WBBSE Notes For Class 8 General Science And Environment Chapter 4 Occurrence Of Carbon And Its Compounds In Nature

Chapter 4 Occurrence Of Carbon And Its Compounds In Nature Carbon And Carbon Compounds

Carbon And Carbon Compounds

Carbon and its compounds are widely distributed in nature both in a free state and in a combined state. In the free state, carbon occurs as diamond, graphite, coke, coal, gas carbon, etc.

And in the combined state it occurs as minerals e.g. limestone (CaC0₃), marble (CaC0₃ ), dolomite (MgC0₃.CaC0₃ ), calamine (ZnC0₃ ), magnesite (MgC0₃ ), siderite (FeC0₃ ), and in the gaseous state as carbon dioxide (C0₂ ), LPG, CNG, natural gas, etc.

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It is the chief constituent of all living organisms. It is present in carbohydrates, fats, proteins, cellulose, vitamins, hormones, enzymes, DNA, RNA, etc. Besides these, it plays important role in providing clothing materials like cotton, silk, nylon, rayon, jute, etc.

 

And also provides different medicines example quinine for malaria, penicillin for pneumonia, Chloromycetin for typhoid, etc. and provides mostly used paper, soap, perfumes, pigments and so on.

Chapter 4 Occurrence Of Carbon And Its Compounds In Nature Carbon Cycle

Carbon Cycle

It is the continuous circulation of carbon from the ‘environment’ through organic and inorganic substances, and then ‘back to the environment’ such that the concentration of carbon remains the same in the environment.

Steps involved in the carbon cycle :

1. Removal of carbon from the environment:

Carbon is mostly removed as carbon dioxide from the environment in a various ways, such as

 

 

1.  During photosynthesis, carbon is absorbed as carbon dioxide by the chlorophyll of green plants to synthesise carbohydrates. The process is also known as ‘carbon assimilation’,
2.  Some aquatic creatures like snails, corals, clams, etc. form their shells (made of CaC0₃ ) through the absorption of carbon dioxide dissolved in water.
3. Limestone, marbles, dolomite, etc. are the ores in which carbon is stored as carbonate. These metallic carbonates are also formed by the absorption of C02 from the atmosphere. For example, in limestone caves, columns of stalactites (hang as like as inverted conical shape) and stalagmites (rise upwards originating from the base of the cave),
4.  A portion of atmospheric carbon dioxide dissolved in rainwater falls down the seawater where some of the aquatic animals convert it into limestone and chalk.

2. Addition of carbon to the environment :

Carbon comes to the environment by

1.  Animal respiration
2.  Decay of plants and animals.
3.  Burning of coal and other carbonaceous substances.
4.  Volcanic eruptions.
5.  Fermentation of plants and plant products.
6.  Forest fire.
7.  Heating shells of marine organisms.

 

Chapter 4 Occurrence Of Carbon And Its Compounds In Nature Allotropy

 

Some elements like carbon, sulphur, phosphorus, etc. may exist in more than one form, having more or less different properties. For example, carbon exists as diamond, graphite, coal, coke, gas carbon, charcoal, etc.

sulphur exists in five different crystalline varieties, phosphorus as red and white phosphorus, oxygen molecules exist in oxygen gas (0₂ ) and ozone gas (0₃ ), etc. Such different forms are termed as allotropes and the element is said to exhibit the property known as allotropy.

Definition :

The property by virtue of which an element can exist In two or more forms having different physical properties but same chemical properties is called allotropy and the different forms are called allotropes.

Causes of allotropy :

The property of allotropy happens to occur due to the

1. The difference in the crystal structure.
2. The difference in the number of atoms in the molecules.
3. The difference in internal energy of the molecules.

All the allotropic forms of an element arc in the same physical state, i.e, if one allotrope be solid, then all other allotropes would be solid. Similarly, if one allotrope be gas, all other allotropes would be gas. Each of the allotropes bears the same basic characteristic features by which we can easily identify them as one and the same element.

Chapter 4 Occurrence Of Carbon And Its Compounds In Nature Allotropes Of Carbon

 

The allotropes of carbon are mainly divided into two groups—

1. Crystalline (having well-defined crystal structures) and
2. Amorphous (having no definite structure or micro-crystalline).

