Optional Science Book Class 10

5. pOH : The negative logarithm of molar concentration of hydroxyl ions is called pOH.

6. pOH scale: The scale of hydroxyl ions concentration which is used to express acidic
and basic strength of an aqueous solution is called pOH scale.

7. pH meter: The scientific device which is used to measure the exact pH value of a
solution without colour matching in the pH chart is called pH meter.

8. Neutralization reaction: The chemical reaction in which an acid loses its acidic
property and the base also loses its basic property to give neutral products is called
neutralization reaction.

Introduction

In 1800 AD Volta had performed an experiment by passing current in water. As a result, he
found bubbles of hydrogen gas from the cathode and bubbles of oxygen gas from anode. Here,
electric current split water into hydrogen and oxygen gases. This process is called electrolysis
and water is called an electrolyte. Similarly, in various physical and chemical processes,
electricity is either consumed or produced. Different types of chemical reactions occur in
solution state. For example: neutralization reaction or acid base reaction, chemical reactions in
simple cell, battery, etc. Similarly, some chemical reactions also occur by passing electricity in
the solution. For example: electrolysis, electroplating, electro-refining, electrotyping, etc. These
chemical reactions are studied in a separate branch of chemistry called electrochemistry. Thus,
electrochemistry is a branch of physical chemistry which deals with relationship between
chemical energy and electrical energy and how one is converted into another. In this unit,
we will discuss about the nature of water, pH and pOH and application of neutralization
reactions in our daily life.

Ionic Product of Water

Water is a universal solvent. It dissolves most of the chemical substances

in it. Pure water is a weak electrolyte. It is neutral with pH value 7.0. It

undergoes very less ionization in normal temperature and pressure. Water

undergoes self-ionization into opposite Ion ization +H
ions. After ionization, water produces -------;>
hydrogen ion (H+) and hydroxide ion

(OH-).This self-ionization of water can be Water mol ecule Hydroxyl ion Hydrogen ion
written as: H2O ⇋H+ + OH- [H2 0 I
[OHl [H•)

Memory Plus

We know that hydrogen ion is very reactive species. It reacts further with water molecule

to form hydronium ion(H3O+).
H2O + H+ H3O+

By applying the law of mass action, the ionization constant of water (K) can be given as:

K = [H+][OH—]
[H2O]

or, [H+][OH-] = K×[H2O]

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As we have discussed that, water is a very weak electrolyte. It undergoes very less ionization
to give hydrogen ion and hydroxyl ion. As a result, the concentration of non-dissociate water
molecules [H2O] is more or less constant. Therefore, the product of ionization constant of
water (K) and concentration of water [H2O] gives another constant. This constant is called
ionic product of water (Kw).

Kw= [H+][OH-]

The product of molar concentration of hydrogen ion (H+) and hydroxide ion (OH-) produced
by self-ionization of water at a particular temperature is called ionic product of water.

Memory Plus

Derive the relation of ionic product of water.

Water undergoes self-ionization into opposite ions. As a result, it produces hydrogen ion

(H+) and hydroxide ion (OH-).

This self-ionization of water can be written as:
H2O ⇋ H+ + OH-

By applying the law of mass action, the ionization constant of water (K) can be written as:

K = [H+][OH—]
[H2O]

or, [H+][OH-] = K×[H2O]

Kw= [H+][OH-]

Here, Kw is called ionic product of water. It is the product of ionization constant of water
(K) and concentration of water [H2O].

Water behaves like an acid as well as a base. This is because water undergoes self-ionization
to give hydrogen ion(H+) and hydroxide ion (OH-). The substance which increases the
concentration of hydrogen ion (H+) makes water acidic and the substance which increases
the concentration of hydroxide (OH-) ion makes water basic. However, the pure water always
produces equal amount of hydrogen ion concentration and hydroxide ion concentration.
Thus, the product of molar concentration of hydrogen ion (H+) and hydroxide ion(OH-)
produced by self-ionization of water at a particular temperature is called ionic product of
water. The symbol of ionic product of water is Kw. The value of ionic product of water (Kw)
depends upon temperature. Kw increases with increasing temperature and decreases with
decreasing temperature. Therefore, the concentration of H+ and OH- ions increases with
increasing temperature and decreases with decreasing temperature. The value of Kw at
different temperature is given in the table.

Temperature The value of Kw
0.11 × 10-14
0°C 0.31 × 10-14
10°C 0.86 × 10-14
20°C 1.00 ×10-14
25°C 2.91 × 10-14
40°C 9.61 × 10-14
60°C 7.50 × 10-14
100°C

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From the above table, it is clear that the value of Kw at 25°C is 1 x 10-14. As pure water is neutral
in nature, after auto-ionization the concentration of hydrogen ion [H+] and concentration of
hydroxyl ion [OH-]must be equal.

[H+] = [OH-] = x
or, [H+][OH-] = x2 = 1 × 10-14 at 25°C temperature
or, x = 1 × 10-7 mol/liter

or, [H+] = [OH-] = 1 × 10-7 mol/ liter

Memory Plus

The value of ionic product of water (Kw) at 25°C is 1 x 10-14.

As we have discussed that pure water produces equal amount of hydrogen ion and hydroxyl
ion. Addition of outer substance increases or decreases their concentration. When an acid or
a base is added to water, the ionic concentration product [H+][OH-] remains constant and it
becomes equal to Kw. Meanwhile the concentrations of H+ and OH- ions do not remain equal.
For example, the addition of acid increases the hydrogen ion concentration and decreases the
hydroxyl ion concentration.

So, for acid solution[H+] > [OH-]

or [H+] > 1 × 10-7 mol/l

Similarly, when a base is added to the water, the hydroxyl ion concentration [OH- ] increases
and hydrogen ion concentration [H+] decreases.

