13. Carbon : An Important Element | Science | Class 9

Question 1: Select the proper option and complete the statements

(single, all, double, ionic, carbon, give and take, hydrogen, multiple, share, most, covalent)

a. A carbon atom forms a covalent bond with other atoms. In this bond the two atoms share electrons.
b. All the carbon bonds in a saturated hydrocarbon share electrons.
c. At least one carbon bond in an unsaturated hydrocarbon is multiple.
d. Carbon is the essential element in all the organic compounds.
e. The element Hydrogen is present in most organic compound.

Question 2: Answer the following questions

a. Why are carbon and its compounds used as fuels?

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

b. In which compound forms does carbon occur?

Ans, Carbon occurs in the following compound forms:

  • Hydrocarbons: These are organic compounds made up of carbon and hydrogen.
  • Carbon dioxide (CO2) : Carbon also exists as carbon dioxide (CO2) in the air in free state. Carbon dioxide is present as a salt in chalk and Shahabad tiles/ marble/ limestone. Carbon dioxide is given out in the combustion of wood and the fossil fuel.
  • Methane (CH4): Carbon also exists as methane.

c. Write the uses of diamond.

The following are the uses of diamond:
1. Diamonds are used in glass cutting and rock drilling machines.
2. Diamonds are used in ornaments.
3. Diamond knives are used in the eye surgery.
4. Diamond dust is used for polishing other diamonds.
5. Diamond is used to make windows giving protection from radiation in space and in artificial satellites.

Question 3: Explain the difference:

a. Diamond and graphite.

Ans.

Diamond :

  1. In Diamond every carbon atom is bonded to four neighbouring atoms by covalent bonds.
  2. Diamond has a tetragonal three dimensional structure/
  3. Nature: Colourless and the hardest substance
  4. The density of diamond is 3.5 g/cm3
  5. Diamond does not dissolve in any solvent.
  6. Diamond is a bad conductor of electricity
  7. Refractive index 2.5

Graphite :

  1. Every carbon atom in graphite is bonded to three other carbon atoms
  2. Hexagonal layered structure is formed.
  3. Nature: Soft, greyish and slippery to touch.
  4. The density of graphite is 1.9 to 2.3 g/cm3 .
  5. Graphite dissolve in few solvents.
  6. Graphite is a good conductor of electricity.
  7. No Refractive index, as it is opaque.

b. Crystalline and non-crystalline forms of carbon.

Ans.

Crystalline form of Carbon:

  1. A crystalline form has a regular and definite arrangement of atoms.
  2. They have sharp melting points and boiling points.
  3. A crystalline form has a definite geometrical shape, sharp edges and plane surfaces.
  4. Carbon has three crystalline allotropes, diamond, graphite and fullerene.

Non-crystalline form of Carbon:

  1. A non-crystalline form has an irregular and indefinite arrangement of atoms.
  2. They have melting points and boiling points over a range and are not sharp.
  3. A crystalline form has a irregular geometrical shape.
  4. Carbon has three non-crystalline forms Coal, charcoal, coke.

Question 4: Write scientific reasons

a. Graphite is a conductor of electricity.

Ans. As there are free electrons present between the layers of graphite, these free electrons continuously move within the entire layer and result in conduction of electricity. Hence Graphite is a conductor of electricity.

b. Graphite is not used in ornaments.

Ans. Graphite is a soft, brittle and slippery material. Also it does not have any metallic lusture like gold or silver which is a desired quality in any ornaments . Hence graphite is not used in making ornaments.

c. Limewater turns milky when CO2 is passed through it.

Ans. When carbon dioxide gas is passed through lime water, the following reaction takes place: (CaOH)2 + CO2 —-> CaCO3 + H2O.

This is a chemical test for detection test for carbon dioxide. The lime water turns milky due to the formation of calcium carbonate.

d. Biogas is an eco-friendly fuel.

Ans. Biogas is an eco-friendly fuel because it is generated from waste like animal dung dry leaves, wet garbage. Biogas serves as a cheap fuel that is convenient to use and, in addition to this, a very good manure is also produced as a side product of the process. It also helps to cut the dependence on fossil fuels which are fast depleting.

Question 5: Explain the following.

a. Diamond, graphite and fullerenes are crystalline forms of carbon.