The naturally occurring crystalline allotropes of carbon are

1. Diamond.
2. Graphite and
3. Fullerene. [The allotrope fullerenes are discovered in the laboratory in the late 20th century.]

On the other hand, the amorphous forms of carbon are of four types—

1. Charcoal, carbon.
2. Coke,
3. Lamp black and
4. Gas carbon.

Charcoal has two different distinct forms depending on the source such as

1. Plant charcoal and
2. Animal charcoal.

Plant charcoal has also two different forms like

1. Wood charcoal and
2. Sugar charcoal.

Animal charcoal is also divided into

1. Bone charcoal and
2. Blood charcoal.

 

Let us now describe the allotropes of carbon one by one:

Diamond :

It is the second hardest solid (the hardest is crystalline boron) of high density (3.51 g. cm^-3) and high melting point. The crystals of a diamond is octahedral in which each carbon atom is linked to four neighbouring carbon atoms by covalent bonds (the distance, the C—C bonds are very tightly bound in its structure and that’s why diamond is so hard.

It is a colourless, transparent, lustrous substance. It has a high refractive index (2.415), due to which it produces a maximum total internal reflection of light rays incident on it, that’s why it looks so bright. When properly cut and polished, it acquires amazing lustre. One of the world-famous gems is Kohinoor of weighs 1.6 carats (1 carat = 0.200g).

 

 

Uses Of Diamond :

1. Diamond is used as a precious gem due to its lustre.
2. The black or dark variety of diamonds (called boart or carbonado) is comparatively harder and tough and has no value as a gem. It is used for cutting, drilling and polishing glass, stones, rocks, metals, etc.
3. Natural diamond is transparent to X-rays, whereas diamond-like lustrous imitations are opaque. Using this property, a natural diamond can be identified from glass and imitations with the help of X-rays.

Graphite :

Like diamonds, graphite is also a pure form of carbon. It is a dark grey soft solid, greasy to the touch having a metallic lustre. It marks white paper black and is hence called ‘black lead’. It is lighter than a diamond.

 

 

In graphite crystals, each carbon atom is linked to three adjacent carbon atoms forming different parallel layers one over another. The length of C—C bonds in different layers is about 1.42A.

1. Graphite is a good conductor of heat and electricity at room temperature. Diamond is a very good conductor of heat, but a poor conductor of electricity.
2. Graphite is chemically inert, but it is slightly more reactive than diamond

Uses Of Graphite :

1. Graphite is used for making the cores of lead pencils used for writing or marking,
2. It is used as a lubricant in the industry because of its slippery nature.
3. It is used in making electrodes, and electric furnaces because it is a good conductor of electricity.
4.  It is also used.in making high temperature-resistant crucibles

Fullerenes :

In 1985, another crystalline allotropic form of carbon, other than diamond and graphite, was discovered. It contains about 60, 70 or even greater number of carbon atoms joined together.

These are called fullerenes. The smallest fullerene C-60 is the most common. The shape of it is like that of a hollow football, called a ‘bucky ball’. Fullerenes are in a stage of research. It is believed that these may find applications in electronics and medicine.

Amorphous allotropes of Carbon :

 

Destructive distillation of wood and others. carbonaceous matter of plant origin or animal origin in absence of oxygen results in the formation of charcoal. It is a black porous substance. Its density being very less, it can float on water.

Due to its porosity, the wood charcoal can absorb the solute of a substance and poisonous gases and also for purifying water. That is why it is used for making gas masks.

Destructive distillation of bones and animal blood brings out the products bone charcoal and blood charcoal respectively. Animal charcoal is used for purifying and producing white sugar in sugar industries, for making ivory black (paints used by artists), etc.

Sugar charcoal is the purest form of carbon and it is used as a reducing agent. Lamp black is prepared by burning rich carbonaceous substances such as kerosene oil, turpentine oil, etc. in a limited supply of air and collecting the soot on wet blankets.

It is used for making carbon papers, shoe polishes, Japanese paint, etc. And in vulcanising rubber. Coke, another form of amorphous carbon, is used in the manufacture of fuel gases. Gas carbon has a wide use for making electrodes as it is a good conductor of electricity.

