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Nature of Oxides: Introduction, Classification, Nature of Oxides, Acid-Base Behaviour Trends, Practice Problems, FAQs

Nature of Oxides: Introduction, Classification, Nature of Oxides, Acid-Base Behaviour Trends, Practice Problems, FAQs

Our earth has a surrounding atmosphere with a mixture of gases- mainly Nitrogen, oxygen with a small amount of carbon dioxide and other gases. Being electronegative, oxygen is more active and almost has formed with all elements on earth giving oxides..

You must be familiar with the gaseous molecule carbon dioxide as an exhaust gas from animal breathing, a pollutant from fuel burning greatly affecting the earth's environment as a greenhouse gas. On the other hand, carbon dioxide is much consumed by plants during photosynthesis. Carbon dioxide is an oxide, as its name would indicate.

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Likewise, oxygen can interact with both metals and nonmetals to generate oxides..

Numerous elements are readily oxidised in air or water. This explains why several oxides are found as mineral resources in nature. Oxides make up the majority of the solid crust of the planet. Since oxygen is a highly electronegative element and forms bonds with nearly all other elements, it naturally produces stable compounds called oxides.

But all the oxides do not have the same properties. Let us learn about these oxides' nature. following

Table of Content

  • Introduction of Oxides
  • Classification of Oxides
  • Nature of Oxides
  • Acid-Base Behaviour Trends
  • Practice Problems
  • Frequently Asked Questions(FAQs)

Introduction to Oxides

Oxygen combines with different elements according to their valencies and forms oxide compounds having a different number of oxygen atoms in the formula of the compound. Thus the formula of the oxides could be different. Oxygen being highly electronegative next only to fluorine gives a polar character to the oxides. The presence of two lone pairs of electrons adds to the reactivity of oxygen. Properties of the oxides are determined by both oxygen and the other atom combined with it.

Classification of Oxides:

Oxides are classified based on multiple characteristics like the number of oxygen atoms present in a molecule, acidic, basic and amphoteric character etc.

A) Based on the Number of Oxygen Atoms in the Chemical Formula


A chemical molecule known as an oxide comprises at least one oxygen atom and one additional element. O2- the oxygen dianion is also referred to as oxide. At least one dianion of oxygen is found in all oxide compounds.

Examples: Zinc oxide(ZnO), Carbon monoxide(CO), Calcium oxide(CaO), Magnesium oxide (MgO), Sodium oxide (Na2O) etc.


The name dioxide refers to an oxide that has two oxygen atoms that are each chemically bound to an atom of a different element.

Examples: Carbon dioxide(CO2), Sulphur dioxide (SO2) etc


The name trioxide refers to an oxide that has three oxygen atoms that are each chemically bound to an atom of a different element.

Examples: Sulphur trioxide(SO3), Chromium trioxide(CrO3), Tellurium trioxide(TeO3) etc.

B) Based on Valency of the other Constituent Element:

On the basis of the valency of the other elements present in the oxide, the following types of oxides can be distinguished.

1. Simple oxides
2. Mixed oxides

Simple oxides:

A single metal or semimetal and oxygen are the components of simple oxides. These oxides contain the maximum number of oxygen atoms permitted by the element's or metal's usual valency.

Examples: SiO2, MgO, CaO, H2O etc

Mixed or Compound Oxides:

When simple oxides interact further, mixed oxides are created. These mixed oxides may be made of two separate metals or the same metal (element).


  1. Red Lead is made when Lead dioxide (PbO2) and Lead monoxide (PbO) are mixed (Pb3O4).
  1. The mixed oxide of Ferro-ferric oxide or magnetic oxide (Fe3O4) is composed of ferric oxides (Fe2O3) and ferrous oxide (FeO).
  1. Iron oxide (FeO) and titanium oxide (TiO2) are combined to form ilmenite (FeTiO3).
  1. Strontium oxide (SrO) and Titanium oxide (TiO2) combine to form the compound strontium titanate (SrTiO3).

C) Based on Metallic or Nonmetallic Nature of the Combining Element:

Based on the metallic nature of the other element that combines with oxygen to form an oxide,, oxides can be classified into the following categories.

1. Metallic oxides
2. Nonmetallic oxides

Metallic Oxides:

Metal and oxygen join together to form metallic oxides. These are typically found as minerals in the natural world. These are created when metals get oxidised.

Examples: Na2O, BaO, ZnO, CaO, MgO, Fe3O4 etc.

Na(s)+O2(g)Na2O (s)

A metallic oxide known as sodium oxide is produced when oxygen molecules react with sodium metal.

The elements in the s-block are metals and they typically react with oxygen to create metallic oxides.