So, for alkaline solution [OH-] > [H+]

or, [OH-] > 1 × 10-7 mol/l

From the above discussion it is clear that, if hydrogen ion concentration is more than 1× 10-7 mol/l,
the solution becomes acidic. Similarly, if hydrogen ion concentration is less than 1 × 10-7 mol/l,
the solution becomes alkaline and if hydrogen ion concentration is equal to 1 × 10-7 mol/l, the
solution becomes neutral.
The different values of hydrogen ion concentration and the nature of the solution is given in
the table.

The value of hydrogen ion concentration [H+] Nature of Water

10-0, 10-1, 10-2, 10-3, 10-4, 10-5, 10-6 Acidic
10-7 Neutral
10-14, 10-13, 10-12, 10-11, 10-10, 10-9, 10-8 Alkaline

Similarly, the different values of hydroxyl ion concentration and the nature of the solution is
given in the table.

The value of hydroxyl ion concentration [OH-] Nature of Water
0-14, 10-13, 10-12, 10-11, 10-10, 10-9, 10-8 Acidic
10-7 Neutral
10-0, 10-1, 10-2, 10-3, 10-4, 10-5, 10-6 Alkaline

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At 25°C temperature, the product of concentration of hydrogen ion and hydroxyl ion is
always constant and it is equal to 1×10-14 mol/l. If hydrogen ion concentration is more, then
the hydroxyl ion concentration will be less. From the above discussion, it is clear that every
aqueous solution contains both H+ and OH- ions and the product of their concentrations is
always constant, i.e., equal to 1 × 10-14 .

For example:
If [H+] = 10-3 mol/l, then [OH-] = 10-11 mol/l
Thus, the product of [H+][OH-] = 10-3 × 10-11 = 10-14
As hydrogen ion concentration is more than 10-7 mol/l , the solution is acidic.
Similarly, if [H+] = 10-12 mol/l, then [OH-] = 10-2 mol/l
Thus, the product of [H+][OH-] = 10-12 × 10-2 = 10-14
As hydrogen ion concentration is less than 10-7 mol/l , the solution is alkaline.

Fact with reason

Hydrochloric acid increases hydrogen ion concentration in water. How?

Hydrochloric acid ionizes in water to give hydrogen ion and chloride ion. Since it
produces hydrogen ion in water the concentration of hydrogen ion increases in water.

pH and pOH of a solution

Normally, there are three types of solutions. They are acidic solution, basic solution and neutral
solution. The nature of a solution is expressed in terms of hydrogen ions concentration. As we
know that pure water is a weak electrolyte, it produces very less concentration of hydrogen
ion and hydroxyl ion in the solution. So, it is very difficult to express in the general number. To
remove the difficulty, Sorensen in 1909 AD, introduced the popular term called pH and pOH.

pH

We know that strong acids undergo almost complete ionization to give a large amount of
hydrogen ion concentration and weak acids undergo very less ionization. The nature of the
solution, i.e. acidity, alkalinity or neutral can be expressed in terms of the hydrogen ions
concentration [H+]. The measure of hydrogen ions concentration present in a given solution
is called pH.

In the solution, the concentration of hydrogen ions is very small. It is inconvenient to use.
So, mathematically, it is expressed as a negative power to the base 10. Thus, the negative
logarithm of molar concentration of hydrogen ions is called pH. Form the above definition, it
is clear that,

pH = -log[H+]

In pure water, the molar concentration of hydrogen ions is 1 × 10-7 mol/l at 25°C. Thus, the pH
of pure water can be expressed as,
pH = -log[H+]

pH = -log[10-7 ]

=7

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Memory Plus

More hydrogen ions concentration indicates less pH value and less hydrogen ions
concentration indicates more pH value.

Therefore, the pH value of pure water is 7. This pH value also indicates that the solution is
neutral. In a given solution, if molar concentration of hydrogen ions is more than 10-7 mol/l, or
pH value less than 7, it indicates that the given solution is acidic in nature. Similarly, in a given
solution if molar concentration of hydrogen ions is less than 10-7 mol/l, or pH value more than
7, it indicates that the given solution is alkaline in nature.

If pH value is equal to seven It is a neutral solution
If PH value is less than seven It is an acidic solution
If pH value is more than seven It is an alkaline solution

pH Scale

An acidic solution has more hydrogen ions concentration and a basic solution has more
hydroxyl ions concentration. The relation between hydrogen ions concentration and pH value
is just opposite to each other. More hydrogen ion concentration means less pH value and vice-
versa. Therefore, to express the nature of solution in terms of hydrogen ions concentration,
a simple, convenient but standard scale is introduced. It is called pH scale. Thus, the scale of
hydrogen ions concentration which is used to express acidic and basic strength of an aqueous
solution is called pH scale.

pH scale is calibrated from zero to fourteen. The middle point of this scale is seven. It is a
neutral point and it indicates a neutral solution. The pH value more than seven indicates that
the solution is alkaline and the pH value less than seven indicates acidic solution. The pH
value zero is for the strongest acid and pH value fourteen is for the strongest base.

pH=0 pH=1 pH=2 pH=3 pH=4 pH=5 pH=6 pH=7 pH=8 pH=9 pH=10 pH=11 pH=12 pH=13 pH=14

Acidic solution Neutral solution Basic solution

Example: 1

Calculate pH of a solution whose hydrogen ions concentration is 10-2 mol/l.

Solution: Given,
Hydrogen ions concentration = 10-2 mol/l.
pH value = ?
Now, pH = -log[H+]
pH = -log[10-2 ]
=2

Example: 2

An aqueous solution of sulphuric acid has hydrogen ions concentration 10-2 mol/l. Calculate
its pH.

