NCERT Class 8 Chemistry Chapter 9 Carbon Notes

Chapter 9 Carbon

Present in every living being, carbon is a very important element. It has many uses. The food we eat has compounds of carbon. Coal and hydrocarbons are widely used as fuels. Carbon compounds are also used in medicines. The clothes we wear have compounds of carbon. In the form of a diamond, carbon is used as a gemstone.

In the form of graphite, carbon finds a wide application in the industry. And you will learn in higher classes that in the form of carbon nanotubes, it plays a great role in nanotechnology.

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In one form or another, carbon forms a significant part of the mineral world.

Carbon Occurrence

In the free or combined state, carbon is widely distributed on earth.

In the Free State

Carbon exists in the free state in coal, diamond and graphite.

1. Coal:

Coal is a decomposition product of plants buried millions of years ago due to some natural phenomena. Plants contain carbon compounds, slowly converted their buried remains into carbon. The conversion of a carbon compound into carbon is called carbonisation.

2. Diamond and graphite:

Diamond and graphite are the crystalline forms of carbon found in nature. Graphite is more abundant than diamond

In the Combined State

Carbon is widely distributed in the combined state.

1. Carbon dioxide:

Air contains about 0.03% carbon dioxide (CO₂).

2. Living organisms:

All living organisms—plants and animals— have carbon compounds. Hence, everything we eat, which is derived from plants and animals, contains carbon compounds. The essential ingredients of food—carbohydrates, proteins, fats and vitamins—are compounds of carbon.

3. Minerals:

All carbonate minerals contain carbon. For example, limestone, calcite and marble are calcium carbonate (CaCO₃), and dolomite is a mixed carbonate of magnesium and calcium (MgCO₃CaCO₃).

4. Natural gas and petroleum

Natural gas and petroleum contain mainly hydrocarbons, i.e., compounds which have only carbon and hydrogen. Natural gas is mostly methane (CH₄), and petroleum is a mixture of various hydrocarbons containing a large number of carbon atoms.

Some common compounds which have carbon:

Common Compounds Which Have Carbon

Compounds containing carbon are of two types

1. Organic
2. Inorganic

All carbon-containing compounds, except carbon monoxide, carbon dioxide, carbonates and hydrogencarbonates, are called organic compounds.

And all noncarbon compounds, along with carbon monoxide, carbon dioxide, carbonates and hydrogencarbonates, are called inorganic compounds

Some examples of organic and inorganic compounds are given in Table

Carbon Allotropy

Before we take up allotropy, let us learn about crystalline and amorphous solids.

Crystalline and Amorphous Solids

Solids are divided into two classes:

1. Crystalline or true solids
2. Amorphous solids or pseudosolids.

1. Characteristics of crystalline solids:

1. The particles constituting a crystalline solid are arranged in an ordered manner in three dimensions.

2. When crystalline solids are broken or cut with a sharp knife, we get pieces with sharp edges and plane faces.

2.  Characteristics of amorphous solids:

1. The particles constituting an amorphous solid are not arranged in an ordered manner.

2. When amorphous solids are broken or cut with a sharp knife, we get pieces with curved faces.

What is Allotropy?

The phenomenon of some elements existing in different forms is called allotropy

The term allotropy has been derived from the Greek words alios (meaning ‘other’) and tropos (meaning ‘form’).

The different forms of an element are known as allotropes or allotropic modifications. The allotropes of an element are chemically the same but different from each other in structure, atomicity or both.

Carbon, oxygen, phosphorus and sulphur are some common elements that show allotropy. Diamond, graphite, the fullerenes etc., are allotropes of carbon. Dioxygen (O₂) and ozone (O₃) are allotropes of oxygen. You will learn about the allotropy of other elements in higher classes.

The allotropes of an element differ in physical properties. And though they are chemically the same, they differ in some chemical properties too.

For example:

Graphite is a good conductor of heat and electricity, whereas diamond is not. And, though both of them are chemically carbon, graphite burns in air to give carbon dioxide at 700°C, whereas diamond does so at 900 °C. Again, ozone (O₃) absorbs UV rays, whereas dioxygen (02) does not. And though both of them are oxygen, ozone is a much stronger oxidising agent than dioxygen.

Allotropy of Carbon

Carbon exists in crystalline as well as amorphous forms.

• Diamond, graphite and the fullerenes are the crystalline forms.
• And charcoals, lampblack, coke and gas carbon are the amorphous forms.
• The amorphous forms of carbon are found to contain extremely small crystals of graphite, and hence they are also called microcrystalline forms

The crystalline forms of carbon:

In its crystalline forms, a carbon atom is bonded to three or four other carbon atoms. These atoms, in turn, are bonded to other carbon atoms. In the different crystalline forms of carbon such as diamond and graphite, the atoms are arranged in a different manner.