Ans. The crystalline forms of carbon have the following properties:

(i) regular and definite arrangement of atoms
(ii) high melting points and boiling points
(iii) a definite geometrical shape and sharp edges and plane surfaces

As these properties are exhibited by the three allotropes of Carbon i.e. diamond, graphite and fullerenes, they are said to be crystalline forms of carbon.

b. Methane is called marsh gas.

Ans. As Methane is found at the surface of marshy places (a form of wetland dominated by herbacious rather than woody plants) it is called marsh gas.

c. Petrol, diesel, coal are fossil fuels.

Ans. Petrol, diesel, coal are fossil fuels because:

(i) Petrol, diesel, coal are formed from the decayed living organisms (plants and animals) present in the earth’s crust since long time.

(ii) They can be used as a source of energy i.e. they release light and energy when burnt.

d. Uses of various allotropes of carbon.

The uses of various allotropes of Carbon are:

(i) Diamond:

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

(ii) Graphite:

  • Graphite is used for making lubricants.
  • Graphite is used for making carbon electrodes.
  • Graphite is used in pencils for writing.
  • Graphite is used in paints and polish.
  • Graphite is used in arc lamps which give a very bright light.

(iii) Fullerene:

  • Fullerenes are used as insulators.
  • Fullerenes are used as a catalyst in water purification.
  • At a certain temperature fullerene exhibits superconductivity.

e. Use of CO2 in fire extinguisher.

Ans. CO2 obtained by chemical reaction or kept under pressure is used as fire extinguishers for the following reasons:

Chemical reaction:

co2

(i) The CO2 based fire extinguishers do not cause corrosion and are non-conductors of electricity hence can be used when electrical and electronic equipment catches fire.

(ii) The CO2 based extinguishers are used in small scale fires.

f. Practical uses of CO2

Ans. The practical uses of Carbon are as follows;

  1. To make aerated drinks
  2. Solid carbon dioxide is used in cold storage and also to keep milk and milk products
    and frozen substances cool during transport.
  3. For special effects of a mist in dramas and movies.
  4. CO2 obtained by chemical reaction or kept under pressure is used in fire extinguishers.
  5. Liquified CO is used to remove caffeine from coffee.
  6. Liquid CO2 is used as solvent in modern eco-friendly dry cleaning.
  7. Plants use CO2 in air for photosynthesis.

Question 6. Write two physical properties each.

a. Diamond

Ans. The two important physical properties of diamond are:

1. It is the hardest natural substance known.
2. The density of diamond is 3.5 g/cm 3 .

b. Charcoal

Ans. The two important physical properties of charcoal are:

  1. High porosity,
  2. Bad conductor of heat and electricity

c. Fullerene

Ans. The two important physical properties of fullerene are:

  1. Molecules of fullerenes are found in the form of buckyballs and buckytubes.
  2. Exhibit high electrical conductivity, high ductility, high heat conductivity.

Question 7. Complete the following Chemical reactions.

1. CH4 + 2O2 ——–> CO2 + 2H2 O + Heat

2. CH4 + Cl2 ——–> CH3 Cl + HCl

3. 2NaOH + CO2 ——–> Na2CO3 + H2O

8. Write answers to the following in detail.

a. What are the different types of coal? What are their uses?

1. Coal : Coal is a fossil fuel. It contains carbon, hydrogen and oxygen. It also contains nitrogen, phosphorus and sulphur. It occurs in the solid state. It is of four types.
a. Peat : Formation of peat is the first step in the formation of coal. It contains a high proportion of water and less than 60% of carbon. Therefore, not much heat can be obtained from peat.
b. Lignite : Peat was transformed into L ignite due to increased pressure and temperature inside the earth. It contains 60 to 70% of carbon. L ignite is the second step of the formation of coal.
c. Bituminous coal : Bituminous coal was formed as the third step of formation of coal. It contains 70 to 90% of carbon.
d. Anthracite : Anthracite is known as the pure form of coal. T his coal is hard and contains about 95% of carbon.

Uses of coal :
1. Coal is used as fuel in factories and homes.
2. Coal is used to obtain coke, coal gas and coal tar.
3. Coal is used in thermal power plants for generation of electricity.

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