Video Lesson

Lesson Objectives

At the end of this lesson, you will be able to:

• Define inorganic compounds;
• Classify inorganic compounds as oxides, acids, bases, and salts.
• Define and classify oxides as acidic, basic, neutral, amphoteric, and peroxides
• Explain the chemical properties of acidic oxides, basic oxides, amphoteric oxides, neutral oxides, and peroxides.

Brainstorming Question

• 1. What is the basis for classifying compounds as organic and inorganic?
• 2. What are the sources of inorganic compounds?
• 3. How does oxygen exist in nature?
• 4. Which elements react with oxygen?
• 5. What do you call the product when oxygen reacts with an element?

Key terms

Lesson Overview

This lesson covers the fundamental nature, classification, and chemical behavior of the 5 types of oxides, providing examples for each and highlighting their importance and applications.

1.1 Introduction

The term inorganic compound refers to all compounds that do not contain carbon except simpler compounds of carbon like oxides (eg CO₂), carbonates (eg CaCO₃) and carbides (eg SiC). Inorganic compounds are compounds consisting of mineral constituents of the earth or generally found in nonliving things. Compounds found in living things are generally organic compounds (see unit 6 for the details).

The majority of metal compounds are inorganic. Inorganic compounds are mostly found in nature as silicates, oxides, carbonates, sulphides, sulphates, chlorides, nitrates, etc. Inorganic compounds are generally classified into four groups namely oxides, acids, bases and salts.

We are familiar with the utility and importance of oxides, acids, bases, and salts in our daily life. For example, we know that the carbon dioxide (CO₂) gas in CO₂-fire extinguishers is an oxide, the sour taste of vinegar is due to acetic acid (CH3COOH), sodium hydroxide (NaOH) is a base used in the preparation of soaps, and sodium chloride (NaCl) is common salt which is useful as a food ingredient. This unit deals with the chemical nature and formation of more oxides, acids, bases, and salts.

1.2 Oxides

Oxygen reacts directly with almost all elements except the noble gases and inactive metals like gold, platinum, and palladium. Such compounds of oxygen are called oxides. Oxides are binary compounds containing oxygen and any other element (metal, non-metal or metalloid). Note that binary compounds are those consisting of only two elements.

               Oxygen + Element (Metal, Non-metal or Metalloid) → Oxide

Based on their properties Oxides are classified as:

A. Acidic oxides

B. Basic oxides

C. Amphoteric oxides

D. Neutral oxides

E. Peroxides

A. Acidic oxides

Acidic oxides are the oxides formed by the chemical combination of oxygen with non-metals.

These oxides are also called acid anhydrides since they form acidic solutions when reacted or dissolved in water. Acid anhydride means acid without water. Generally speaking, acidic oxides are non-metal oxides.

Examples of acidic oxides include carbon dioxide, CO₂, nitrogen dioxide, NO₂, and Sulphur dioxide, SO₂. However, it is very important to note that all non-metal oxides are not necessarily acidic oxides. For example, carbon monoxide, CO, and di-nitrogen monoxide, N₂O, are non-metal oxides, but they are neutral oxides which will be discussed later .

There are acidic oxides that do not react directly with water. The acids of such oxides are formed by other methods. For example; SiO₂ is not soluble in water, but it neutralizes basic oxides, thus it is acidic.

                              SiO₂ + Na₂O → Na₂SiO₃ (sodium silicate)

The acid of SiO₂ (H₂SiO₃) is prepared by the reaction of HCl with Na₂SiO₃ as shown below:

                Na₂SiO₃ (aq) + HCl (aq) → H₂SiO₃ (aq) + 2NaCl (aq)

Chemical properties of acidic oxides

1. Acidic oxides (acid anhydrides) dissolve in water to form an acidic solution (acid).

                          Acid anhydride + Water → Acid

                       CO₂ + H₂O → H₂CO₃ (Carbonic acid)

                       SO₂ + H₂O → H₂SO₃ (Sulfurous acid)

2. Acidic oxides react with basic or metallic oxides to form salt.

                          Acidic oxide + Basic oxide → Salt

                      CO₂ + Na₂O → Na₂CO₃ (sodium carbonate)

                      SO₃ + CaO → CaSO₄ (calcium sulphate)

3. Acidic oxides react with bases to form salt and water. This reaction is called the neutralization reaction.

                        Acidic oxide + Base → Salt + Water

                          SO₂ + 2NaOH → Na₂SO₃ + H₂O

                          CO₂ + 2LiOH → Li₂CO₃ + H₂O

B. Basic Oxides

Basic oxides are oxides that are composed of metals and oxygen. Most metals form oxides which exhibit basic properties and dissolve in water to give alkaline solutions. However, all metal oxides are not necessarily basic oxides; for example, Al₂O₃ and ZnO are amphoteric oxides, which will be discussed in part (C).

