2O \), Nitrogen (II) oxide, \( NO \), dinitrogen (I) oxide, \( N_2O \) .

2. Basic Oxides:

   These are oxides formed when oxygen reacts with metals. They react with acids to form salts and water. They typically contain metal cations.

   $$ 4Na + O_2 \rightarrow 2Na_2O $$ $$ 2Ca + O_2 \rightarrow 2CaO $$

   Basic oxides combine with acids to form salt and water only. They are called basic anhydrides.

   $$ CaO + 2HCl \rightarrow CaCl_2 + H_2O $$ $$ Na_2O + 2HCl \rightarrow 2Nacl + H_2O $$

   Oxides of sodium, calcium, barium potassium dissolve in water to yield alkalis (soluble hydroxides)

   $$ Na_2O + H_2O \rightarrow 2NaOH $$ $$ K_2O + H_2O \rightarrow 2KOH $$
3. Acidic Oxides:

   These are oxides formed when oxygen reacts with non-metals. They are called acid anhydrides because they dissolve in water to form acids. $$ CO_2 + H_2O \rightarrow H_2CO_3 $$ $$ SO_2 + H_2O \rightarrow H_2SO_3 $$ $$ NO_2 + H_2O \rightarrow HNO_3 $$

   Acidic oxides combine with alkalis to form salt and water only.

   $$ CO_2 + 2NaOH \rightarrow Na_2CO_3 + H_2O $$ $$ SO_2 + 2NaOH \rightarrow Na_2SO_4 + H_2O $$ $$ NO_2 + 2NaOH \rightarrow NaNO_3 + NaNO_2 + H_2O $$

   Note: \( NO_2 \) is mixed anhydrous because it produces two salts.

4. Amphoteric Oxides:

   They oxides of metals can react with both acids and bases. They often contain elements capable of accepting or donating protons. Oxides of lead, tin, aluminum and zinc are amphoteric in nature.

   They behave as basic oxides with acids to form salt and water only.

   $$ ZnO + H_2SO_4 \rightarrow ZnSO_4 + H_2O $$ $$ PbO + 2HCl \rightarrow PbCl_2 + H_2O $$

   They also behave as acidic oxides with alkalis to form salt and water only. $$ ZnO + NaOH + H_2O \rightarrow Na_2Zn(OH)_4 $$ $$ Al_2O_3 + 2NaOH + 3H_2O \rightarrow 2NaAl(OH)_4 $$

5. Higher oxides: These are oxides which a higher proportion of oxygen as compared to other oxides. They are classified into:
   1. Peroxides:

      A peroxide is a metallic oxide which gives hydrogen peroxide by the action of dilute acids. They contain more oxygen than the corresponding basic oxide, e.g., sodium, calcium and barium peroxides. $$ K_2O_2 + 2HCl \rightarrow 2KCl + H_2O_2 $$ $$ Ba_2O_2 + H_2SO_4 \rightarrow Baso4 + H_2O_2 $$

   2. Dioxides:

      These oxides do not produce acids on reacting with dilute acids. They decompose on hearing to yield oxygen. $$ 2PbO_2 \rightarrow 2PbO + O_2 $$

   3. Superoxides:

      Often Potassium, Rubidium, and Cesium react with excess oxygen to produce the superoxide, \(MO_2 \) . with the oxidation number of the oxygen equal to -1/2. $$ Cs + O_2 \rightarrow CsO_2 $$

   4. Compound oxides:

      Compound oxides are metallic oxides that behave as if they are made up of two oxides, one that has a lower oxidation and one with a higher oxidation of the same metal, e.g., $$ Pb_3O_4 \rightarrow PbO_2 + PbO $$

1. Direct combination of elements: Many metals and nonmetals burn in oxygen or air to yield oxides. Examples of such elements include: carbon, hydrogen, calcium, sodium, etc $$ 2Ca + O_2 \rightarrow 2CaO $$ $$ 2Na + O_2 \rightarrow Na_2O $$ $$ C + O_2 \rightarrow CO_2 $$
2. Oxidation of elements: Trioxonitrate (V) acid oxidizes some metals like Copper and non-metals like carbon to produce oxides. $$ C + 4HNO_3 \rightarrow CO_2 + 4NO_2 + 2H_2O $$ $$ 3Cu + 8HNO_3 \rightarrow 3Cu(NO_3)_2 + 4H_2O + 2NO $$
3. Oxidation of sulphides in the presence of heat : Sulphides like zinc sulphide and lead sulpide are oxidized by heating to yield corresponding oxides. $$ 2PbS + 3O_2 \stackrel{\Delta}{\rightarrow} 2pbO + 2SO_2 $$ $$ 2ZnS + 3O_2 \stackrel{\Delta}{\rightarrow} 2ZnO + 2SO_2 $$
4. Combustion of organic compounds: Organic compounds burn in air or oxygen to yield carbon (IV) oxide.
$$ C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O $$
5. Thermal decomposition of compounds: Certain compounds like nitrates, carbonates and hydroxides decompose on heating to yield oxides. $$ CaCO_3 \stackrel{\Delta}{\rightarrow}\ CaO + CO_2 $$ $$ Cu(OH)_2 \stackrel{\Delta}{\rightarrow} CuO + H_2O $$