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Carbon and its compound of Class 8


Hydrocarbons are compounds that contain only the elements hydrogen and carbon. As hydrocarbons contain carbon, they are called organic compounds. Many hydrocarbons occur naturally in coal, natural gas and petroleum (oil). Hydrocarbons are used in industry to make many different products, such as dye, glue, gasoline, plastic and wax.

There are two basic groups of hydrocarbons - aliphatics and aromatics. In aliphatic hydrocarbons, the carbon atoms are usually linked in chains. The hydrogen atoms are then linked to these chains of carbon atoms. Methane, ethane, propane etc. are examples of aliphatic hydrocarbons.

Aromatic hydrocarbons are made up of rings. The ring structure that characterises aromatics is called a benzene ring. A benzene ring (C6H6) includes six carbon atoms. Toluene is an example of an aromatic hydrocarbon. It belongs to the benzene series. Toluene's chemical formula is C6HSCH3.

Methane is also called marsh gas as it is formed in marshy places due to the decomposition of vegetable matter under water.NCERT solutions for class 8 Science prepared by Physics Wallah will help you to solve your NCERT text book exercise. 

Petroleum gas: It is the gaseous fraction of petroleum and consists of hydrocarbons. The boiling point of petroleum gas is below 400C.

Liquefied petroleum gas (LPG): Petroleum gas gets liquefied when it is subjected to high pressure at room temperature. This is liquefied petroleum gas commonly known as LPG. Its main content is butane. It is highly volatile in nature and changes back into a gas as soon as the pressure is relieved. LPG is used as a household fuel.

Advantages of LPG:

  • It has high calorific value and low ignition temperature.
  • It burns with a flame and produces no smoke.
  •  It is easy to handle and convenient to store.

Note: LPG is stored and marketed in steel cylinders, each containing about 14 kg of gas. The liquefied gas is mixed with a small amount of a strong smelling substance, called ethyl mercaptan(C2H5SH), which helps to detect any leakage of the gas. 

Natural gas: Natural gas is another fossil fuel obtained from oil wells. It occurs in a layer reserve just above the layer of crude petroleum, trapped between two layers of impervious rocks. Along with petroleum, natural gas was formed millions of years ago from the anaerobic carbonization of marine plants and animals. A mixture of gaseous hydrocarbons, its main constituent is methane. Gases like ethane,propane and butane constitute about 5% of natural gas.


  • Natural gas is the cheapest gaseous fuel and it has wide ranging uses as domestic and industrial fuel.
  • It is used as a source of hydrogen and carbon because, when it is heated strongly, it breaks up into carbon and hydrogen, which can be collected.

CNG: Now adays compressed natural gas has become a popular fuel for three wheelers, cars and buses. It does not cause much pollution and is a relatively cheap fuel.

Advantages of CNG: 

  • It is lead free and its use reduces harmful emission of gases into the air.
  • In case of a leakage, it rises and gets diffused into the atmosphere i.e., does not form ground hugging puddles as happens in the case of petrol. Obviously CNG is lighter than air.
  • It has a moderate ignition temperature.

Biogas (Gobar gas):

Biogas is a mixture of gases obtained by the degradation and the decomposition of animal and plant matter. It is chiefly composed of methane. The other gases present in biogas are carbon dioxide, hydrogen, traces of hydrogen sulphide and water vapour.

Formation of biogas: Animal dung and plant wastes contain fats, carbohydrates, proteins and anaerobic bacteria. The waste material is mixed with water and allowed to ferment in the absence of air for 30–40 hours. The anaerobic bacteria slowly decompose the complex organic compounds to form biogas. The residue contains all the necessary plant nutrients and is therefore used as manure.

Advantages of biogas:

  • Biogas is a fuel with a high calorific value. It does not produce any smoke on burning.
  •  It can be easily ignited. Moreover, it undergoes complete combustion.
  • It can be directly supplied through pipes to our homes.
  • It is a cheap fuel and generates manure as a by product.


This gas is also found in coal mines. Coal gas which is a mixture of methane, hydrogen, carbon monoxide, nitrogen, ethylene, acetylene, carbon dioxide and oxygen is formed when coal is heated in the absence of air. Coal gas is used as a fuel. Natural gas contains about 90% methane. It is formed in swampy areas by slow decomposition of organic matter in the absence of oxygen.

NOTE: Methane gas is also formed in the intestines of ruminating animals such as cows and buffaloes and in the bodies of termites.

  • Structure of Methane: Methane is a tetrahedral molecule. In it, hydrogen atoms are arranged at the four corners of a regular tetrahedron. Bond angle in methane is 109°28′.



  •  Laboratory Preparation of Methane:In the laboratory, methane can be prepared by heating a mixture of sodium acetate and soda lime (a mixture of sodium hydroxide and calcium oxide) in a boiling tube:

CH3COONa + NaOH Hydrocarbon CH4 + Na2CO3

Sodium acetate Sodium hydroxide Methane Sodium carbonate



Physical Properties of Methane:

  •  It is odourless and colourless gas.
  •  It is insoluble in water but soluble in organic solvents like alcohol.
  • It is lighter than air.
  • It can be made liquid at 00 C and 150 atmospheric pressure or at −161.40 C and normal pressure.

Chemical Properties of Methane :

(i) Combustibility : On being ignited, it burns with a blue flame, forming carbon dioxide and water vapours.

CH4(g) +2O2 (g) ⎯→ CO2(g) + 2H2O(l) + Energy 

Methane Oxygen Carbon dioxide Water 

(ii) Action with chlorine:

CH4 +   Cl2(g) ⎯→ CH3Cl + HCl

Methane     Chlorine Methyl chloride Hydrochloric acid

(iii) Pyrolysis : When methane is heated in the absence of air to 1000oC, it decomposes to form carbon and hydrogen. The process of decomposition of an organic compound into elements on heating is called pyrolysis.


(e) Uses of Methane:Methane is an excellent fuel for cooking.

  • Methane is used to manufacture fertilizers, formaldehyde and methanol.
  •  Methane is used for preparing carbon black which is used in rubber and tyre industry. 
  •  Methane in liquefied form is used as an automobile fuel.


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