Chapter 13 - Carbon : An Important Element
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 _____ bond with other atoms. In this bond the two atoms _____ electrons.
Ans: covalent, share
b. All the carbon bonds in a saturated hydrocarbon _____ electrons.
Ans: share
c. At least one carbon bond in an unsaturated hydrocarbon is _____
Ans: double
d. _____ is the essential element in all the organic compounds.
Ans: Carbon
e. The element hydrogen is present in _____ organic compound.
Ans: all
2. Answer the following questions
a. Why are carbon and its compounds used as fuels?
Ans: Carbon and its compounds have a lot of energy stored in their bonds, so when these compounds undergo combustion, a lot of this energy (light and energy) is released in the form of heat. This is mainly because carbon compounds have greater calorific values. Also, they burn with a clean flame, and no smoke is produced.
b. In which compound forms does carbon occur?
Ans: Carbon is found in nature in both its free and compound states. Carbon in the free state is found as diamond and graphite, and in the combined state in the following compounds:
(i) As carbon dioxide and in the form of carbonates such as calcium carbonate, marble, and calamine (ZnCO₃).
(ii) Fossil fuels: coal, petroleum, and natural gas
(ii) Carbonaceous nutrients: carbohydrates, proteins, and fats
(iv) Natural fibers: cotton, wool, silk
(v) Hydrocarbons: compounds of carbon and hydrogen.
c. Write the uses of diamond.
Ans: The uses of diamonds are:
(i) Diamonds are used in glass cutting and rock drilling machines.
(ii) Diamonds are used in ornaments.
(iii) Diamond knives are used in eye surgery.
(iv) Diamond dust is used for polishing other diamonds.
(v) Diamond is used to make windows that give protection from radiation in space and in artificial satellites.
3. Explain the difference:
a. Diamond and graphite.
Ans:
| Diamond | Graphite |
|---|---|
(i) A diamond is a brilliant, hard, and crystalline allotrope of carbon. | (i) Graphite is a black, soft, brittle, and slippery crystalline allotrope of carbon. |
(ii) In diamonds, every carbon atom is bonded to four neighbouring atoms by covalent bonds, forming a tetragonal, three-dimensional structure that makes it very hard. | (ii) In graphite, every carbon atom is bonded to three other carbon atoms by covalent bonds in such a way that a hexagonal layered structure is formed. A graphite crystal is made of many such layers of carbon atoms. These layers slip over each other when you apply pressure. |
(iii) The density of a diamond is 3.5 g/cm³. | (iii) The density of graphite is 1.9 – 2.3 g/cm³. |
(iv) Diamond is a bad conductor of electricity as it does not have free electrons. | (iv) Inside each layer of graphite, free electrons move continuously within the entire layer. Hence, graphite is a good conductor of electricity. |
b. Crystalline and non-crystalline forms of carbon.
Ans:
| Crystalline forms of carbon | Non-crystalline forms of carbon |
|---|---|
(i) A crystalline structure has a regular and definite arrangement of atoms. | (i) A non-crystalline form does not have a regular and definite arrangement of atoms. |
(ii) They have high melting points and boiling points. | (ii) They have low melting points and boiling points. |
(iii) A crystalline form has a definite geometrical shape, sharp edges, and plane surfaces. | (iii) They are amorphous; hence, they do not have a definite geometrical shape. |
(iv) Diamond, graphite, and fullerene are different crystalline forms of carbon. | (iv) Coal, charcoal, and coke are different non-crystalline or amorphous forms of carbon. |
4. Write scientific reasons
a. Graphite is a conductor of electricity.
Ans:
(i) In graphite, each carbon atom is bonded to three other carbon atoms in such a way that a hexagonal layered structure is formed.
(ii) Due to this structure, graphite has free electrons available.
(iii) These free electrons move continuously within the entire layer.
(iv) Hence, graphite is a good conductor of electricity.
b. Graphite is not used in ornaments.
Ans:
(i) Graphite is a black, soft, brittle, and dull form of carbon.
(ii) It is neither malleable nor ductile.
(iii) These properties of graphite make it unsuitable for making ornaments.
(iv) Hence, graphite is not used for making ornaments.
c. Limewater turns milky when CO₂ is passed through it.
