ICSE Class 8 Chemistry Chapter 02 Carbon

2 Carbon

Carbon is a very important element. It is present in every living being. The food we eat has compounds of carbon.

Carbon has many uses. Coal and hydrocarbons are widely used as fuels. Carbon compounds are also used in medicines. The clothes we wear have compounds of carbon. In the form of diamond, carbon is used as a gemstone.

In one form or the other, carbon forms a significant part of the mineral world.

Occurrence

In the free or in the combined state, carbon is widely distributed on earth.

In the Free State

1. Coal

As you know, coal is a decomposition product of plants buried millions of years ago due to some natural phenomena. Plants contain carbon compounds, and their buried remains were slowly converted into carbon. We will discuss the formation of coal in greater detail in the next chapter.

The conversion of a carbon compound into carbon is called carbonisation.

In the Combined State

Carbon is widely distributed in the combined state.

1. Carbon Dioxide

Air contains about 0.03% carbon dioxide (CO2).

2. Living Organisms

All living organisms - plants and animals - have carbon compounds. Hence, everything we eat, which is derived from plants and animals, contains carbon compounds. The essential ingredients of food - carbohydrates, proteins, fats and vitamins - are compounds of carbon.

3. Minerals

All carbonate minerals contain carbon. For example, limestone, calcite and marble are calcium carbonate (CaCO3), and dolomite is a mixed carbonate of magnesium and calcium (MgCO3 - CaCO3).

4. Natural Gas and Petroleum

Natural gas and petroleum contain mainly hydrocarbons, i.e., compounds which have only carbon and hydrogen. Natural gas is mostly methane (CH4), and petroleum is a mixture of various hydrocarbons containing a large number of carbon atoms.

Teacher's Note

Carbon is essential to all life on Earth and forms the backbone of countless materials we use daily, from the diamonds in jewelry to the coal that powered the industrial revolution.

2. Diamond and Graphite

Diamond and graphite are the crystalline forms of carbon found in nature. Graphite is more abundant than diamond.

Common Compounds Which Have Carbon

You have learnt that all carbon-containing compounds, except carbon monoxide, carbon dioxide, carbonates and hydrogencarbonates, are called organic compounds. And all noncarbon compounds, along with carbon monoxide, carbon dioxide, carbonates and hydrogencarbonates, are called inorganic compounds. Some examples of organic and inorganic compounds are given in Table 2.1.

Teacher's Note

Understanding the difference between organic and inorganic carbon compounds helps explain why carbon is fundamental to both the non-living minerals in our environment and the living organisms that inhabit it.

Allotropy

Before we take up allotropy, let us learn about crystalline and amorphous solids.

Crystalline and Amorphous Solids

Solids are divided into two classes - crystalline or true solids, and amorphous solids or pseudosolids.

Characteristics of Crystalline and Amorphous Solids

Teacher's Note

The difference between crystalline and amorphous solids explains why a piece of quartz crystal breaks cleanly while glass shatters into fragments with curved edges.

What Is Allotropy?

The phenomenon of some elements existing in different forms which vary in physical and chemical properties is called allotropy.

The different forms of an element are known as allotropes or allotropic modifications. The allotropes of an element differ from each other in atomicity, structure or both.

Carbon, oxygen, phosphorus and sulphur are some common elements that show allotropy. Diamond, graphite and the fullerenes are allotropes of carbon. Dioxygen (O2) and ozone (O3) are allotropes of oxygen. You will learn about the allotropy of other elements in higher classes.

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