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Solution:
Given,

Hydrogen ions concentration = 10-2 mol/l.
pH value = ?
Now, pH = -log[H+]
pH = -log[10-2 ]

=2

Memory Plus

In pH, the p stands for potenz. It is a German word meaning power. Thus, pH indicates
power of hydrogen ion concentration.

pOH

We have discussed that, pH value of a neutral solution is 7. If pH value increases from 7 to
14, it represents an increase of hydroxyl ion concentration in the solution. So, similar to the
pH scale, the acidic and basic strength of an aqueous solution can also be represented in term
of hydroxyl ions concentration (OH-). Thus, the negative logarithm of molar concentration of
hydroxyl ions is called pOH. Form the above definition, it is clear that,

pOH = -log[OH-]

In pure water, the molar concentration of hydroxyl ions is 1 × 10-7 mol/l at 25°C. Thus, the pOH
of pure water can be expressed as,

pOH = -log[OH-]

pOH = -log[10-7 ]

=7

Therefore, the pOH value of pure water is 7. This pOH value also indicates that the solution
is neutral. In a given solution, if molar concentration of hydroxyl ions is more than 10-7 mol/l,
or pOH value less than 7, it indicates that the given solution is alkaline in nature. Similarly, in
a given solution if molar concentration of hydroxyl ions is less than 10-7 mol/l, or pOH value
more than 7, it indicates that the given solution is acidic in nature.

If pOH value is equal to seven It is a neutral solution
If POH value is less than seven It is an alkaline solution
If pOH value is more than seven It is an acidic solution

pOH Scale

A basic solution has more hydroxyl ions concentration and an acidic solution has less hydroxyl
ions concentration. The relation between hydroxyl ions concentration and pOH value is just
opposite to each other. More hydroxyl ion concentration means less pOH value and vice-
versa. Therefore, to express the nature of solution in terms of hydroxyl ions concentration, a
simple, convenient but standard scale is introduced. It is called pOH scale. Thus, the scale of
hydroxyl ions concentration which is used to express acidic and basic strength of an aqueous
solution is called pOH scale.

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pOH scale is calibrated from 0 to fourteen. The middle point of this scale is seven. It is a
neutral point and it indicates a neutral solution. The pOH value more than seven indicates
that the solution is acid and the pOH value less than seven indicates basic solution. pOH value
zero is for the strongest base and pOH value fourteen is for the strongest acid.

pOH pOH pOH pOH pOH pOH pOH pOH pOH pOH pOH pOH pOH pOH pOH
= 0 = 1 = 2 = 3 = 4 = 5 = 6 = 7 = 8 `= 9 = 10 = 11 = 12 = 13 = 14

Basic solution Neutral solution Acid solution

Example: 1

Calculate pOH of a solution whose hydroxyl ions concentration is 10-3 mol/l.
Solution: Given, Hydroxyl ions concentration = 10-3 mol/l

pOH = ?
Now, pOH = -log[OH-]
pOH = -log[10-3 ]

=3
Therefore, the solution is basic in nature.

Example: 2

Calculate pOH of an aqueous solution of potassium hydroxide whose hydroxyl ions
concentration is 10-2 mol/l.

Solution: Given, Hydroxyl ions concentration = 10-2 mol/l
pOH = ?
Now, pOH = -log[OH-]
pOH = -log[10-2 ]

=2

Therefore, the solution is basic in nature.

Relation between pH and pOH
Pure water undergoes auto-ionization to give hydrogen ion and hydroxyl ion. From these
hydrogen and hydroxyl ions, we can express ionic product of water.

H2O ⇋H+ + OH-
[H+] [OH-] = Kw
We know that at 25°C, the value of Kw is 10-14
Taking -log on both sides of the equation, we have,
- log [H+] +(- log [OH- ]) = - log [Kw ]
- log [H+] +(- log [OH- ]) = - log [10-14 ]

pH + pOH = 14

The above expression is a relation between pH and pOH.
Therefore, for pure water the sum of pH and pOH is equal to 14. If a solution has pH value 3
than the pOH value will be 11.

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Example: 1

Calculate pOH of a solution whose hydrogen ions concentration is 10-3 mol/l.
Solution: Given, Hydrogen ions concentration = 10-3 m/l

The value of pOH = ?
According to the formula,

pH =- log [H+]
pH =- log [10-3 ]
pH = 3
Now, pH + pOH = 14
3 + pOH = 14
The value of pOH = 11
The solution has pH value less than 7 or pOH value more than 7. So, it is an acidic solution.

Example: 3

An aqueous solution of potassium hydroxide has hydroxyl ions concentration 10-2 mol/l.
Calculate pOH and pH value.
Solution: The hydroxyl ions concentration = 10-2 m/l.

The value of pOH = ?
The value of pH = ?
From the formula

pOH = -log[OH-]
pOH = -log[10-2 ]

=2
Now, pH + pOH = 14

pH + 2 = 14
pH = 12
Hence, the given solution is basic in nature as it has pH value more than 7 and pOH value
less than 7.

pH meter

The lab technician measures the pH value of different body fluids like blood, urine, etc. in the
pathology. A chemist measures pH value of different chemicals in the chemistry laboratory.
Similarly, a gardener or agriculturist measures the pH value of soil. While measuring pH value
of the different substances, it is necessary to find the exact value of pH. Generally, we use pH
paper to find their pH value. While measuring pH value through pH paper, the colour change
in pH paper is matched with the colour chart of the pH scale. The matching of colour of pH
paper with pH colour chart, it is very difficult to find the exact result. So, we use a scientific
device to measure the pH value of the solution. It is called pH meter. Thus, the scientific
device which is used to measure the exact pH value of a solution without colour matching in
the pH chart is called pH meter.

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CHEMISTRY

The pH meter contains a rectangular box, pH rod and conducting wire. I
While measuring the pH value of a given solution, the rod is dipped inside fig:pH meter
the solution and the switch is turned on. After the switch turned on, the pH
value of the solution comes directly on the number.

The pH Value of Some Substances is given in the table

S.N. Substances or Compounds pH

1. Hydrochloric acid (HCl) 1.0
2. Sulphuric acid (H2SO4) 1.2
3. Lemon juice 2.5
4. Juice of apples, vinegar and carbonic acid 3.0
5. Butter 6.0
6. Ethanol, water, salt and sugar solution 7.0
7. Human blood 7.3
8. Baking soda 8.5
9. Ammonium hydroxide (NH4OH) 10.0
10. Sodium carbonate or washing soda (Na2CO3) 11.5
11. Caustic soda or sodium hydroxide (NaOH) 13

Neutralization reaction

Take a test tube and put some amount of hydrochloric acid and sodium hydroxide. After their
reaction, we will get sodium chloride (NaCl) and water (H2O) called an acid-base reaction. As
a result of this reaction, acid and base lose their own properties. Hence,this reaction is also
called a neutralization reaction.

