Non metalic oxides react with water to give acidic oxides, they give hydronium ions (H3O) in aqueous solution.
But not all metallic oxides are acidic, such as CO.
An example that explains the acidic nature of non metalic oxides is, SO3. 

Non-metal oxides can be classified as either acidic or neutral. This means that they do not have the ability to exhibit basic properties. Let's delve into the reasons behind this classification.

Acidic Nature of Non-Metal Oxides:
Non-metal oxides generally form acidic solutions when dissolved in water. This is due to their reaction with water, which leads to the formation of acids. The acidic nature of non-metal oxides can be attributed to the presence of non-metallic elements in their composition. These elements tend to gain electrons and form negative ions, resulting in the generation of acidic properties.

Explanation of the Acidic Nature:
When non-metal oxides react with water, they undergo a chemical reaction known as hydrolysis. This process involves the exchange of ions between the non-metal oxide and water molecules. The non-metal oxide reacts with water to form an acid. For example, nitrogen dioxide (NO2) combines with water to produce nitric acid (HNO3):

NO2 + H2O → HNO3

Similarly, sulfur dioxide (SO2) reacts with water to form sulfuric acid (H2SO4):

SO2 + H2O → H2SO4

These reactions demonstrate the acidic nature of non-metal oxides. They release hydrogen ions (H+) when dissolved in water, which gives rise to an acidic solution.

Examples of Acidic Non-Metal Oxides:
1. Carbon dioxide (CO2): When carbon dioxide dissolves in water, it forms carbonic acid (H2CO3).
2. Nitrogen dioxide (NO2): Nitrogen dioxide reacts with water to produce nitric acid (HNO3).
3. Sulfur dioxide (SO2): Sulfur dioxide combines with water to form sulfuric acid (H2SO4).

Conclusion:
Non-metal oxides are classified as acidic because they react with water to produce acids. This is due to the presence of non-metallic elements in their composition, which tend to gain electrons and form negative ions. Examples of acidic non-metal oxides include carbon dioxide, nitrogen dioxide, and sulfur dioxide.