Ans:
(i) Limewater traditionally means a weak solution of the alkali calcium hydroxide Ca(OH)₂.
(ii) When CO₂ is passed through lime water, it reacts with calcium hydroxide to form insoluble particles (precipitate) of calcium carbonate (CaCO₃).
(iii) Calcium carbonate is a weak basic salt, and this gives a milky white precipitate.
Chemical Equation:
CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O
Carbon Calcium Calcium Water
dioxide hydroxide carbonate
(Lime water)
d. Biogas is an eco-friendly fuel.
Ans:
(i) Biogas is a very cheap fuel option which meets the demand for cooking gas.
(ii) Biogas is eco-friendly as it contains about 55% to 60% of methane and the rest is carbon dioxide, hence, on combustion it does not produce harmful gases which cause pollution.
(iii) Biogas is a fuel which is convenient to use and in addition to this it produces a very good manure as a side product of the process.
(iv) Hence, biogas is an eco-friendly fuel.
5. Explain the following.
a. Diamond, graphite and fullerenes are crystalline forms of carbon.
Ans:
(i) Diamond, graphite and fullerene are the three crystalline forms of carbon.
(ii) A crystalline form has a definite and regular geometric pattern.
(iii) The structure of all the three crystalline forms has a definite shape, sharp edges and plane surfaces.
(a) Graphite has a hexagonal layered structure.
(b) Diamond has a tetragonal three-dimensional structure.
(c) Fullerene exists in the form of buckyballs and buckytubes.
b. Methane is called marsh gas.
Ans:
(i) Methane is formed by the decomposition of plant and animal matter in swamps or marshy areas.
(ii) As methane gas bubbles out from marshy area, it is called as marsh gas.
c. Petrol, diesel, coal are fossil fuels.
Ans: Petrol, diesel, coal are fossil fuels because:
(i) They are main source of heat and energy.
(ii) They release heat, light and energy when burnt in air.
(iii) They are formed from the decayed living organisms (plants and animals) that are present in the earth’s crust.
d. Uses of various allotropes of carbon.
Ans: Uses of various allotropes of carbon:
(i) Diamond: It is used in making ornaments, rock drilling, glass cutting, artificial satellites, etc.
(ii) Graphite: It is used in pencils, paints, polishes, and for making carbon electrodes, lubricants, etc.
(iii) Fullerene: It is used as an insulator and as a catalyst in water purification.
(iv) Coal: Coal is used in thermal power plants and is used as fuel in factories and industries.
(v) Coke: It is used as a reducing agent and a domestic fuel.
e. Use of CO₂ in fire extinguisher.
Ans:
(i) Fire is caused by the three elements i.e. fuel, oxygen, and heat. If one of the three elements is removed, the fire will stop.
(ii) Carbon dioxide removes one element, i.e., oxygen.
(iii) CO₂, being heavier than oxygen, covers the fire like a blanket.
(iv) Since the contact between the fuel and oxygen is cut off, the fire is controlled.
(v) Therefore, CO₂ is used in fire extinguishers.
(vi) CO₂-based fire extinguishers do not cause corrosion and are not conductors of electricity.
(vii) Therefore, these are used when electrical and electronic equipment catches fire.
f. Practical uses of CO₂
Ans: Uses of carbon dioxide include:
(i) CO₂ is used to make aerated drinks.
(ii) Solid carbon dioxide is used in cold storage and to keep milk, milk products, and frozen substances cool during transport. It is also used to get the special effects of a mist in dramas and movies.
(iii) CO₂ obtained by chemical reaction or kept under pressure, is used in fire extinguishers.
(iv) Liquid CO₂ is used to remove caffeine from coffee.
(v) Liquid CO₂ is used as a solvent in modern eco-friendly dry cleaning.
(vi) Plants use CO₂ in the air for photosynthesis.
6. Write two physical properties each.
a. Diamond
Ans: The properties of a diamond are:
(i) A brilliant and pure diamond is the hardest known natural substance.
(ii) The density of diamond is 3.5 g/cm³.
(iii) The melting point of a diamond is 3500 °C.
(iv) When a diamond is heated at 800 °C in the presence of oxygen, CO₂ is given off. In this process, no other product besides CO₂ is formed.
(v) Diamond does not dissolve in any solvent.