1. Diamond:

Diamond is the costliest gemstone and the hardest natural substance known. It is so hard that it can only be cut by another diamond.

• Diamonds are formed at the high temperature and pressure that exist over
• 100 km below the Earth’s surface. They are brought to the surface along with the carrier rock —kimberlite—by volcanic action.
• They form only one part in over 15,000,000 (i.e., 15 million) parts of the rock.
• Diamonds are found mainly in Australia, Botswana and South Africa
• Diamond is artificially produced by heating graphite at a high temperature (say, 5000 °C) and pressure (say, 100,000 atmospheres).
• Diamond is generally colourless. However, the coloured varieties —yellow, brown, red, green, blue, grey or even black—are also found in nature.

The colour arises due to metallic impurities. The less costly varieties—grey and black —have no use as gemstones, and are used for cutting glass and drilling rocks

Diamond Properties:

1. Diamond is the hardest solid known.

2. It has a density of 3.51 g/cm3.

3. A properly cut diamond bends back a great percentage of the light falling on it. That is why it sparkles.The ability of a substance to bend light depends upon a property called refractive index. Diamond has a very high refractive index.

4. It has a very high melting point—3930 °C.

5. It is a bad conductor of electricity, i.e., it does not allow electric current to pass through it.

6. When ignited, it burns in air at 900 °C and in oxygen at 700 °C to give carbon dioxide

Diamond Structure:

In diamond, each carbon atom is bonded to four other carbon atoms. As shown in, the central carbon atom is bonded to four carbon atoms placed at the vertices of a tetrahedron. The other carbon atoms, in turn, are also tetrahedrally bonded to four carbon atoms each.

This kind of bonding results in the formation of a giant molecule, in which the carbon atoms are packed closely. The fact that the carbon atoms are so closely packed in diamond accounts for its high density and hardness. The strong chemical bonding between the atoms gives diamond its high melting point.

Diamond:

2. Graphite

Graphite is a black, opaque solid, found in large deposits in many countries like China, South Korea and India It is artificially prepared by strongly heating coke with silica in an electric furnace.

Graphite Properties:

• Graphite has a density of 2.2 g/cm3.
• Unlike diamond, it is very soft.
• Graphite melts at 3700°C.
•  It is a good conductor of heat andelectricity.
• It bums in air at 700 °C to give carbon dioxide

Graphite Structure:

Graphite contains layers of hexagonal rings of carbon atoms, joined together.

• Each carbon atom is shared by three rings.
• These rings occur in different planes, arranged parallel to each other.
• Each layer is held by the adjacent layer with weak forces.
• So, the adjacent carbon layers can slide over one another.
• As you can see, graphite and diamond have different structures. This accounts for the difference in their properties.

You will learn more about this in higher classes.

Graphite Uses:

•  Graphite electrodes are widely used.
• It is used as a solid lubricant for machines that work at high temperatures,
  • Example: The internal combustion engine of a motor vehicle.
• Graphite leaves a mark on paper. The ‘lead’ of a pencil is graphite mixed with clay. Being very soft, graphite has to be mixed with clay. The greater the proportion of graphite, the softer the pencil. The term graphite is derived from the Greek word grapho, meaning ‘I write’.
•  It is used for making crucibles for casting metals.
• A mixture of graphite and linseed oil is used for painting things made of iron
• It is used in nuclear reactors.

Fullerenes and carbon nanotubes

• Fullerenes and carbon nanotubes are crystalline forms of carbon.
• Nanotubes have played a great role in the development of an altogether new technology known as nanotechnology.
• You will leam about these allotropes in higher classes

The amorphous forms of carbon:

Unlike the crystalline form, the amorphous form of a substance contains loosely held particles.

• These particles easily separate and make available a large surface area.
• When a piece of a substance breaks into two, two new surfaces are created . If the pieces keep on breaking, newer surfaces appear. So, the more powdery a substance, the larger is the surface area.
• Because of its larger surface area, the amorphous form of a substance is generally more active than the crystalline form

The charcoals:

Charcoals are prepared by a process known as destructive distillation or pyrolysis. In destructive distillation, a substance is heated in the absence of air with a view to breaking larger molecules into smaller ones. During the process, certain substances distil out, and may be collected.

Wood charcoal:

Wood charcoal is prepared by the destructive distillation of wood. A mixture of gases and vapours evolves, and charcoal is left as a residue.

The mixture of gases (CO₂, CO, CH₄ and H₂) is combustible, and is known as wood gas. On being condensed, the vapours separate into a tar and a liquid. The liquid, called pyroligneous acid, contains some organic compounds.

The destructive distillation of wood can be carried out on a small scale in the laboratory.

Activity:

You can make wood charcoal at home, too. Put some wood shavings in a can, and cover it with a lid.