For example:

Acid + Base → Salt + Water

HCl + NaOH → NaCl + H2O

H2SO4 + KOH →K2SO4 + H2O

The chemical reaction in which an acid loses its acidic property and the base also loses its basic
property to give neutral products is called neutralization reaction. Normally, in neutralization
reaction, an acid and a base react together to form salt and water. The salt thus produced may
be acidic, basic or neutral depending upon the nature of the acid and base. The neutralization
actually involves the combination of H+ ions and OH- ions to produce water. The products of
neutralization reaction after the combination of a strong acid and a strong base have the pH
value 7. The neutralization products of a strong acid and a weak base have pH value less than
7. Similarly, when a strong base reacts with a weak acid, it gives the products having pH value
greater than 7. The above discussion can be summarized in the following box.

S.N. Acid Base Resultant Salt Resultant pH value

1. Strong acid Strong base Neutral salt pH value equal to 7
2. Strong acid Weak base Acidic salt pH value less than 7
3. Weak acid Strong base Basic salt pH value more than 7
4. Weak acid Weak base Neutral salt pH value equal to 7

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Memory Plus

The most common strong acids are HCl, H2SO4, HNO3, etc. Similarly, the strong bases are
LiOH, NaOH, KOH, RbOH, Ca(OH)2 Sr(OH)2, Ba(OH)2, etc.

i) The neutralization reaction between strong acid and strong base
HCl(aq) +NaOH(aq) ⇋ NaCl(aq) + H2O(l)

Observe the above chemical reaction between hydrochloric acid and sodium hydroxide. In
this reaction, hydrochloric acid is a strong acid and sodium hydroxide is a strong base. When
equal amount of hydrochloric acid and sodium hydroxide are mixed together, they give
sodium chloride and water. Sodium chloride is a neutral salt and water is a neutral liquid. So,
both have pH value seven. The ionic combination of this reaction can be expressed as:

H++Cl−+Na++OH− ⇋ NaCl +H2O

ii) The neutralization reaction between weak acid and weak base
H2CO3(aq) +2 NH4OH(aq) ⇋ (NH4)2CO3(aq)+2H2O(l)

Observe the chemical reaction between carbonic acid (H2CO3) and ammonium hydroxide
(NH4OH). In this reaction, carbonic acid is a weak acid and ammonium hydroxide is a weak
base. When equal amount of carbonic acid (H2CO3) and ammonium hydroxide (NH4OH)are
mixed together, they give ammonium carbonate and water. The actual ionic combination of
this reaction is given below.

2H++CO3−− + NH4++OH−⇋ (NH4)2CO3(aq)+ H2O(l)

Here, ammonium carbonate is a neutral salt. Its aqueous solution has pH value seven. Similarly,
water is a neutral liquid with pH value seven. Thus, when a weak acid and a weak base fully
neutralize, the products have pH value 7.

Applications of the neutralization reaction

Soil Test

The top most fertile layer of the earth is called soil. We use soil to grow crops. There are
different kinds of crops which grow in the soil of different nature. We know that different
soil contain different kinds of nutrients. That is why the pH value of the soil is different in
different places. It is observed that the soil of a certain pH value is suitable for the healthy
growth and development of the plants. It would be better to grow the crops after knowing the
concentration of the nutrients and pH value of the soil. But, it is also found that the pH value
and the concentration of the nutrients change as we grow the same crop continuously for a
long time. The pH value also changes due to the different kinds of environmental pollutions,
such as acid rain, solid wastes, chemical fertilizers, etc. Therefore, it is necessary to test the soil
before sowing the crops.

Measurement of pH value of the Soil

In the pH scale, the pH value ranges from 0 to 14. The pH value seven represents neutral
solution. The pH value below seven represents acidity and pH value above seven represents
alkalinity. Zero pH value represents severe acidity and fourteen is an extreme alkalinity.

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Plants require a specific pH range for their healthy growth and development. The pH value of
a soil is the measure of its acidity and alkalinity. The pH of the soil in the crop-land should be
in the range of 6.0 to 7.5. If pH value becomes more or less than the specific range, it directly
affects the presence of nutrients in the soil. So, it is necessary to test the soil time and again
to find the pH value and concentration of the nutrients. If pH value changes, we need to add
certain chemicals to balance it. Similarly, if concentration of the nutrients changes, we need to
add organic or chemical fertilizers to maintain the concentration of the nutrients.

Memory Plus

The favourable pH range of the soil for the healthy growth and development of the plants
is 6.0 to 7.5

Neutralization of the soil

It is necessary to neutralize soil in order to promote the healthy growth and development of
the plants. The pH value of the surrounding soil affects the absorption of nutrients from the
soil into the roots of the plants. Different kinds of environmental pollution, solid wastes, acid
rain, etc. increase the acidity of the soil. The acid present in the soil must be neutralized by
using lime or calcium oxide (CaO). The process of adding lime to reduce acidity of the soil
is called liming. In the liming process, agricultural limestone, i.e. calcium carbonate (CaCO3)
along with some amount of magnesium carbonate (MgCO3) is used to neutralize acidity of
the soil. In some area, the soil becomes too basic. To reduce the alkalinity, we add some acid
substances such as calcium sulphate (CaSO4), sulphur (S2), etc. to the soil.

Memory Plus

The acidity of the soil is reduced by using agricultural limestone, i.e. calcium carbonate
(CaCO3) along with some amount of magnesium carbonate (MgCO3). Similarly, alkalinity
is reduced by adding calcium sulphate (CaSO4), sulphur (S2), etc.

