Solution-Class 9-Science-Chapter-13-Carbon: An Important Element-Maharashtra Board

a. A carbon atom forms a covalentbond with other atoms. In this bond the two atoms shareelectrons.

b. All the carbon bonds in a saturated hydrocarbon singleelectrons.

c. At least one carbon bond in an unsaturated hydrocarbon is multiple.

d. Carbonis the essential element in all the organic compounds.

e. The element hydrogen is present in allorganic compound.

Carbon and its compounds are used as fuels because most of the carbon compounds produce lot of heat and energy when burnt in air i.e. they have high calorific value.

Fuels are classified on the basis of their physical state as follows :

Solid fuels : Wood-Cellulose (C₆H₁₀O₅)n, Coal-carbon, Coke-carbon

Liquid fuels : Kerosene-carbon, ethanol-carbon, petrol, diesel-carbon

Gaseous fuels : Methane-carbon, liquefied petroleum gas (LPG) -butane-isobutane, natural gas - methane, water gas - (CO + H₂), producer gas - CO.

Above all fuels are combustible carbon compounds and contain combustible carbon, therefore, carbon and its compounds are used as fuels.

In the combined state carbon occurs in the following compounds :

• Carbon dioxide from air, in the form of carbonates such as calcium carbonate, marble and calamine (ZnCO₃).
• Fossil fuels : Coal, petroleum, biogas and natural gas.
• Carbonaceous nutrients : carbohydrates, proteins, fats.
• Natural fibres : cotton, wool, silk, etc.

Uses of diamonds are as follows:

• used in ornaments
• used in glass cutting and rock drilling machines
• diamond dust is used for polishing other diamonds
• diamond is used to make windows giving protection from radiation in space and in artificial satellites
• diamond knives are used in eye surgery

Graphite is a good conductor of electricity because there are free electrons present between its layers. These free electrons move continuously within the entire layer and thus lead to conduction of electricity.

• Graphite is not used in making ornaments because it is soft, britlle and slippery.
• It cannot be moulded like gold and silver
• In making ornaments hard metals are used Since graphite is not hard and neither does it possess luster which is a desired characteristics in jewelry.

When carbon dioxide gas is passed through lime water, it turns milky due to the formation of calcium carbonate.

• Biogas contains about 55% to 60% methane and the rest is carbon dioxide.
• Biogas is a very cheap fuel, which meets the demand for cooking gas.
• Biogas burns almost instantaneously to provide heat quickly.
• The combustion of biogas can be easily controlled.
• It does not produce any solid Waste products.
• It can be transported easily and can be supplied through pipes to limited distant place.
• It has high heat content.
• Hence biogas is an eco-friendly fuel.

Diamond, graphite and fullerene are crystalline forms of carbon.

• They are allotropes of carbon and this property of carbon is termed as allotropy.

• These crystalline forms of carbon have a regular and definite geometrical shape, sharp edges and plane surfaces.
• They have high melting and boiling points.

Methane is formed by the decomposition of plants and animal matter at the surface of marshy lands. Hence, methane is called marsh gas.

Petrol, diesel, coal are fossil fuels because,

• Millions of years ago the remains of plants and animals got buried under the earth. Due to action of microorganisms, the gaseous substances in these remains were released into atmosphere and what was left behind were energy rich carbon compounds.
• After this, under tremendous pressure, the liquid substances in these compounds drained away as petroleum, which on fractional distillation yields petrol and diesel, and the remaining mineral solidified into compact and hard rock and this material is called coal.
• Therefore, petrol, diesel, coal are fossil fuels.

CO₂ is used in fire extinguishers for various reasons but the main reason is that the carbon dioxide gas displaces the oxygen that's necessary to maintain combustion. It means that it cuts of the supply of oxygen which is required for combustion and also it has a cooling effect when it is released under high pressure. Other uses of carbon dioxide in fire extinguishers is

• it does not conduct electricity thus making it an ideal fire suppressant for use in electrical instruments like computers etc.
• it does not create any toxic or other by-products when used to suppress a fire.

The practical uses of CO₂ are:

• Carbon dioxide is used in the manufacture of aerated drinks.
• Solid carbon dioxide is used in cold Storage and also keep milk and milk products and frozen substances cool during transport. It is used for getting special effects of a mist in dramas and movies.
• Carbon dioxide obtained by chemical reaction or kept under pressure is used in fire extinguisher.
• Liquefied CO₂ is used to remove caffeine from coffee, as solvent in dry cleaning.

Physical properties of diamond:

• It is the hardest naturally occurring substance.
• Pure diamond is colourless, transparent, but brittle solid.

Physical properties of charcoal:

• It is a highly porous and brittle material 
• It is a bad conductor of heat and electricity.

1. Physical properties of fullerene:

• They exist in the form of buckyballs and buckytubes.
• They are made up of 60 C atoms.

CH₂ + 2O₂  → CO₂ + 2H₂O + Heat

CH₄  + CL₂ → CH₃Cl + HCl

2 NaOH + CO₂ → Na₂CO₃ + H₂O

The different types of coal are:

• Peat: It is the first step of coal formation. It has high water content and less than 60% carbon content.
• Lignite: Lignite is a transformed form of peat and has around 25-35% of carbon content. It is a low quality brown coal with very little percentage of carbon in it. It is however used in the generation of electricity.

• Bituminous: It is the most commonly found variety of coal in India is bituminous and has a carbon content of 70-90%.
• Anthracite: It is the best known variety of coal found in India. It has a carbon content of more than 80% which makes it suitable for iron smelting.

Uses : (i) Coal is used as fuel in factories and homes. (ii) Coal is used to obtain coke, coal gas and coal tar. (iii) Coal is used in thermal power plants for generation of electricity. 

Graphite is a good conductor of electricity can be proved from the following experiment. 
We would require some apparatus- pencil, electrical wires, battery/cell, small bulb.

• Take a battery, open plug key, a bulb used in a torch, a piece of graphite and copper wire. Connect these components in a series.
• Now complete the circuit using the plug key (closing the key). You will find that bulb glows. This shows that graphite is a good conductor of electricity.

Carbon exhibits allotropy. It exists in more than one form. The chemical properties of these different forms are the same but their physical properties are different.

Carbon allotropes have two forms : crystalline form and amorphous form.

• Crystalline forms : (i) It has a regular and definite arrangement of atoms. (ii) They have high melting points and boiling points. (iii) A crystalline form has a definite geometrical shape, sharp edges and plane surfaces. (iv) Carbon has three crystalline allotropes : 1. Diamond 2. Graphite 3. Fullerene.
• Non-crystalline/ Amorphous forms : The arrangement of carbon atoms is irregular. Examples : Coal, charcoal, coke.

Carbon allotropes have two forms : crystalline form and amorphous form.

Crystalline forms : Carbon has three crystalline allotropes :

• 1. Diamond 2. Graphite 3. Fullerene.

Non-crystalline/ Amorphous forms : Amorphous form : Carbon has following amorphous forms :

• Coal : (i) Peat (ii) Lignite (iii) Bituminous (iv) Anthracite
• Charcoal
• Coke.

• When a burning candle is placed in a glass jar containing carbon dioxide, the flame extinguishes, therefore it shows that carbon dioxide, neither burns nor supports burning.
• When carbon dioxide is passed through fresh lime water, white precipitate of calcium carbonate is formed due to which lime water turns milky, from this test the existence of carbon dioxide is easily identified.
• When moistened blue and a red litmus are added to a gas jar of carbon dioxide, blue litmus turns red, from this it is confirmed that CO2 is an acidic oxide, however, there is no change observed in moistened red litmus.