• Make a hole in the lid, and heat the can on a stove.
• Hold a lighted match to the mixture of gases emerging from the hole. It should burn with a steady flame
• Place a can full of ice near the hole.
• Substances in the mixture that have a low melting point will condense around this can. Turn off the stove when the gases stop evolving. Examine the cans after they have cooled.
• The wood shavings would have converted to charcoal. And there will be tar on the surface of the second can as well as on the inside surfaces of the first.

Being a form of carbon, wood charcoal shows the general properties of carbon, which we will study soon.

Activated charcoal:

Activated charcoal is prepared by heating wood charcoal at 900°C in a limited supply of air or steam.

• Any tar in the wood charcoal is removed, and the surface area of the charcoal increases greatly.
• 1 kg of activated charcoal in powder form has a surface area of 1 km².
• Hence, activated charcoal is much more active than simple wood charcoal.

Bone, or animal, charcoal:

Animal bones are first boiled with water to remove fatty substances and then subjected to destructive distillation in a retort.

The solid product in the retort is washed thoroughly with hydrochloric acid. The residual substance is bone charcoal.

Sugar charcoal:

Sugar charcoal is the purest form of carbon. It is prepared by the destructive distillation of cane sugar or by the dehydration of sugar with concentrated sulphuric acid

⇒ \(\begin{aligned}
&\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11} \rightarrow 12 \mathrm{C}+11 \mathrm{H}_2 \mathrm{O}\\
&\text { cane sugar }
\end{aligned}\)

Lampblack (soot):

When substances like oil and wax burn, soot is given out.

Activity:

You can prepare lampblack at home by burning mustard oil with the help of a wick and collecting the soot on a dish. The soot is used for preparing kajal and printer’s ink.

Coke:

Coke is obtained when coal is heated strongly in the absence of air. Coke is more porous and, therefore, more active than coal.

Gas carbon:

Gas Carbon deposits on the walls of a retort when a hydrocarbon is heated in it in the absence of air. Carbon is a good conductor of electricity and is, therefore, used as an electrode.

Some Useful Properties Of Carbon

We have already discussed some properties and uses of the different forms of carbon. Let us talk about a few more of its properties and uses.

Adsorption—A Physical Property

You have learnt that in adsorption, a thin layer of a substance is formed on the surface of another substance.

• A substance on the surface of which adsorption takes place is called an adsorbent.
• Adsorption is a surface phenomenon in which a thin layer of the adsorbed substance is formed only at the surface of the adsorbent.
  • For example: Moisture is adsorbed at any solid surface that is exposed to moist air. The formation of a thin film of oil over the surface of water is another example of adsorption.
• Being a surface phenomenon, adsorption is dependent on surface area.
• It is found that the larger the surface area of the adsorbent, the greater is the adsorption.

Adsorption by charcoals

As charcoals have a large surface area in powder form, they are good adsorbents.

The following uses of charcoals are based on this property:

• Wood charcoal is a good adsorbent of gases. It is, therefore, used in gas masks to remove poisonous gases. An active form of charcoal prepared from coconut shells is especially suitable for gas masks.
•  Bone charcoal (or animal charcoal) is used to remove the brown impurities from unrefined sugar.
• Many impurities of water are removed by filtration through charcoal. This is done in municipal water-treatment plants as well as in domestic UV purifiers.
• Activated coconut charcoal is used for separating a mixture of noble gases (helium, neon, argon, krypton and xenon). The different noble gases are adsorbed by the charcoal at different temperatures.
• Activated charcoal facilitates certain chemical reactions and is, therefore, used as a catalyst. For example, chlorine adsorbed on activated charcoal readily combines with hydrogen in the dark.

The reaction between chlorine and hydrogen in the dark is otherwise extremely slow

Chemical Properties and Their Uses

1. Reaction with oxygen or air:

On being lit, carbon burns in an excess of oxygen or air to form carbon dioxide

C + O₂ → CO₂+ heat

In an insufficient supply of air, carbon monoxide is formed.

2C + O₂ → 2CO + heat

Carbon monoxide also burns in air to form carbon dioxide

2CO + O₂ → 2CO₂ + heat

The heat produced in these reactions makes carbon a good fuel.

2. Reducing properties:

Carbon has a great affinity for oxygen. So, it combines with the oxygen present in many compounds, and thus acts as a reducing agent.