Treatment of Hyper-acidity

Our stomach produces hydrochloric acid which helps in the digestion of food without harming
stomach. During indigestion, large amount of hydrochloric acid accumulates in the stomach.
This acid causes pain and irritation. It also causes heart burn, sour fluid vomiting, bombast,
etc. This problem can be solved by using bases present in the tablets called antacids. Antacids
contain aluminium hydroxide, magnesium hydroxides, magnesium carbonate, magnesium
trisilicate, etc. In the market, antacids are present in various brand-names. They are available
in tablets as well as in liquids. Some antacids are combined with another medicine called
simeticone which helps to reduce wind (flatulence).

Memory Plus

Antacids contain aluminium hydroxide, magnesium hydroxides (milk of magnesia),
magnesium carbonate, magnesium trisilicate, etc.

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Treatment of insect stings
Have you ever been stung by a honeybee? Bee-sting leaves an acid which causes pain and
irritation. This acid can be neutralized by using chemicals containing mild-base such as
sodium bicarbonate or baking soda. Similarly, wasp sting also injects alkaline venom in our
body. Since venom is alkaline in nature, baking soda does not work in this situation. In this
case, mild acids such as vinegar can relieve the pain.

Memory Plus

Bee-sting gets relief by using sodium bicarbonate and wasp sting by using vinegar.

Answer writing skill

1. Define ionic product of water. Write down its equation.

Ans: The product of molar concentration of hydrogen ion (H+) and hydroxide ion (OH-)
produced by self-ionization of water at a particular temperature is called ionic product
of water. The equation of ionic product of water is Kw = [H+][OH-]

2. What are pH and pOH scale?

Ans: The scale of hydrogen ions concentration which is used to express acidic and basic
strength of an aqueous solution is called pH scale. Similarly, the scale of hydroxyl ions
concentration which is used to express acidic and basic strength of an aqueous solution
is called pOH scale.

3. What is the main difference between pH paper and pH meter?

Ans: pH paper is a device which is used to measure pH value of the solution comparing its
colour change with the colour chart. But, pH meter is a scientific device which is used to
measure the exact pH value of a solution without colour matching in the pH chart.

4. What is acid base reaction? Why is it called a neutralization reaction?

Ans: The chemical reaction in which an acid reacts with base to give salt and water is called
acid-base reaction. In acid-base reaction, an acid loses its acidic property and the base
also loses its basic property to give neutral products. So, it is also called a neutralization
reaction.

5. How does hyper acidity occur in our stomach? What can be done to relief it?

Ans: Our stomach produces hydrochloric acid which helps in the digestion of food without
harming stomach. During indigestion, large amount of hydrochloric acid accumulates
in the stomach causing heart burn, sour fluid vomiting, bombast, etc. This problem can
be solved by using mild bases present in the tablets or liquids called antacids. Antacids
contain aluminium hydroxide, magnesium hydroxides (milk of magnesia), magnesium
carbonate, magnesium trisilicate, etc.

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6. Derive the relation to show that [H+] = [OH-] = 1 × 10-7 mol/ liter

Ans: At 25°C temperature, the value of ionic product of water (Kw) is 1 × 10-14. After self-
ionization, the concentration of hydrogen ion [H+] and concentration of hydroxyl ion
[OH-] must be equal.
Thus, [H+] = [OH-] = x
or [H+][H+] = x2 {Since, in value[H+] = [OH-] }
or [OH-][OH-] = x2
or, [H+][OH-] = x2 = 1 × 10-14 at 25°C temperature
or, x = 1 × 10-7 mol/l
or, [H+] = [OH-] = 1 × 10-7 mol/ liter

7. Derive the relation between pH and pOH.

Ans: Pure water undergoes self-ionization to give hydrogen ion and hydroxyl ion. From these
hydrogen and hydroxyl ions, we can express ionic product of water.
H2O ⇋H+ + OH-
[H+] + [OH-] = Kw
We know that at 25°C, the value of Kw is 10-14
Taking -log on both sides of the equation,
We have,
- log [H+] +(- log [OH- ]) = - Kw [10-14 ]
- log [H+] +(- log [OH- ]) = - log [10-14 ]

pH + pOH = 14

8. Write down any three application of the neutralization reaction.
Ans: The three applications of the neutralization reaction reactions are:

i. The acidity of the soil is reduced by using agricultural limestone, i.e. calcium
carbonate (CaCO3) along with some amount of magnesium carbonate (MgCO3).
Similarly, alkalinity of the soil is reduced by adding calcium sulphate (CaSO4),
sulphur (S2), etc.

ii. The hyperacidity in the stomach can be solved by using antacids which contain
aluminium hydroxide, magnesium hydroxides (milk of magnesia), magnesium
carbonate, magnesium trisilicate, etc.

iii. Bee-sting gets relief by using sodium bicarbonate and wasp sting by using vinegar.

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9. An aqueous solution of a compound has hydrogen ions concentration 10-2 mol/l.
Calculate its pH and pOH value. Also, mention its chemical nature.

Solution:
Given, Hydrogen ions concentration = 10-2 mol/l.
pH value = ?
pOH value = ?
Now, pH = -log[H+]
pH = -log[10-2 ]
=2
Again, pH + pOH = 14
2 + pOH = 14
pOH = 12
Here, pH value is less than 7 and pOH value is more than 7. So, the given chemical
compound is an acid.

10. Describe an activity to test the pH value of the different soil samples.
Ans: i. Collect different samples of soil from the different places.

ii. Put about 2 g of soil in each test tube separately.
iii. Put distilled water in each sample of soil and shake well.
iv. Allow the solution for sedimentation.
v. Filter the solution and separate the clear liquid.
vi. Dip a pH paper in the clear solution.
vii. Match the colour change in the pH paper with the colour chart and find the pH

value of the soil sample.

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Exercise

1Step

1. Define the following terms.

i. Ionic product of water (Kw)
ii. pH
iii. pH scale
iv. pOH
v. pOH scale
vi. pH meter
vii. Neutralization reaction

2. Very short answer questions

1. What is pH range? Show in the pH scale.
2. Write down the value of hydrogen ion concentration in pure water.
3. Write down the equation for the ionic product of water.
4. Write down the pH and pOH value of pure water.
5. What happens when a strong acid reacts with a strong base?
6. What kind of products can be obtained when weak acid reacts with a weak base?
7. Write down the pH value of the products when strong acid reacts with strong base?
8. What are antacids? Write down any two examples.