Coal, the main form of amorphous carbon, is mainly used as fuel in steam engines, steam boilers, etc. On destructive distillation in absence of air, coal gives a number of products such as coal gas, coal, tar, perfumes, coke, etc.

Chapter 4 Occurrence Of Carbon And Its Compounds In Nature Calorific Value Of Fuels

Fuels are the substances that produce heat and light energy on burning in air or oxygen. The commonly used fuels are wood, coal, kerosene, petrol, diesel, LPG, CNG, gobar gas, etc.

These are used in various purposes such as heating, cooking, running automobiles, generating of electricity and so on. Some fuels produce more energy than others. The choice of fuel depends upon its cost, availability, and the extent of pollution it may cause.

Solid fuels	Liquid fuels	Gaseous fuels
Wood, coal, charcoal, cow-dung cakes, etc.	Kerosene, petrol, diesel etc.	Petroleum gas, natural gas, biogas, LPG, CNG, etc.

 

Types of fuels especially, its heating value or calorific value. The calorific value of a fuel is defined as the amount of heat produced on complete burning or combustion of unit mass of the fuel.

That is, the higher the calorific value of a fuel, the better is the fuel as it produces more heat on burning in air. Thus, an ideal fuel should have a high calorific value. Calorific value of a fuel is usually expressed in kilojoules per gram (KJ/g).

1.  Hydrogen has the highest calorific value and hence, it is considered the best fuel.
2.  LPG and methane have fairly high calorific values. Thus, they are considered as ideal fuel for domestic use.
3.  Petrol, and diesel are mainly used in automobiles.
4.  For cooking and other usual domestic purposes, wood, cow dung cakes, kerosene, biogas, etc. are used.

Hazardous effects on the environment due to the burning of fossil fuels :

 

 

The use of fossil fuels like petrol, diesel, coal, etc. adds many undesirable harmful substances (called pollutants) in our environment.

Fuels	Calorific Value (KJ/g)
Cow dung cakes	7
Wood	17
Coal	25-33
Coke	33
Petrol	47
Kerosene	48
Diesel	47-48
LPG	50
Methane	55
Hydrogen	150

 

 

 

These pollutants adversely affect the life of plant and animal kingdom, such as—

 

 

1.  An increase in the level of C0₂  in the air gives rise to the increase of atmospheric temperature which is called global warming. It leads to changes in climatic patterns worldwide, causing the melting of polar ice caps, and rising the sea level.
2.  The incomplete burning of fossil fuels produces carbon monoxide gas, which can cause headaches, and dizziness and even lead to coma or death.
3.  Oxides of sulphur (SO_X ), and oxides of nitrogen (NO_X ) released into the atmosphere by the burning of coal, petrol, diesel, etc. react with oxygen and water vapour to produce sulphuric acid and nitric acid respectively, which fall on the earth as acid rain. The soil gets acidic, in turn, it reduces the fertility of the soil and also damages aquatic lives.

Energy crisis:

Most of our energy requirements are met by the burning of fossil fuels. But their stocks are very limited. So, we are to be careful to conserve fossil fuels to the extent possible. This can by done

1.  By the judicious use of the existing resources and
2. To use ‘alternative sources of energy’ such as solar energy, wind energy, water energy, geothermal energy, biogas, nuclear energy, etc. to meet up our energy needs.

Chapter 4 Occurrence Of Carbon And Its Compounds In Nature Alternative Sources Of Energy :

1. Solar energy:

 

It is the cleanest form of energy. It causes absolutely no environmental ‘ pollution. Today scientists are able to make solar cookers, solar water heaters and many other devices which work on solar energy. Solar cells are used to generate electricity on a small scale example in watches, transistors, calculators and for domestic purposes in remote village areas.

Solar batteries are used in space flight. To obtain more electricity and more electric power, a large number of solar cells are joined together to form a solar panel. These are used in artificial satellites, space stations, water pumps, street lighting, etc.

2. Wind energy :

 

The energy obtained from very fast-moving wind is called wind energy. The kinetic energy of wind is converted into electrical energy by the windmills.