Reduction of metal oxides. When heated with coke or charcoal, the oxides of metals below aluminium in the activity series (e.g., zinc, iron, tin, lead and copper) are reduced to the corresponding metals

1. \(\underset{\text { zinc(II) oxide }}{\mathrm{ZnO}}+\mathrm{C} \xrightarrow{\text { heat }} \underset{\text { zinc }}{\mathrm{Zn}}+\mathrm{CO} \uparrow\)

2. \(\underset{\text { iron(III) oxide }}{\mathrm{Fe}_2 \mathrm{O}_3}+3 \mathrm{C} \xrightarrow{\text { heat }} \underset{\text { iron }}{2 \mathrm{Fe}}+3 \mathrm{CO} \uparrow\)

3. \(\underset{\text { tinc(I) oxxide }}{\mathrm{SnO}_2}+2 \mathrm{C} \xrightarrow[\text { heat }]{\mathrm{Sn}}+2 \mathrm{CO} \uparrow\)

4. \(\underset{\text { copper(II) oxide }}{\mathrm{CuO}}+\mathrm{C} \xrightarrow{\text { heat }} \underset{\text { copper }}{\mathrm{Cu}}+\mathrm{CO} \uparrow\)

This kind of reduction, called carbon reduction, is of great importance in metallurgy, i.e., the science of extracting metals from their ores and modifying them for use.

Reduction of water:

When steam is passed over red-hot coke, a mixture of carbon monoxide and hydrogen is formed. This mixture is called water gas

⇒ \(\underset{\text { lead(II) oxide }}{\mathrm{PbO}}+\mathrm{C} \xrightarrow{\text { heat }} \underset{\text { lead }}{\mathrm{Pb}}+\mathrm{CO} \uparrow\)

⇒  \(\underset{\text { copper(II) oxide }}{\mathrm{CuO}}+\mathrm{C} \xrightarrow{\text { heat }} \underset{\text { copper }}{\mathrm{Cu}}+\mathrm{CO} \uparrow\)

Water gas is of great industrial importance

Carbon Dioxide (CO₂)

Carbon dioxide is present in the air (-0.03%), and its amount is greater in cities than in the countryside.

Carbon dioxide is used by plants for photosynthesis and given out by plants and animals during respiration. It is present in the form of carbonates in minerals like limestone or marble (CaCO₃), dolomite (MgCO₃.CaCO₃), calamine (ZnCO₃), etc.

Obtaining CO₂

Carbon dioxide is formed in the following reactions

1. Burning of C and C-based fuels:

CO₂ is formed when C is burnt in an excess of air or oxygen

⇒ \(\mathrm{C}+\underset{\text { (excess) }}{\mathrm{O}_2} \xrightarrow{\text { burn }} \mathrm{CO}_2\)

In an insufficient supply of air, carbon monoxide is formed, which is a poisonous gas

⇒ \(2 \mathrm{C}+\mathrm{O}_2 \xrightarrow{\text { burn }} \mathrm{CO}\)

Carbon-based fuels like CNG (compressed natural gas, which is mainly methane) and LPG (liquefied petroleum gas, which is mainly butane) burn smoothly to form CO₂ and H₂O (vapour).

⇒ \(\underset{\substack{\text { methane } \\(\mathrm{CNG})}}{\mathrm{CH}_4}+2 \mathrm{O}_2 \xrightarrow{\text { burn }} \mathrm{CO}_2+2 \mathrm{H}_2 \mathrm{O}\)

⇒ \(\underset{\substack{\text { butane } \\ \text { (LPG) }}}{2 \mathrm{C}_4 \mathrm{H}_{10}}+13 \mathrm{O}_2 \xrightarrow{\text { burn }} 8 \mathrm{CO}_2+10 \mathrm{H}_2 \mathrm{O}\)

Fuels like petrol, kerosene and diesel are mixtures of hydrocarbons containing larger numbers of C atoms. They also burn to give CO₂ and H₂O

2. Thermal decomposition of carbonates:

The thermal decomposition of metal carbonates (except a few like Na₂CO₃ and K₂CO₃) gives CO₂

1. \(\underset{\begin{array}{c}
\text { magnesium } \\
\text { carbonate }
\end{array}}{\mathrm{MgCO}_3} \xrightarrow{\text { heat }} \underset{\substack{\text { magnenium } \\
\text { oxide }}}{\mathrm{MgO}}+\mathrm{CO}_2\)

2. \(\underset{\text { limestone }}{\mathrm{CaCCO}_3} \xrightarrow{\text { heat }} \underset{\text { quicklime }}{\mathrm{CaO}}+\mathrm{CO}_2\)

3. Action of an acid on a carbonate or a hydrogencarbonate:

When a carbonate or a hydrogencarbonate is treated with an acid, CO₂ is evolved with effervescence

1. \(\underset{\substack{\text { sodium } \\ \text { carbonate }}}{\mathrm{Na}_2 \mathrm{CO}_3}+\underset{\substack{\text { sulphuric } \\ \text { acid }}}{\mathrm{H}_2 \mathrm{SO}_4} \rightarrow \underset{\substack{\text { sodium } \\ \text { sulphate }}}{\mathrm{Na}_2 \mathrm{SO}_4}+\mathrm{CO}_2 \uparrow+\mathrm{H}_2 \mathrm{O}\)

2. \(\underset{\substack{\text { sodium } \\ \text { ndrogencarbonate }}}{\mathrm{NaHCO}_3}+\underset{\substack{\text { hydrochloric } \\ \text { acid }}}{\mathrm{HCl}} \underset{\mathrm{NaCl}}{ }+\mathrm{CO}_2 \uparrow\)

4. Fermentation of a sugar:

CO₂ is evolved during the fermentation of a sugar (in the presence of yeast), a process in which alcohol is formed. (This is how ethanol —an alcohol—is manufactured from the molasses obtained from the sugar industry.)