2Step

1. Give reason.

i. Water is a neutral solution.
ii. Hydrochloric acid increases hydrogen ion concentration in water. How?
iii. Sodium hydroxide increases hydroxyl ion concentration in water. How?

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2. Draw a pH scale showing acid, base and neutral range.
3. What is pOH scale? Show with pOH scale.
4. What is pH meter? Why is it better than pH paper?
5. What treatment can be done in bee sting?

3Step

1. What is hyperacidity? Describe the process to reduce it.
2. Write down any three application of the neutralization reaction.
3. If the pH of a solution is 4, what is its pOH?
4. Calculate pOH and pH value of aqueous solution of potassium hydroxide whose

hydroxyl ions concentration is 10-3 mol/l.
5. Calculate pH and pOH of a solution whose hydrogen ions concentration is 10-4 mol/l.
6. Calculate pOH and pH of a solution whose hydroxyl ions concentration is 10-3 m/l.
7. Calculate pOH and pH of aqueous solution of calcium hydroxide whose hydroxyl

ions concentration is 10-3 mol/l.

4Step

1. What is pH meter? How does it work?
2. Describe the importance of pH value of soil. How does pH value of soil change?

How can we balance the pH value of the soil?
3. Derive the relation to show that H+] = [OH-] = 1 × 10-7 mol/ liter
4. Derive the relation between pH and pOH.
5. Describe an activity to test the pH value of the soil sample.

Multiple choice questions:

1. The ionic product of pure water at 25°C is

a. 10-7 mol/l b. 10-14mol/l

c. 10-10mol/l d. 10-5mol/l

2. What is the pH range of crop land?

a. pH 2 to 7 b. pH 6 to 8
c. pH 5.5 to 7.5 d. pH 6 to 7.5

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3. What is the pOH value of pure water?

a. 1 b. 7
d. 0
c. 14
4. pH is the measure of

a. Percentage of water molecules b. Percentage of hydrogen atoms

c. Percentage of hydroxyl ions d. Hydrogen ion concentration
5. pOH is the measure of

a. Hydrogen ion concentration b. Percentage of hydrogen atoms

c. Percentage of hydroxyl ions d. Hydroxyl ion concentration
6. Which acid causes hyperacidity in the stomach?

a. Sulphuric acid b. Nitric acid
d. Acetic acid
c. Hydrochloric acid
7. Pure water is

a. Slightly acidic b. Slightly alkaline

c. Neutral d. Highly acidic

8. Which chemical is added to reduce acidity of soil

a. Calcium sulphate b. Calcium carbonate
d. Calcium borate
c. Calcium nitrate
9. Which is an antacid?

a. Sodium chloride b. Lactic acid
d. Magnesium hydroxide
c. Citric acid
10. Which is milk of magnesia

a. Aluminium hydroxide, b. Magnesium carbonate
c. Magnesium trisilicate, d. Magnesium hydroxide

Project Work

1. Collect sample of soil from your surroundings and calculate their pH value
using pH paper and pH colour chart.

2. Collect a citrus fruit and extract juice from it. With help of pH paper and pH
colour chart find out its pH value.

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UNIT ORGANIC CHEMISTRY

11

·······················································---------.............................................................

Vladimir Vasilyevich Markovnikovborn (in 1838 AD-1904) was born in Nizhny
Novgorod, Russia and died in Moscow. He formulated a theory called Markovnikov
addition theory. It is used in addition of hydrogen halides to the carbon-carbon double
bond or unsymmetrical alkenes.

••••••••••••••••••••••••••••••••••••••., •••••••• • - - - - - - u •- •••••••••• •• ., ••••••••••••••••••••••••••••••••••••••••••••••••••••••

Syllabus issued by CDC Learning objectives:
• Introduction to organic reactions After completing the study of this unit, students
will be able to:
* Substitution reaction
* Addition reaction * Introduce substitution reaction, addition reaction
* Eliminationreaction and elimination reaction in organic chemistry.
• Electrophyles and nucleophyles
• Types of chemical reactions • Explain electrophyles and nucleophyles.
• Markovnikov rule • Explain markovnikov rule and Saytziff’s rules
• Saytziff’s rules

Key terms and terminologies of the unit

1. Organic chemistry: The branch of chemistry in which we study about structure,
properties, composition, reactions and preparation of the carbon-containing
compounds is called organic chemistry.

2. Organic compounds: The chemical compounds which contain carbon as one of the
primary elements are called organic compounds.

3. Organic reactions: Those chemical reactions which involve organic compounds and
production of organic compounds are called organic reactions.

4. Substitution reactions: Those chemical reactions in which an atoms or groups of
atoms in a molecule are replaced by another atoms or groups of atoms are called
substitution reactions.

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5. Addition reactions: Those chemical reactions in which atoms or groups of atoms are
added to the organic molecule which has one or more multiple covalent bonds are
called addition reactions.

6. Elimination reactions: Those chemical reactions in which atoms or group of atoms
are eliminated from the organic molecules to give unsaturated product are called
elimination reactions.

7. Nucleophiles: Nucleophile are atoms or groups of atoms which provide a pair of
electrons to form a new covalent bond.

8. Electrophiles: Electrophiles are atoms or groups of atoms which accept a pair of
electrons to form a new covalent bond.

9. Markovnikov's rule: According to Markovnikov's rule "when an unsymmetrical
alkene reacts with hydrogen halide to give an alkyl halide, the hydrogen adds to the
carbon of the alkene that has the greater number of hydrogen atoms and the halogen
to the carbon of the alkene with the fewer number of hydrogen atoms".

10. Saytjeff’s rule: According to Saytjeff’s rule "the most substituted product is more
stable and is favoured over other products."