A windmill is used :

 

1. To run a water lifting pump to pull water from wells ;
2. To run flour mills ;
3. To produce electricity by means of the wind generator. Windfarms are generally established at places where wind blows continuously with high speed.

The advantages of wind energy are :

 

1.  Wind energy is a natural resource. It is available in high-wind regions without any cost.
2.  Wind energy causes no pollution.

3. Geothermal energy :

 

It is the energy obtained from the heat of the core of the earth. Magma (hot molten lava) below the surface of the earth can heat nearby rocks and water as hot as 400-500°C. Some of such hot water reaches the earth’s surface as hot springs (what we see in Bakreshwar). Using a turbine, the hot water can be used to generate electricity.

 

4. Tidal energy :

 

It is the energy obtained from rising tides in coastal areas. High tide and ebb of ocean water constituting a water current can be used to rotate the turbine to produce electricity. Modern scientists are working to tidal energy on a large scale.

5. Biomass :

A mixture of waste materials and dead plants of living beings is known as biomass. Biomass concludes excreta of animals, crop residue, garbage, sewage, wood fillings and industrial wastes. Biomass can be used as a source of energy, likewise,

1.  Dry biomass, like cow-dung cake, is burnt to produce heat energy, which is then used for cooking purposes.
2. In biogas plants, cow dung and sewage are used to produce biogas. The residue is used as manure in the fields.

Advantages and disadvantages of renewable resources in comparison to Non-renewable resources :

 

Advantages :

Renewable resources

1.  Can be replenished,
2.  Are easily available,
3. Cause less pollution.

Disadvantages :

In comparison with non-renewable resources, the renewable resources

1.  Produce less amount of energy,
2. Cannot be used by simpler mechanisms.

6. Biogas :

When the slurry of cattle-dung and water (or crop residue, sewage or other waste materials, poultry droppings, etc.) is allowed to make fermentation in absence of oxygen, they produce a mixture of methane, carbon dioxide, hydrogen and traces of hydrogen sulphide. This mixture is called biogas.

The fermentation is carried out by anaerobic bacteria which decompose the carbon compounds present in biomass into methane gas. Some of these bacteria decompose biomass to form carbon dioxide, hydrogen and other components. Finally, these combine to form methane and water.

It causes no environmental pollution. It can be used for street lighting and for running engines. The waste products such as nitrogen and phosphorus left from biogas plants are used as fertilisers.

7. Nuclear energy :

A large amount of energy is produced when the nucleus of an atom splits or when two or more nuclei of different lighter atoms combine together to form heavier nuclei. Energy stored inside heavier elements like uranium, radium, thorium, etc. is nuclear energy. This energy is used to generate electricity. It is a non-renewable energy.

Chapter 4 Occurrence Of Carbon And Its Compounds In Nature Carbon Dioxide

Occurrence :

Carbon dioxide is present in nature both in the free state and in the combined state. It is present in the atmosphere to the extent of about 0.03% by volume. Being slightly soluble in water, it remains dissolved in natural water.

In the combined state, it occurs as minerals (carbonates and bicarbonates) such as limestone (CaC0₃ ), dolomite (MgC0₃  -CaC0₃  ) magnesite (MgC0₃  ), Calamine (ZnC0₃  ), etc.

Uses Of Carbon Dioxide In Chemical Industries :

It is used—

1. In the manufacture of urea—a nitrogenous organic fertilizer essential for good production of crops. In the laboratory, carbon dioxide reacts with ammonia gas at 200°C temperature under a pressure of about 200 atm to form urea.
2.  In the manufacture of washing soda (Na₂  CO_{3  .}10H₂  O) by Solvay process.
3.  In the preparation of different soft drinks example. soda water, lemonade, etc.
4.  To extinguish a fire in fire extinguishers.
5.  In the preparation of dry ice, which is used as a refrigerant for the preservation of food. Carbon dioxide liquefies under a pressure of about 70 atm at normal temperature. On the sudden release of pressure, the liquid suddenly starts to evaporate to be transformed into solid carbon dioxide (whose temperature is about – 78°C), it looks like white ice, but it does not contain water. Such solid carbon dioxide is known as dry ice.
6. ln, the manufacture of glass.