⇒ \(\underset{\text { (from the molasses) }}{\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6} \xrightarrow{\text { yeast }} \underset{\text { ethanol }}{2 \mathrm{C}_2 \mathrm{H}_5 \mathrm{OH}}+2 \mathrm{CO}_2\)

Laboratory preparation

Principle CO₂ is prepared in the laboratory by the action of dilute hydrochloric acid on marble (CaCO₃) chips.

⇒ \(\underset{\substack{\text { calcium carbonate marble } \\ \text { } \\ \text { }}}{\mathrm{CaCO}_3}+\underset{\substack{\text { hydrochloric acid (dilute) } \\ \text { } \\ \text { }}}{2 \mathrm{HCl}} \rightarrow \underset{\substack{\text { calcium chloride} \\ \text { }}}{\mathrm{CaCl}_2}+\mathrm{CO}_2 \mathrm{}+\mathrm{H}_2 \mathrm{O}\)

As the gas is heavier than air (~1.6 times), it is collected by the upward displacement of air.

Procedure:

Some marble chips are placed in a conical flask and covered by water. The flask is fitted with a thistle funnel and a delivery tube bent at 90° .

Dilute HCl is poured through the thistle funnel into the flask. A brisk reaction takes place. The gas evolved displaces the air inside the flask and the delivery tube first, and then collects in the gas jar by displacing the air upwards.

To test whether the jar is filled with CO₂, a lighted matchstick is brought near its mouth from time to time. When the flame gets extinguished, we infer that the jar is filled with CO₂. The delivery tube is taken out and the jar is closed by putting the lid in place.

(You could use sodium carbonate in place of marble chips and then dilute H₂SO₄ in place of dilute HCI. The reaction with marble is, however, smoother.)

Could we use H₂SO₄ instead of HCI with marble?

No, because calcium sulphate (CaSO₄) would be formed then. And the salt, being insoluble, would deposit on the chips , preventing them from coming in contact with the acid.

⇒ \(\mathrm{CaCO}_3+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \underset{\text { (insoluble) }}{\mathrm{CaSO}_4}+\mathrm{CO}_2 \uparrow+\mathrm{H}_2 \mathrm{O}\)

How about using H₂SO₄ with Na₂CO₃?

This is all right because the salt Na₂SO₄ formed is soluble.

Does the action of dilute HCI on marble chips give pure CO₂ ?

No, the gas is contaminated with HCI as the latter gets volatilised by the evolving CO₂. The gas can be freed from HCI vapours by passing it through a small amount of water before collection. Water dissolves its own volume of CO_2, but large volumes of HCl.

Properties of CO₂

Na₂CO₃ CO₂ has the following properties

Physical properties:

•  It is a colourless, odourless gas—1.67 times heavier than air
• At atmosphere pressure, the gas directly solidifies at-78.5 °C. The solid is called dry ice as it sublimes at this temperature (i.e., vaporises without turning into liquid).
• Under ordinary conditions, water dissolves its own volume of the gas. But under high pressures, the gas is highly soluble in water —the property that is used in making soda water or a fizzy drink. The dissolved CO₂ bubbles out vigorously when the pressure is released.

Chemical properties:

1. Combustion CO₂ is neither combustible nor a supporter of combustion. So, it is used in extinguishing a fire. Being heavier than air, the gas displaces air from the vicinity, and the fire gets extinguished. You can easily understand this by doing the following activity.

Activity:

Take a spoonful of baking powder in glass A and add some vinegarto it.

• CO₂ is evolved with effervescence (1).
• Let the effervescence subside (2).
• Hold glass A almost horizontally above glass B for a short while, taking care that no liquid flows from A to B (3) .Now, hold glass B in a ‘pouring mode’ over a lighted candle kept in a bowl.
• The flame is extinguished (4).

This shows that CO₂ is heavier than air. And so it can be ‘poured’ from one glass to another and then over the flame, which gets extinguished.

2. Reaction with active metals:

An active metal like Na, K or Mg abstracts oxygen from CO₂ (setting C free) when burnt in the gas.

Experiment:

• Take a gas jar full of CO₂ and introduce a burning piece of magnesium into it.
• The metal continues to bum in the gas. As a result, white smoky scales of magnesium oxide (MgO) are deposited on the inner walls of the jar.