Introduction

In our surrounding we see different kinds of substances. These substances are called matter.
The study of physical and chemical properties of these matters is called chemistry. In chemistry
there are mainly three branches. They are physical chemistry, inorganic chemistry and
organic chemistry. In inorganic chemistry we study about inorganic substances, in organic
chemistry we study about organic substances and in physical chemistry we study about
physical properties of the organic and inorganic substances. Thus, the branch of chemistry
in which we study about structure, properties, composition, reactions and preparation of the
carbon-containing compounds is called organic chemistry. It has broad area. It covers not only
the compounds made up of hydrocarbon but also the compounds with other elements like,
nitrogen, oxygen, halogens, phosphorus, silicon, sulphur, etc.

Memory Plus

Urea is an organic compound and naturally it is produced in the kidneys of human beings.

HCl is produced in our stomach but it is an inorganic compound because it does not
contain carbon atom.

Previously, organic chemistry was limited to the chemical compounds which were produced
within the living organisms. But, after the formation of urea in the laboratory, organic
chemistry became a vast branch of science. This credit goes to the chemist Friedrich Wohler.
Now, organic chemistry covers the man-made compounds like monomers, polymers, plastics,
dyes, drugs, etc. and natural compounds like protein, carbohydrates, fat, vitamins, etc. So,
the range of application of organic compounds is very high. It includes pharmaceuticals,
petrochemicals, food, explosives, paints, cosmetics, textiles, etc. In this unit, we will discuss
about some organic compounds and the related chemical reactions.

CHEMISTRY 0Optional Science - 10 219

Organic compounds
The chemical compounds which contain carbon as one of the primary elements are called
organic compounds. It includes large number of chemical compounds like methane, ethane,
propane, alcohol, ether, carbohydrates, proteins, fat, vitamins, plastics, dyes, drugs, etc. But,
organic chemistry does not include some chemical compounds which contain carbon as one of
the elements. For example, carbides, carbonates, cyanides, etc. There are two types of organic
compounds. They are:

Memory Plus

Propane is known as acyclic compound because it has open chain of carbon atoms.

a. Open chain or acyclic organic compounds

It includes those compounds which contain open chain of carbon atoms. For example,

methane, ethane, propane, butane, alcohol, ether, etc. They are also called aliphatic

compounds.

H HHH

I III

H- C - H H-C-C-C-H

I

H 1II
HHH

Methane Propane

b. Closed chain or cyclic compound

It includes those organic compounds which have closed ring of carbon atoms. For

example: cyclopropane, cyclobutane, cyclopentane, cyclohexane, etc. Cyclic compounds

are also known as ring compounds. --H2C
--H2CI
/ \CH2 H2C CH2 CH2
\
I I CH2
;
H2C CH2
H2C CH2 CH2

Cyclopropane Cyclobutane Cyclopentane

Both acyclic and cyclic compounds have covalent bond. Thus, organic compounds must
have two properties. They are:

i. Organic compounds must have carbon atom as one of the elements.

ii. There must have covalent bond between the carbon atoms.

The organic compounds differ from inorganic compounds in many respects. Such

Faasctcowmitphosrietiaosno, nisomerism, solubility, melting point, boiling point, etc. Some organic

Cacrobmidpeos,ucnadrsbowniathtems, oclyeacnuildaresf,oermtc.ulha,asvteruccatrubroanl faotrommulbau, tcothnedyenasredcafollremduilnaoarngadntihcree
comdipmoeunnsdiosn. aWl hstyru? ctures are given below.

. Carbides, carbonates, cyanides, etc. have carbon atom but they are called inorganic
compounds because they do not have covalent bond in them.

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Some of the organic compounds along with their molecular formula, structural formula,
condensed formula and the three dimensional structure are given below.

Organic Molecular Condensed expanded 3-D structural
structural formula formula
Compounds formula structural formula

ethane C2H6 CH3CH3

butane C4H10 CH3CH2CH2CH3

cyclohexane C6H12

ethene C2H4 CH2 CH2

ethyne C2H2 HC CH H—C C—H

Uses of organic compounds

There are large number of natural and man-made organic compounds which are very
important for both plants and animals. They are the pillars to the economic growth of the
country. Economic growth becomes fast if the country can manufacture different kinds of
organic compounds like plastics, rubber, fuel, pharmaceutical, cosmetics, detergent, dyes,
agrochemicals, etc. Some of the major uses of the organic compounds are given below:

1. Industrial use of organic compounds: Organic compounds are used for manufacturing
different kinds of organic materials like dynes, detergent, soap, fuel, plastic, rubber,
cosmetics, pharmaceutical goods, pesticides, fertilizers, etc.

2. Domestic use of organic compounds: Large numbers of organic compounds are also
used in our home. For example: sugar, oil, ghee, etc.

3. Medicinal use of organic compounds: Various organic compounds are used for the
treatment of diseases.

4. Religious use of organic compounds: Some organic compounds are religiously very
important. For example: chandan plant is used as tika in Hindu religion.

5. Laboratory use of organic compoundsSome organic compounds are used in the
laboratory as the laboratory reagents. They are used in chemical test, research, etc.

On the basis of use and study of organic compounds, the organic chemistry also has many
branches. For example: biochemistry, biotechnology, medicine, pharmaceutical, etc.

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Memory Plus

Nobel Prize was established in 1895 in name of Swedish chemist, engineer and inventor
Alfred Noble. It honors the people in six different fields. They are: chemistry, physics,
medicine, economics, literature and peace.

Organic Reactions

In inorganic chemistry we have studied various kinds of chemical reactions like addition
reaction, decomposition reaction, displacement reaction, neutralization reaction, etc. In
organic chemistry, we will study about addition reactions, elimination reactions, substitution
reactions, pericyclic reactions, rearrangement reactions, photochemical reactions, redox
reactions, etc. The production of many man-made chemicals such as drugs, plastics, food
additives, fabrics also depend up on organic reactions.