Nonmetallic Oxides:

Non-metal and oxygen mix to generate non-metallic oxides. Typically, these appear in nature as gases like carbon dioxide. These are created when non-metals oxidise.

Examples: CO2, SO2, CO, P2O5 etc.


Carbon dioxide is the name of the oxide of carbon that is produced when carbon is allowed to burn in the atmospheric oxygen.


Sulphur dioxide gas is produced when sulphur burns in the atmosphere.

Since the majority of the elements in the p-block are non-metallic, they typically mix with oxygen to create non-metallic oxides.

Nature of Oxides:

1. Acidic Oxide:

Acidic oxide is made when an oxide combines with water to produce an acid. So, when water and acidic oxide react, an acid is produced. This means that when a blue piece of litmus paper is dipped in an acidic oxide and water solution, it becomes red. Most acidic oxides have a non-metallic composition, but some metal oxides with high oxidation states can also be acidic. Therefore, some metallic oxides like, Mn2O7, CrO3 etc. are acidic.

Examples: Sulphur trioxide (SO3), Carbon dioxide (CO2) etc

1. Sulphur trioxide (SO3) dissolves in water to form acid called Sulphuric acid (H2SO4).

SO3(g) + H2O (l)  H2SO4 (aq)

2. Carbon dioxide (CO2) dissolves in water to form an acid called Carbonic acid (H2CO3).

CO2(g) + H2O (l)  H2CO3 (aq)

2. Dichloro heptoxide (Cl2O7) dissolves in water to form an acid called Perchloric acid (HClO4).

CI2O7(s) + H2O (l)  2HCIO4 (aq)

2. Basic Oxide:

Basic oxide is generated when an oxide combines with water to form a base. So, when water and basic oxide react, a base is produced. It indicates that red litmus paper will become blue when immersed in a solution of basic oxide and water.

Examples: MgO, CaO, BaO etc.

When magnesium ribbon is burned, it combines with oxygen in the atmosphere to produce magnesium oxide, a chemical with a grey colour. Now, when we dissolve this grey substance in water and dip a piece of red litmus paper in it, it transforms the red litmus into blue as it reacts with the water to generate magnesium hydroxide. The involved reactions are listed below.

2 Mg(s) + O2 (g) → 2 MgO (s)
MgO(s) + H2O (l) → Mg(OH)2 (aq)

3. Amphoteric Oxides:

A metallic oxide with double behaviour is called an amphoteric oxide. It exhibits both acidic and basic oxide behaviours. Additionally, it interacts with both acids and bases.


1. While zinc oxide interacts with hydrochloric acid and acts as basic oxide, it also reacts with sodium hydroxide and acts as acidic oxide. Below are some reactions:

ZnO (s) + 2 H2O (l) + NaOH (aq)→ Na2 Zn (OH)4(aq) + H2 (g)
ZnO (s) + 2 HCl (aq) → ZnCl2 (aq) + H2O (l)

2. Al2O3 or aluminium oxide, is also another type of amphoteric oxide. It behaves as an acid when it combines with sodium hydroxide, whereas it behaves as a base when it combines with sulfuric acid. Below are some reactions:

Al2O3(s) + 3 H2SO4 (aq) → Al2(SO4)3 (aq) + 3 H2O (l)
Al2O3 (s) + 2 NaOH (aq) → 2 NaAlO2 (aq) + H2O (l)

4. Neutral Oxide:

Oxides that exert neither acidic nor basic characteristics are referred to as neutral oxides. When they interact with an acid or base, they don't produce any salt. The P H of neutral oxide is 7 at 25o C.

Examples: Carbon monoxide(CO), Nitrous oxide(N2O), Nitric oxide(NO) etc.

5. Peroxides and Superoxides:

Lithium and sodium frequently react with too much oxygen to form the peroxide M2O2 with the oxygen's oxidation state equal to -1.

Example: Hydrogen peroxides(H2O2), Sodium peroxide(Na2O2), Barium peroxide(BaO2) etc.

Frequently, high oxygen reacts with rubidium, caesium and potassium to form the superoxide,MO2 with the oxygen's oxidation state equal to -1/2.

Examples: Potassium superoxide(KO2), Rubidium superoxide(RbO2), Cesium superoxide (CsO2).