Chapter 4 Occurrence Of Carbon And Its Compounds In Nature Preparation Of Carbon Dioxide Gas :

 

Carbon dioxide gas can be prepared using metal carbonates and bicarbonates in two ways:

1. By the action of dilute mineral acids on metal carbonates and bicarbonates :

2. By heating metal carbonates and bicarbonates :

3. By burning carbon (for example coke, charcoal) in the air:

c + o_{2  →} co₂

 

Chapter 4 Occurrence Of Carbon And Its Compounds In Nature Laboratory Preparation Of Carbon Dioxide :

 

Chemicals required:

Pieces of marble stone or calcium carbonate (CaC0₃  ) and dil. hydrochloric acid (HCl).

Apparatus required :

Woulfe bottle, thistle funnel, a delivery tube and gas jar.

Principle:

Carbon dioxide is prepared in the laboratory by the action of cold and dil. HCl on marble pieces.

Procedure :

Small pieces of marble are taken in a preparation of carbon dioxide Woulfe bottle fitted with a thistle funnel and a delivery tube. The other end of the delivery tube reaches almost to the bottom of a gas jar.

Marble pieces are covered with water. Dilute hydrochloric acid is poured into the Woulfe bottle through the thistle funnel. As the acid comes in contact with the marble, a chemical reaction takes place with the effervescence of carbon dioxide which escapes out through the delivery tube.

Sulphuric acid is not used in this method because it reacts with marble to form insoluble calcium sulphate thereby forming a layer on the marble pieces. This insoluble layer prevents the marble from coming in contact with the acid and soon the reaction ceases.

CaC0₃ + H₂  S0₄ →  CaS0₂  + C0₂  ↑ + H₂  0

Collection:

As carbon dioxide is about 1.5 times heavier than air, it is collected by the downward displacement of air. Carbon dioxide is soluble in water, so it can not be collected by the displacement of water.

Physical Properties Of Carbon Dioxide

1.  It is a colourless, properties odourless gas with a slight sour taste.
2.  It is about 1.5 times heavier than air. Invert a gas jar filled with C0₂  over an air-filled gas jar and remove the cover. After some time, remove the upper gas jar. Put a burning candle in the bottom gas jar. The candle extinguishes, as C0₂  from the upper gas jar comes down to the lower gas jar displacing air upwards. This proves the fact that C0₂  is heavier than air.
3. It is non-poisonous but does not help in respiration. Animals die in suffocation in excessive C0₂  gas due to lack of oxygen.
4.  It is fairly soluble in water in the ratio of the same volume at normal conditions. The solubility of C0₂  in water increases with an increase in pressure. The solution is acidic to litmus—it turns blue litmus paper red.
5. The gas can be liquified by the application of a pressure of 70 atm at ordinary temperature.

Chapter 4 Occurrence Of Carbon And Its Compounds In Nature Chemical Properties Of Carbon Dioxide :

1. Combustibility:

It is neither combustible nor a supporter of combustion. For example, a burning wood chip when inserted into a gas jar filled with C02, it extinguishes and also the gas does not burn.

Certain active metals such as sodium, potassium and magnesium continue burning in a jar filled with C02 gas.

4Na + 3C0₂  →   2Na₂  C0₃  + C
4K + 3C0₂  → 2 K₂  C0₃ + C
2Mg + C0₂  → 2MgO + C

2. Acidic property :

Carbon dioxide is an acidic oxide, as it dissolves in water and carbonic acid (a weak acid) is formed.
co₂  + H₂  O→ H₂  CO₃
carbonic acid

3. Reaction with bases or alkalis :

Because of acidic properties, C0₂ reacts with alkalis like sodium hydroxide (NaOH) and sodium carbonate (Na₂C0₃) to form salt and water.

When C0₂  gas is allowed to pass through clear lime water, it turns milky as insoluble calcium carbonate (salt) is formed. The milkiness however disappears on the passage of excess of C0₂  through it, when soluble calcium bicarbonate (salt) is formed.

4. Oxidising property:

On introducing a burning magnesium wire into a jar of C0₂, it burns with a dazzling light. In the reaction, Mg is oxidised to magnesium oxide and C0₂  is reduced to black carbon.