In the reaction, some black carbon particles are also formed, which can be seen more clearly if the smoky scales of MgO are dissolved in dilute HCl.

3. Action on litmus:

The aqueous solution of the gas is weakly acidic and turns blue litmus wine-red. The acidic nature of the gas is generally tested by placing a moist blue litmus paper in the gas. An aqueous solution of C02 contains carbonic acid

⇒ \(\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \rightleftharpoons \underset{\text { carbonic acid }}{\mathrm{H}_2 \mathrm{CO}_3}\)

4. Reaction with metal oxides:

Metal oxides are generally basic in nature, and CO₂ is acidic. So, CO₂ slowly reacts with metal oxides to form the metal carbonates, which are salts.

1. \(\underset{\substack{\text { sodium } \\ \text { oxide }}}{\mathrm{Na}_2 \mathrm{O}}+\mathrm{CO}_2 \rightarrow \underset{\substack{\text { sodium } \\ \text { carbonate }}}{\mathrm{Na}_2 \mathrm{CO}_3}\)

2. \(\underset{\substack{\text { magnesium } \\ \text { oxide }}}{\mathrm{MgO}}+\mathrm{CO}_2 \longrightarrow \underset{\substack{\text { magnesium } \\ \text { carbonate }}}{\mathrm{MgCO}_3}\)

3. \(\underset{\substack{\text { calcium } \\ \text { carbonate }}}{\mathrm{CaOClcium}}+\mathrm{CO}_2 \rightarrow \underset{\text { oxide }}{\mathrm{CaCO}_3}\)

5. Reaction with limewater:

Limewater is a dilute solution of calcium hydroxide. The reaction of CO₂ with limewater may be studied in three parts.

1. The clear limewater turns milky when CO₂ is passed through it. This is because the insoluble, white substance calcium carbonate is formed.

⇒ \(\underset{\text { limewater }}{\mathrm{Ca}(\mathrm{OH})_2}+\mathrm{CO}_2 \longrightarrow \underset{\substack{\text { calcium } \\ \text { carbonate } \\ \text { (causing milkiness) }}}{\mathrm{CaCO}_3 \downarrow}+\mathrm{H}_2 \mathrm{O}\)

2. The milkiness disappears when an excess of CO₂ is passed through the liquid. This is because the soluble, colourless compound calcium hydrogencarbonate is formed.

⇒ \(\mathrm{CaCO}_3+\mathrm{H}_2 \mathrm{O}+\mathrm{CO}_2 \rightarrow \underset{\substack{\text { calcium hydrogencarbonate } \\ \text { (soluble) }}}{\mathrm{Ca}\left(\mathrm{HCO}_3\right)_2}\)

3. The milkiness reappears when the above solution is boiled. This is because the hydrogencarbonate, on being heated, decomposes back to give the carbonate, causing the milkiness.

⇒ \(\mathrm{Ca}\left(\mathrm{HCO}_3\right)_2 \xrightarrow{\text { heat }} \mathrm{CaCO}_3 \downarrow+\mathrm{H}_2 \mathrm{O}+\mathrm{CO}_2 \dagger\)

Carbon Dioxide and the Environment

In low concentrations in the air, CO₂ is essential for life but in high concentrations, it is a cause of great concern. Let us see how.

1. Photosynthesis:

We know that green plants use C02 for photosynthesis and manufacture their food glucose in the process

Thus, life seems to have originated on this planet from C02 and moisture.

2. The greenhouse effect:

A greenhouse is a glasshouse inside which we can grow plants. During the day, the rays enter the glasshouse and keep the plants warm. But the glass traps a part of the heat of the rays and keeps on radiating it back, even during the night, keeping the plants warm

The carbon dioxide present in the Earth’s atmosphere plays the same role as the glass in a greenhouse. The CO₂ traps the heat of the rays and keeps radiating it back so that the warmth of the Earth is retained. The rays heat the Earth directly, but a large portion of the rays are reflected back. So, without the warmth provided by the C02 of the atmosphere, the Earth would have been much colder during the nights.

A gas that traps heat in the environment and keeps the surroundings warm is known as a greenhouse gas and the whole effect as the greenhouse effect. Thus, CO₂ is a greenhouse gas and other such gases in our environment are mainly methane (emitted by cattle dung) and flurocarbons (used as coolants in fridges and air conditioners).

Global warming

What will happen if the concentration of CO₂ increases in our environment?

• If this happens
• The average temperature of the Earth will increase
• The polar ice caps and the glaciers will melt,
• The sea level will be raised, submerging several islands and coastal cities, and
• There will be drastic climatic changes.

All this will happen due to a rise in Earth’s average temperature, i.e., due to global warming.

Afforestation can bring down the dangers of global warming by using up the increased CO₂ in photosynthesis. But the opposite activity, i.e., deforestation, is on the increase due to increased population.

