Difference Between Acidic and Basic Oxides

Main Difference – Acidic vs Basic Oxides

An oxide is any chemical compound that contains one or more oxygen atoms. Oxides can be either acidic or basic depending on their chemical composition, reactions, and the pH. Acidic oxides react with water, forming an acidic solution. They can react with a base to form a salt. Basic oxides react with water, forming a basic solution and they can react with an acid to form a salt. Acidic oxides have low pH where basic oxides have a high pH. However, the main difference between acidic oxides and basic oxides is that acid oxides form acids when dissolved in water where basic oxides form bases when dissolved in water.

Key Areas Covered

1. What is an Acidic Oxide
     – Definition, Chemical Properties, Nonmetal Oxides, Examples
2. What is a Basic Oxide
    – Definition, Chemical Properties, Metal Oxides, Examples
3. What is the Difference Between Acidic and Basic Oxides
    – Comparison of Key Differences

Key Terms: Acid, Acid Anhydrides, Acidic Oxide, Base, Base Anhydrides, Basic Oxide, Nonmetal Oxide, Metal Oxide, Oxide, pH, Salt

What is an Acidic Oxide

Acidic oxides are compounds that can form an acidic solution when dissolved in water. Acidic oxides are formed when a non-metal reacts with oxygen. Sometimes acidic oxides are formed when metals (with higher oxidation states) react with oxygen as well. Acidic oxides react with water and produce aqueous acids.

Acidic oxides are categorized as acid anhydrides. That is because they produce the acid compound of that oxide when dissolved in water. For example, sulfur dioxide is called sulfurous anhydride and sulfur trioxide is called sulfuric anhydride. Acid oxides can react with a base to produce its salt.

Usually, acid oxides have low melting points and low boiling points except for oxides like silicon dioxide which tend to form giant molecules. These oxides will dissolve in bases and form a salt and water. When an acidic oxide is dissolved in water, it will decrease the pH of the water sample due to the formation of H+ ions. Some common examples for acidic oxides are CO2, P2O5, NO2, SO3, etc.

Figure 1: SO3 is a Nonmetal Oxide (an Acidic Oxide)

Nonmetal Oxides

Nonmetal oxides are oxide compounds formed by nonmetal elements. Most p block elements are nonmetals. They form various oxide compounds. Nonmetal oxides are covalent compounds since they share electrons with oxygen atoms to form oxide molecules. Most nonmetal oxides give acids after reacting with water. Therefore, nonmetal oxides are acidic compounds. For example, when SO3 is dissolved in water, it gives H2SO4 solution, which is highly acidic. Nonmetal oxides react with bases to form salts.

What is a Basic Oxide

Basic oxides, also called base anhydrides, are compounds that can form a basic solution when dissolved in water. Basic oxides are formed as a result of the reaction of oxygen with metals. Due to the difference in electronegativity between oxygen and metals, most basic oxides are ionic in nature. Thus, they have ionic bonds between atoms.

Basic oxides react with water actively, producing basic compounds. These oxides react with acids and form a salt and water. When a basic oxide is added to water, the pH of water increases due to the formation of hydroxyl ions (OH–). Some examples of common basic oxides are, Na2O, CaO, MgO, etc. Therefore, metal oxides are mainly basic oxides.

Figure 2: MgO is a Basic Oxide. It is a Metal Oxide.

Metal Oxides

Metal oxides are chemical compounds containing a metal and one or more oxygen atoms. Here, the oxidation number of oxygen is -2, and it is essentially the anion whereas metal is the cation. Alkali metals (group 1 elements), alkaline earth metals (group 2 elements) and transition metals (some d block elements) form basic oxides. But metals showing high oxidation states can form oxides with a covalent nature. They are more acidic than being basic.

The number of oxygen atoms that bind with the metal ion depends on the oxidation number of the metal ion. Alkali metals form only monovalent cations. Therefore, they form only M2O type oxides (where M is the metal ion and O is oxide anion). Alkaline earth metals form divalent cations. Therefore, they form MO type oxides. These compounds are basic.

Difference Between Acidic and Basic Oxides

Definition

Acidic Oxides:  Acidic oxides are compounds that can form an acidic solution when dissolved in water.

Basic Oxides: Basic oxides are compounds that can form a basic solution when dissolved in water.

Formation

Acidic Oxides: Acidic oxides are formed when oxygen reacts with non-metals.

Basic Oxides: Basic oxides are formed when oxygen reacts with metals.

Reaction with Water

Acidic Oxides: Acidic oxides react with water forming acidic compounds.

Basic Oxides: Basic oxides react with water forming basic compounds.

Reaction with Acids

Acidic Oxides: Acidic oxides do not react with acids.

Basic Oxides: Basic oxides react with acids forming a salt.

Reaction with Bases

Acidic Oxides: Acidic oxides react with bases forming a salt.

Basic Oxides: Basic oxides does not react with bases.

Bonds

Acidic Oxides: Acidic oxides have covalent bonds.

Basic Oxides: Basic oxides have ionic bonds.

Effect on pH

Acidic Oxides: When acidic oxides are dissolved in water it decreases the pH.

Basic Oxides: Dissolving of basic oxides in water cause increasing the pH.

Other Names

Acidic Oxides: Acidic oxides are also known as acid anhydrides.

Basic Oxides: Basic oxides are also called as base anhydrides.

Conclusion

Oxides are compounds having at least one oxygen atom bonded to another element. This element can be a metal or a non-metal. Oxides can be acidic or basic according to their properties. If a particular oxide can react with an acid but not with a base, it is called a basic oxide. If an oxide reacts with a base but not with acids, it is an acidic oxide. The key difference between acidic and basic oxides is that acid oxides form acids when dissolved in water whereas basic oxides form bases when dissolved in water.

Reference:

1. “Acidic oxide.” Wikipedia, Wikimedia Foundation, 29 Dec. 2017, Available here.
2. Libretexts. “Oxides.” Chemistry LibreTexts, Libretexts, 23 Aug. 2017, Available here.

Image Courtesy:

1. “SO3 Sulfur trioxide” By Yikrazuul – Own work (Public Domain) via Commons Wikimedia
2. “Magnesium oxide” By Walkerma assumed – Own work assumed (based on copyright claims) (Public Domain) via Commons Wikimedia

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