This reaction proves that

1.  C0₂ acts as an oxidising agent
2.  C0₂ contains C
3.  C0₂ allows combustion under forcing conditions.

In the following reactions, C02 is reduced to carbon.

C0₂ + C →2CO
Fe + C0₂→ FeO + CO
Zn + C0₂→ZnO + CO

Chapter 4 Occurrence Of Carbon And Its Compounds In Nature Greenhouse Effect

 

The greenhouse is a house made of glass walls and a glass roof, which allows the sun’s heat to go in freely but do not allow the inside heat to go out. That is, it traps the sun’s heat inside and keeps the plants inside it warm, even in winter.

Carbon dioxide and methane, the main greenhouse gases, when accumulated in the atmosphere, form a blanket above the earth. The blanket allows the infrared radiation of short wavelengths from the sun to reach the earth to warm it, but it does not allow infrared radiation of long wavelengths from the earth to escape into the space.

 

The retention of heat by these gases in the atmosphere is called the greenhouse effect. The increase of atmospheric temperature due to the greenhouse effect is called global warming. Global warming has introduced tremendous changes in the weather worldwide.

It has caused the melting of glaciers and polar ice caps which led to the rise in the sea level. Some scientists believe that an intensified greenhouse effect could alter the ecological balance on earth.

Chapter 4 Occurrence Of Carbon And Its Compounds In Nature  Carbon Containing Polymers And Their Uses

A polymer is a very large or giant molecule formed by the union of a very large number of smaller molecules. The term ‘polymer’ is derived from the Greek word ‘Poly’ means ‘many’ and ‘meros’ means ‘parts’. Nowadays, polymers are also known as macromolecules.

The smaller molecules which form the complex polymers are known as monomers. The formation of a polymer from single units is called polymerisation. For example, molecules of polythene, are made up of thousands of ethylene or ethene (C₂H₄) molecules together in the form of long chains. Hence, polythene is the ‘polymer’ and ethene is the ‘monomer’.

Polymers are of two kinds—

1. Natural polymers (for example cellulose, proteins, carbohydrates and their derivatives, nucleic acids, etc.) and
2. Synthetic or man-made polymers (for example ‘ polythene, plastic, refill of ball-pens, nylon, Teflon, thermocol, terylene, PVC pipe, etc.).

Natural polymers are biodegradable—as they are naturally decomposed by micro¬organisms like fungi and bacteria, whereas the synthetic polymers are non-biodegradable—as the molecules of such polymers are not decomposed by any bacteria or fungi.

Besides these, there are special type of bacteria which can produce certain biodegradable polymers, which when discarded, microbes decompose them slowly. Such polymers are used to make biodegradable plastics. There are very much eco-friendly.

Now, let us explain the properties and uses of some widely used synthetic polymers.

Name of Polymers	Properties	Uses
1. Polythene (Polymer of ethene)	Partially solid, partially flexible, whitish, translucent of moderate strength, insulator of heat* and electricity, non-reactive with chemicals.	In preparing buckets, cups, discs, water pots, carry bags, flexible containers, bottles, toys,’ packing films, table cloths, electric cables, etc.
2. Teflon (Polymer of tetrafluoro ethelene)	Very tough material, resistant towards heat, a bad conductor of electricity, inactive toward acids or bases, non-sticky towards water or oil.	For making non-stick cooking utensils, gaskets, pump packings, chemical; equipment, electrical insulators, valves, seals, non-lubricated bearings, etc.
3. PVC (Full name. polyvinyl chloride; polymer of vinyl chloride)	Hard horny material, thermoplastic polymer, plasticity can be increased by the addition of plasticises, inactive towards water, oil or other chemicals.	In the manufacture of raincoats, curtain clothes, gum boots, gramophone records,- slippers, artificial flooring, electrical goods, water pipes, etc.
4. Thermocol	Solid but not hard, not extensible, water or oil resistant.	In making food dishes, plates; bowls, etc.

 

Harmful hazards of using excessive polythene :

 

Due to excessive garbage of polythene, ‘ the porosity of soil is destroyed. Polythene being a non-biodegradable material, does not undergo bacterial decomposition and does not mix with the soil.

On burning, it produces pollutant gases, which cause air pollution. So, the limitless use of polythene becomes a dangerous problem to us.