Short Answer

When an acid reacts with a metal oxide, it forms a salt and water. Metal oxides are generally basic, so they neutralize acids similar to a base in a neutralization reaction.

For example, hydrochloric acid reacts with copper(II) oxide to form copper(II) chloride and water:
CuO + 2HCl → CuCl₂ + H₂O
This reaction is important in chemistry for preparing salts and for neutralization of acidic solutions.

Detailed Explanation :

Reaction of Acid with Metal Oxide

Metal oxides are generally basic in nature. When these oxides react with acids, they undergo a neutralization reaction forming a salt and water. Unlike metals, metal oxides contain oxygen and metal ions, which allow them to react directly with acids without releasing hydrogen gas.

The general equation is:

Metal Oxide + Acid → Salt + Water

This is a type of acid-base reaction where the oxide acts as a base.

Examples of Acid-Metal Oxide Reactions

1. Copper(II) oxide + Hydrochloric acid
   CuO + 2HCl → CuCl₂ + H₂O

• Produces copper(II) chloride (salt) and water.

1. Magnesium oxide + Sulfuric acid
   MgO + H₂SO₄ → MgSO₄ + H₂O

• Produces magnesium sulfate (salt) and water.

1. Calcium oxide + Nitric acid
   CaO + 2HNO₃ → Ca(NO₃)₂ + H₂O

• Produces calcium nitrate (salt) and water.

1. Aluminium oxide + Hydrochloric acid
   Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O

• Produces aluminium chloride and water.

Mechanism of Reaction

1. Metal oxide acts as a base and reacts with H⁺ ions from the acid.
2. The basic oxide neutralizes the acidic solution, forming water.
3. Remaining metal cations combine with the anions of the acid to form a salt.

Example:

• CuO + 2HCl → Cu²⁺ + 2Cl⁻ + H₂O
• The hydrogen ions neutralize the oxide, forming water.

Factors Affecting the Reaction

1. Strength of Acid

• Strong acids (HCl, H₂SO₄, HNO₃) react faster with metal oxides.
• Weak acids (CH₃COOH) react slowly.

1. Nature of Metal Oxide

• Oxides of highly reactive metals (MgO, CaO) react vigorously.
• Oxides of less reactive metals (CuO, PbO) react more slowly.

1. Concentration of Acid

• Higher acid concentration increases the reaction rate and salt formation.

1. Temperature

• Higher temperatures increase kinetic energy → faster reaction.

Observations During Reaction

1. Dissolving of metal oxide in acid.
2. Heat evolution – reaction is exothermic.
3. Formation of salt – solution becomes clear as solid oxide dissolves.
4. No gas evolution – unlike acid-metal reactions, hydrogen gas is not produced.

Applications of Acid-Metal Oxide Reactions

1. Salt Preparation

• This reaction is widely used to prepare salts such as MgSO₄, CuCl₂, and Ca(NO₃)₂ in laboratories and industries.

2. Neutralization of Acidic Waste

• Metal oxides can be used to neutralize acidic industrial effluents, making them safe for disposal.

3. Fertilizer Industry

• Metal oxides react with acids to produce salts used as fertilizers, such as calcium nitrate or magnesium sulfate.

4. Laboratory Demonstrations

• This reaction demonstrates acid-base neutralization and exothermic reaction.

Comparison with Other Acid Reactions

Reaction Type	Products	Gas Evolution?
Acid + Metal	Salt + H₂	Yes
Acid + Metal Oxide	Salt + H₂O	No
Acid + Metal Carbonate	Salt + H₂O + CO₂	CO₂ gas

This shows that metal oxides behave like bases, neutralizing acids to produce salt and water only.

Conclusion

When an acid reacts with a metal oxide, a salt and water are formed. The metal oxide acts as a base and neutralizes the acid without releasing hydrogen gas. This reaction is exothermic and widely used for salt preparation, industrial neutralization, fertilizer production, and laboratory demonstrations. The nature of the metal oxide, strength of the acid, concentration, and temperature affect the rate and completeness of the reaction.