(vi) Acids and bases have no effect on diamond.
(vii) Diamond is a bad conductor of electricity as it does not have free electrons.
b. Charcoal
Ans: The properties of charcoal are:
(i) Charcoal is solid at room temperature.
(ii) It is black in colour.
(iii) It has a low density because of its porosity.
(iv) It is a bad conductor of heat and electricity.
(v) It produces smoke and heat on combustion.
(vi) It has high adsorptive power.
(vii) It is insoluble in water.
c. Fullerene
Ans: The properties of fullerenes are:
(i) Molecules of fullerenes are found in the form of buckyballs and buckytubes.
(ii) There are 30 to 900 carbon atoms in one molecule of a fullerene.
(iii) Fullerenes are soluble in organic solvents such as carbon disulphide and chlorobenzene.
7. Complete the following Chemical reactions.
1. _____ + _____ → CO₂ + 2H₂O + Heat
2. _____ + _____ → CH₃Cl + HCl
3. 2 NaOH + CO₂ → _____ + _____
Ans:
1. CH₄ + 2O₂ → CO₂ + 2H₂O + Heat
2. CH₄ + Cl₂ → CH₃Cl + HCl
3. 2 NaOH + CO₂ → Na₂CO₃ + H₂O
8. Write answers to the following in detail.
a. What are the different types of coal? What are their uses?
Ans: 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 has four types;
(i) Peat: The 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.
(ii) Lignite: Peat was transformed into lignite due to increased pressure and temperature inside the earth. It contains 60 to 70% carbon. Lignite is the second step in the formation of coal.
(iii) Bituminous coal: bituminous coal was formed as the third step in the formation of coal. It contains 70 to 90% carbon.
(iv) Anthracite: Anthracite is known as the pure form of coal. This coal is hard and contains about 95% carbon.
Uses of coal:
(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 the generation of electricity.
b. How will you prove experimentally that graphite is good conductor of electricity?
Ans: Apparatus required: A lead pencil, electrical wires, a battery or cell, a small bulb, etc.
Step-I: Remove the lead from a pencil and assemble the apparatus as shown in the diagram.
Step-II:
(i) Start the electric current in the circuit; the moment the electric current is passed through the circuit, the bulb glows.
(ii) This experiment proves that graphite is a good conductor of electricity, as graphite has free electrons moving continuously within the entire layer, and these free electrons conduct electricity in the lead of the pencil.
c. Explain the properties of carbon.
Ans: Allotropic nature of carbon: Some elements occur in nature in more than one form. The chemical properties of these different forms are the same, but their physical properties are different. This property of elements is called allotropy. Carbon shows the property of allotropy.
Carbon allotropes are of two types:
(A) Crystalline forms:
Carbon has three crystalline materials: diamond, graphite, and fullerene.
The properties of crystalline forms of carbon are as follows:
(i) A crystalline form has a regular and definite arrangement of atoms.
(ii) They are made up of only carbon atoms.
(iii) They have high melting points and boiling points.
(iv) A crystalline form has a definite geometrical shape, sharp edges, and plane surfaces.
(B) Amorphous forms or non-crystalline forms:
Coal, charcoal, and coke are the non-crystalline forms of carbon.
The properties of non-crystalline forms of carbon are as follows:
(i) The arrangement of carbon atoms in this form is not regular.
(ii) Apart from carbon atoms, they also contain hydrogen, oxygen, nitrogen, phosphorus, and sulphur atoms.
(iii) Compared to their crystalline form, they have low melting and boiling points.
(iv) Most of them are used as fuels.
d. Classify carbon.
Ans: Carbon is classified as follows:
9. How will you verify the properties of carbon dioxide?
Ans: The properties of carbon dioxide can be verified in the following ways:
(i) When a burning candle is placed in a gas jar of carbon dioxide, it extinguishes, indicating that carbon dioxide is a non-combustible gas and does not support combustion.
(ii) When carbon dioxide gas is passed through lime water, it turns the lime water milky due to the formation of insoluble calcium carbonate.
(iii) Moist blue litmus turns red in a gas jar of carbon dioxide, indicating it is acidic in nature.
(iv) Carbon dioxide is fairly soluble in water, where it dissolves and forms carbonic acid.