With increasing deforestation as well as industrialisation, the proportion of CO₂ in our environment has increased a lot. And we are already facing the dangers of global warming. The world community is making all-out efforts to bring the proportion of CO₂ down to permissible levels.

3. Action on natural water

Natural water is slightly acidic to slightly alkaline (ph 6.5-8.5). pH is a measure of the acidic or alkaline nature of a dilute solution. It is measured with the help of a pH-meter or a pH paper.

Pure water is neutral, i.e., neither acidic nor alkaline and has pH 7.0. Acidic solutions have pH < 7, and alkaline solutions, pH > 7. You know that, on dissolving in water, CO₂ forms the weak acid H₂CO₃.

So, the gas tends to bring down the pH of water that is open to the atmosphere. For drinking purposes, slightly acidic water is better than alkaline water. Acidic water (pH 6.5) is soft and palatable but alkaline water (pH 8) is hard and unpalatable. Also, the pH inside our digestive system is acidic.

On the other hand, marine organisms thrive better in alkaline water than in acidic water. And it has been found that the increasing proportion of dissolved CO₂ is lowering the pH of seawater. This is posing a threat to marine life

Uses of CO₂

1. CO₂ is used in the industry for the manufacture of

• Metal carbonates and hydrogencarbonates,
• Example: Soda ash (Na₂ CO₃), washing soda (Na₂ CO₃. 10H₂ O), potassium and baking soda, i.e., sodium hydrogencarbonate (NaHCO₃);
• Urea, CO(NH₂ )₂ ; and
• Ethanol, C₂ H₅ OH.

2. It is used in fire extinguishers.

3. It is extensively used for making soda water and fizzy drinks

4. In welding, CO₂ is used to prevent the oxidation of the metal by air.

5. In the form of dry ice, CO₂ is used as a coolant and for depicting smoke in plays, movies, and so on.

How a fire extinguisher works

Common fire extinguishers produce carbon dioxide by the action of sulphuric acid on a solution of sodium carbonate or sodium hydrogencarbonate.

⇒ \(\mathrm{Na}_2 \mathrm{CO}_3+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{Na}_2 \mathrm{SO}_4+\mathrm{H}_2 \mathrm{O}+\mathrm{CO}_2\)

⇒ \(2 \mathrm{NaHCO}_3+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{Na}_2 \mathrm{SO}_4+\underbrace{2 \mathrm{H}_2 \mathrm{O}+2 \mathrm{CO}_2}_{\text {gushes out }}\)

A solution of sodium carbonate or sodium hydrogencarbonate is placed in a fire extinguisher. A sealed bottle containing sulphuric acid is also placed there. In case of fire, the acid bottle is broken by striking the plunger against a hard surface.

The acid reacts with the sodium carbonate or sodium hydrogencarbonate, and carbon dioxide is formed. A mixture of water and carbon dioxide gushes out, which is directed towards the burning object. Carbon dioxide, being heavier than air, surrounds the burning object and cuts off the supply of air. As a result, the fire is put out

Carbon Monoxide

Carbon monoxide (CO) is a common product of the combustion of carbon-based fuels. Being a poisonous gas, it pollutes air. But, at the same time, the gas is used for many industrial purposes too.

So, we will look into both these aspects of the gas here

Poisonous Nature of CO:

Let us see how the gas affects us and how we can get rid of its effects.

Concentration of CO:

The concentration of a substance, expected to be present in very small quantities in a mixture, is often expressed in parts per million (ppm). This tells us the number of parts of a substance present in 1 million (1,000,000) parts of the mixture.

The concentration of CO in air is also expressed in ppm.

For example:

• A CO level of 100 ppm in a congested city indicates that 100 litres of CO is present in1 million litres of air in that city.
• The CO level in cities generally varies from 5 to 100 ppm.

How does CO affect us?

As you know, blood transports oxygen in our body. When we inhale air, the oxygen of the air combines with haemoglobin of the blood to form oxyhaemoglobin.

• Oxyhaemoglobin runs through blood vessels and gives up its oxygen to cells, which use it for respiration.
• But CO displaces oxygen from oxyhaemoglobin to form carboxyhaemoglobin.
• The affinity of CO for haemoglobin is 325 times greater than that of oxygen. So, the displacement takes place easily.
• The formation of carboxyhaemoglobin cuts off oxygen supply to cells, and adverse effects are seen. The severity of the symptoms depends on the amount of CO in the blood.
• Inhaling air with a high CO level may even be fatal. A person may die of asphyxia in an environment of CO. So, one should not sleep in a closed room heated by a coal fire

Effects of CO poisoning:

Curing CO poisoning

Carboxyhaemoglobin slowly loses CO on being exposed to an excess of oxygen, and oxyhaemoglobin is again formed. So, a person suffering from CO poisoning should be kept on oxygen till he or she recovers

Industrial Uses of CO

Carbon monoxide is an industrial gas. Some important uses of this gas are given below.