Oxides of metals that dissolve in water and react with it to form basic or alkaline solutions are called basic anhydrides. There are metallic oxides which have basic properties but are insoluble in water. These oxides react with acids to give salt and water

                     Example:  FeO + 2HCl → FeCl₂ + H₂O

The oxides of active metals, group IA and heavier members of group IIA, dissolve in water and readily form bases. The term “base” is used to describe both soluble and insoluble basic oxides. Some examples of basic oxides are Li₂O, Na₂O, K₂O, MgO, CaO, BaO, and CuO.

Chemical Properties of Basic Oxides

1. Basic oxides dissolve in water to form alkaline solutions. As they dissolve, they react with water to form the corresponding metal hydroxides.

                   Basic oxide + Water → Base (Alkali)

                 Examples:   Li₂O + H₂O → 2LiOH

                                     CaO + H₂O → Ca(OH)₂

2. Basic oxides react with acidic oxides to form salts.

                                Basic oxide + Acidic oxide → Salt

                  Examples:   BaO + SO₃ → BaSO₄

                                     Na₂O + CO₂→ Na₂CO₃

3. Basic oxides react with acids to form salt and water.

                                Basic oxide + Acid → Salt + Water

                Examples:   CaO + 2HCl → CaCl₂ + H₂O

                                 CuO + H₂SO₄ → CuSO₄ + H₂O

Indicator	Color in aqueous solution of acidic oxide	Color in aqueous solution of basic oxide
Universal Indicator	Yellow-orange (in weakly acidic) and red (in strongly acidic)	Blue (in weakly basic) and purple (in strongly basic)
Litmus	Red	Blue
Phenolphthalein	Color less	Pink (red)
Methyl orange	Red	Yellow

Note that acidic and basic oxides can be identified either by their effects on indicators or the chemical interaction they have with one another. Acidic oxides react with bases while basic oxides react with acids. But acidic oxides do not react with acids and basic oxides do not react with bases.

C. Amphoteric Oxides

An oxide that shows both acidic and basic behavior is called amphoteric oxide.

                   Amphoteric oxide  +  Acid    →     Salt + Water 

                   Amphoteric oxide  +  Base    →     Salt + Water

For example, an oxide of aluminum (Al₂O₃) is amphoteric. This oxide reacts with strong bases as well as strong acids.

        Reactions showing the amphoteric nature of aluminum oxide (Al₂O₃)

                     Al₂O₃  +   HCl  →  AlCl₃ + H₂O

                    Al₂O₃  +   NaOH  →   NaAlO₂ + H₂O

Some other examples of amphoteric oxides are ZnO, PbO, PbO₂, SnO, and SnO₂. It is also important to realize that hydroxides that react with both acids and bases are described as amphoteric hydroxides. For example, aluminum hydroxide, Al(OH)₃, reacts with both acids and bases to form salt and water. So, Al(OH)₃, is amphoteric in nature.

What is the common characteristic of acidic, basic, and amphoteric oxides?

Acidic oxides form salts when reacted with basic oxides and bases. Basic oxides also produce salts in their reactions with acidic oxides and acids. Amphoteric oxides form salts when they react with acids and bases. Thus, acidic oxides, basic oxides, and amphoteric oxides are salt-forming oxides.

D. Neutral Oxides

Neutral oxides react neither with acids nor with bases to form salt and water. Hence, neutral oxides do not show basic and acidic properties. Examples of neutral oxides are water, H2O, carbon monoxide, CO, dinitrogen monoxide, N2O, and nitrogen monoxide, NO. Neutral oxides are very few in number. Neutral oxides are not salt-forming oxides.

E. Peroxides

Compounds that contain oxygen with oxidation number -1 are called peroxides. In peroxides, the two oxygen atoms are linked to each other and with atoms of other elements. Thus, peroxides contain the peroxide, “– O – O –” link. Some examples of peroxides are hydrogen peroxide, H₂O₂, sodium peroxide, Na₂O₂, calcium peroxide, CaO₂, barium peroxide, BaO₂, and strontium peroxide, SrO₂.

Most peroxides of metals are formed by burning the metals in a sufficient amount of oxygen.

Hydrogen Peroxide

Hydrogen peroxide, H₂O₂, is a colorless liquid whose solutions are usually used as bleach and an antiseptic. Hydrogen peroxide decomposes to release oxygen. This reaction is slow but can be speed up by the addition of catalysts like MnO₂ or Pt.

                                     2H₂O₂ (aq) → 2H₂O (l) + O₂ (g)

Hydrogen peroxide is a strong oxidizing agent. Its oxidizing power is responsible for its effectiveness as an antiseptic for mouthwash and cleansing wounds. When hydrogen peroxide is added to a colored dye, the molecule responsible for the color will oxidize and so the color will disappear, showing its bleaching action. For example, if hydrogen peroxide is added to a black dye (paint) that contains lead sulphide, PbS, the black color turns white. This is due to the oxidation of PbS to PbSO₄. The equation for this process is:

                               PbS (s) + 4H₂O₂ → PbSO₄ + 4H₂O (l)