Acid-Base Behaviour Trends:

As one progresses from left to right in a period of the periodic table, the oxides of the elements grow progressively more acidic. For instance, the behaviour of oxides alters as follows in the third period:

Na2O, MgO                              Al2O3                                SiO2,  P4O10, SO3, Cl2O7Basic oxides                  Amphoteric oxide                                Acidic oxides

We may have a better understanding of the acid-base characteristics of oxides if we examine more closely at a particular time period. Examining the physical characteristics of oxides may be useful, but it is not required. On the left side of the periodic table, metal oxides create basic solutions in water (e.g.Na2O, MgO). Acidic solutions are produced by non-metal oxides on the right side of the periodic table (e.g. P4O10, SO3, Cl2O7). Within acid-base behaviour, there is a tendency for basic oxides to be on the left side of the period and acidic oxides to be on the right.

Al2O3 (Aluminium oxide) is amphoteric and exhibits both the acidic and basic characteristics of an oxide. Al2O3 hence designates the transition point from a basic oxide to an acidic oxide. It's crucial to keep in mind that the tendency only holds true for oxides in the higher oxidation states. The trend does not hold if all oxidation states are included, hence each particular element must be in its highest potential oxidation state.

Practice Problems:

Q1. Which one of the following is BaO2?

(A) Oxide
(B) Peroxide
(C) Superoxide
(D) None of the above

Answer: (B)

Solution: It is a peroxide. The oxygen atoms have an oxidation state of -1 because barium has an oxidation state of +2. Consequently, the substance is a peroxide, more specifically known as barium peroxide. In oxides, the oxygen oxidation state is -2 and in superoxides, the oxidation state of oxygen is -½.

Q2. Which substance makes up sand primarily?

(A) Silicon
(B) Silica
(C) Feldspar
(D) Silicon tetraoxide

Answer: (B)

Solution: Silica, commonly known as silicon dioxide, makes up the majority of sand. Not a compound, silicon tetroxide is a silicon oxo-anion. In contrast to feldspar, which is a mineral resource made up of numerous tectosilicate compounds, silicon metal does not naturally develop in nature.

Q3. Among the following, which is amphoteric in nature?

(A) BeO
(B) MgO
(C) Cl2O7
(D) None of the above

Answer: (A)

Solution: As shown by the following reactions, BeO is amphoteric because it combines with both acids and bases to form salts and related compounds.


BeO(s) + 2 NaOH (aq)  Na2BeO2(s) + H2O (l)BeO(s) + 2 HCl (l) BeCl2 (s) + H2O (l)


MgO is basic oxide as it reacts with water to form base and Cl2O7 is acidic oxide as it reacts with water to give acid and the reactions are given below


MgO(s) + H2O (l)  Mg(OH)2 (aq)Cl2O7(s) + H2O (l)  2HCIO4 (aq)


Q4. What results from the reaction between amphoteric oxides and an alkali solution?

(A) Salt and water
(B) No reaction
(C) Acid and water
(D) Salt only

Answer: (A)

Solution: Both acids and alkalis cause a reaction with amphoteric oxides. When exposed to an alkali solution, they behave like acids and go through reactions similar to neutralisation, resulting in the formation of salt and water.

Frequently Asked Questions(FAQs):

Q1. Why is a fluorine-oxygen molecule not regarded as an oxide?

Answer: Oxide is a molecule that is created when oxygen is joined to a less electronegative element and has an oxidation state of (-2). However, when fluorine, the most electronegative element in the periodic table, is present, oxygen becomes a less electronegative element and is often not referred to as an oxide, leading to the production of the substance known as oxygen fluoride, which has the chemical formula OF2.

Q2. Is it possible for oxide to be neither acidic nor basic?

Answer: Yes, an acidic oxide is an oxide that reacts to form an acid. A basic oxide is an oxide that produces a base with water.. An amphoteric solution is one that can react chemically as either an acid or a base. It is also feasible for oxide to be neither acidic nor basic, but rather neutral. Carbon monoxide is one example (CO). When carbon monoxide is reacted with an acid or a base, it does not generate a salt.

Q3.What are the characteristics of p-block oxides?

Answer: The acidic character of the oxides of p-block elements normally diminishes as we advance down the group because the metallic character of the element grows and the electronegativity value lowers. As a result, the acidic character of the oxide reduces as one moves down the group. Let us see the trend of the nature of oxides of group 13 and group 14 elements.

Nature of oxides for group-13 elements

Formula of oxide












Nature of oxides for group-14 elements

Formula of oxide








SnO, SnO2


PbO, PbO2


Q4. What makes Carbon Dioxide not an Organic Compound?

Answer: Because organic compounds do not only include carbon. They are made up of hydrocarbons or carbon bound to hydrogen. Because the C-H bond in carbon dioxide has lower bond energy than the carbon-oxygen bond, carbon dioxide (CO2) is more stable/less reactive than the average organic complex. When evaluating if a carbon compound is organic or not, search for hydrogen in addition to carbon, as well as whether the carbon is bonded to the hydrogen.

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