1. Itis usedasa reducingagentin metallurgy. It can abstract the oxygen from many metal oxides and set the metal free.

⇒ \(\underset{\text { metal oxide }}{\mathrm{MO}}+\mathrm{CO} \xrightarrow{\text { heat }} \underset{\text { metal }}{\mathrm{M}}+\mathrm{CO}_2\)

For example:

The mineral bauxite (which contains Fe203) is reduced by CO in a blast furnace to give iron.

⇒ \(\underset{\text { iron(III) oxide }}{\mathrm{Fe}_2 \mathrm{O}_3}+\mathrm{CO} \xrightarrow[\text {}]{\text { heat }} \underset{\text {iron oxide }}{2 \mathrm{FeO}}+\mathrm{CO}_2\)

⇒  \(\mathrm{FeO}+\mathrm{CO} \xrightarrow{\text { heat }} \underset{\text { iron }}{\mathrm{Fe}}+\mathrm{CO}_2\)

2. Carbon monoxide is also used in metallurgy for the refining of nickel.

3. It is used on a large scale in the industrial preparation of several organic compounds such as

• Methanol
• Acetic acid
• Hydrocarbons, and

Phosgene (COCl₂, used in the synthesis of some polymers)

Fuels

We know that heat (usually accompanied by light) is produced when a substance burns. A substance that is burnt to obtain heat or light from it is called a fuel. Thus, wood, cow-dung cakes, coal, petrol, diesel, kerosene, CNG and LPG are fuels.

Hydrogen is also a good fuel, but it is difficult to handle due to its explosive nature. These fuels (except hydrogen) contain carbon.

Calorific Value of a Fuel:

The amount of heat given out by 1 g of a fuel in air or oxygen is known as the calorific value of the fuel. It is expressed in kj/g.

The calorific value of hydrogen is the highest, and that of common fuels, much lower

The calorific values of some fuels:

That the calorific value of LPG is much higher than that of the traditional fuels can be realised by doing the following activity with the help of an adult.

Activity:

Take the same volume of water in two similar vessels. Heat one of them on an LPG stove and the other on burning cow-dung cakes. You will find that the water boils on the LPG stove much sooner than that on the burning cow-dung cakes.

(You could use thermometers to observe the rate at which the temperature of water increases in the two vessels).

Thus, attaining the cooking temperature on an LPG stove will be faster than on a conventional fire as that of cow-dung cakes

Of the C-based fuels, CNG is mainly methane (CH₄ ) and LPG, butane (C₄ H₁₀ ). Methane and butane contain only hydrogen and carbon, so they belong to a class of compounds known as hydrocarbons. Hydrocarbons burn to give CO₂ and H₂O.

• Petrol, kerosene and diesel are also mixtures of hydrocarbons containing higher numbers (7 and above) of carbon atoms. Sufficient oxygen for complete combustion of the fuel may not be available from the surroundings, and so some CO and soot are also formed.
• Besides, some oxides of nitrogen and sulphur are also formed on the burning of the impurities in them. These gases and soot pollute the air.
• A good-quality coal contains about 90% and a poor-quality one, about 55% carbon. Coal is used as a fuel, but the products of combustion (CO₂ , CO, NO₂ , SO₂ , SO₃ and soot) are undesirable.
• When wood or dung cake is burnt, we get CO₂, H₂O and some CO along with a small amount of the oxides of nitrogen.
• Coal and petroleum are known as fossil fuels as they are formed by a biochemical process called fossilisation.
• Natural gas is a decay product of marine organisms and collects over petroleum in petroleum wells. Petroleum gas is obtained by the cracking (i.e., the breaking of bigger molecules into smaller ones) of petroleum.

Promoting Cleaner Fuels

The use of fuels like wood, cow-dung cake and coal for cooking and of petrol and diesel in motor vehicles is accompanied by the emission of pollutants like

• Carbon monoxide,
• Oxides of sulphur and nitrogen, and
• Soot.

CNG and LPG are considered to be much cleaner fuels than those mentioned above because, on complete burning, they form C02 and H20 only.

• These days, CNG is fast replacing petrol and diesel in motor vehicles, especially the commercial ones.
• LPG is highly recommended for use as a fuel in the kitchen. Extremely poor people can’t afford it.
• On the other hand, a large number of premature deaths occur every year on account of the use of conventional fuels.
• So, in 2016 the Government of India launched a welfare scheme to reach out to the BPL families (i.e., those who are below the poverty line) with free LPG connection. The scheme is known as the Pradhan Mantri Ujjwala Yojana.
• It aims at giving free LPG connections to 50 million women from BPL families —most living in remote rural areas. This will also help keep our environment clean.