The chemical reactions which involve organic compounds and production of organic
compounds are called organic reactions. The organic reactions are divided into two groups.
They are basic organic reactions and advanced organic reactions. The earlier organic reactions
were combustion of organic fuels and saponification of fats to make soap. But, the modern
organic chemistry starts with the Wohler synthesis. In this synthesis, urea was formed by
the thermal decomposition of ammonium cyanide in the laboratory. The invention of some
organic reactions such as Grignard reaction , Diels-Alder reaction, Wittig reaction , olefin
metathesis, etc. have changed the history of organic chemistry. In this unit, we will study
about some basic organic reactions.

i. Substitution reaction ii. Addition reaction iii. Elimination reaction

Fact with reason

Wohler synthesis is considered as a milestone in the history of organic chemistry.
Why?

It is because Wohler synthesized urea in the laboratory from ammonium cyanide.
Thereafter, chemists denied the vital force theory of the organic compounds which was a
milestone in the history of chemistry and extend the area of organic chemistry.

i. Substitution reactions

CH3-CH2-Cl +NaOH  CH3-CH2-OH + NaCl

Observe the above chemical reaction. In this reaction, ethyl chloride reacts with sodium
hydroxide to give ethyl alcohol and sodium chloride. In this reaction, chlorine of ethane
is replaced by hydroxyl group of sodium hydroxide. Such type of reaction is called
substitution reaction. Those chemical reactions in which an atom or groups of atoms
in a molecule are replaced by another atoms or groups of atoms are called substitution
reactions. In substitution reaction, the saturation and unsaturation of the organic
compounds do not change. It means that unsaturated molecules remain unsaturated and
saturated molecules remain saturated after the reaction. For example:

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Example: 1

Ethanol (CH3CH2OH) reacts with hydrogen iodide (HI) to form iodoethane (CH3CH2I)
and water. Here, hydroxyl group (OH) of ethanol has been replaced by an iodine atom (I).

CH3- CH2-OH + HI CH3- CH2-I + H2O

HH HH
H C C OH + HI
H C C I + H2O
HH HH

Example: 2

Ethyl chloride (CH3CH2Cl) reacts with potassium cyanide (KCN) to form ethyl cyanide
(CH3CH2CN) and potassium chloride. Here, chloride group (Cl) of ethyl chloride has
been replaced by cyanide group.

CH3-CH2-Cl +KCN CH3-CH2-CN+ KCl

HH HH
H C C Cl + KCN
H C C CN + KCl
HH HH

Example: 3

The gas ethane (CH3CH3 ) reacts with bromine vapour in presence of light to form
bromoethane (CH3CH2Br) and hydrogen bromide (HBr).

CH3-CH3 +Br2 CH3-CH2-Br + HBr

HH HH

H C C H + Br2 H C C Br + HBr
HH HH

ii. Addition reactions

Observe the above chemical reaction. In this reaction hydrogen reacts with ethene
molecule to give ethane. In this reaction, one hydrogen atom is added to each carbon
atom. Such type of chemical reaction is called addition reaction. Thus, those chemical
reactions in which atoms or groups of atoms are added to the organic molecule which has
one or more multiple covalent bonds are called addition reactions. In addition reactions,
the saturation of the organic molecules increases. It means that alkyne becomes alkene
and alkene becomes alkane. After addition reaction, molecular mass becomes equal to
the sum of molecular mass of the reacting molecules.

Memory Plus

The chemical reactions in which an unsaturated molecule reacts with another
molecule to form a saturated molecule is called addition reaction.

Fact with reason

The saturation of hydrocarbon increases after addition reaction. Why?

After addition reaction the saturation of the hydrocarbon molecules increases because
the number of multiple bonds between carbon atoms decreases.

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Example: 1

When ethene reacts with hydrogen bromide, it produces ethyl bromide. In this reaction,

hydrogen atom combines with one carbon atom and bromine atom combines with

another carbon atom. So, it is an addition reaction.

CH2 = CH2 + HBr  CH3 - CH2Br

-H H HH
H C C Br
C C + HBr
HH HH

Example: 2

Ethyne (C2H2) is an unsaturated hydrocarbon. It has a triple covalent bond between
carbon atoms. It combines with two molecules of hydrogen bromide to give 1, 1

dibromoethane (CH3CHBr2). As a result of this reaction, the two double bonds of ethyne
molecule break down and two atoms of bromine and two atoms of hydrogen combine.

Therefore, it is a kind of addition reaction.

CH≡CH +2HBr  CH3-CHBr2

H C C H +2HBr H Br

H C CH
H Br

Example: 3

When ethene reacts with hydrogen molecule, it produces ehane. It is also a kind of
combination reaction.

CH = CH + H  CH - CH
2 22 33

-H H H2 HH

'C C/ + H C CH
'H/ H HH

iii. Elimination reactions

An elimination reaction is just opposite of addition reaction. In elimination reaction, a
saturated molecule is converted into an unsaturated molecule. During the reaction, a
saturated compound eliminates two atoms from the adjacent carbon atoms. As a result,
multiple covalent bonds are formed. Thus, those chemical reactions in which atoms or
group of atoms are eliminated from the organic molecule to give unsaturated product are
called elimination reactions.

Example: 1

Ethanol (CH3CH2OH), when mixed with concentrated sulphuric acid (H2SO4), it
produces ethene(CH2= CH2). In this reaction, one hydrogen atom (H) and one hydroxyl
group (OH)are eliminated at the same time. After the elimination of these atoms, the

saturated ethanol converted into unsaturated ethene. At the end of elimination reaction,

the molecular mass of product molecules is less than reactant molecules.

CH3-CH2OH conc. H2SO4 CH2 = CH2 + H2O

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CHEMISTRY

H HH OH conc. H2SO4 HH
CC
'C C/ + H2O
HH 'H/ H

Example: 2

When ethyl bromide is boiled with alcoholic solution of potassium hydroxide, it produces

alkene. In this reaction, one hydrogen and one bromine atoms are eliminated to give

ethene. Alcoholic KOH

CH3-CH2Br CH2 = CH2 + HBr

HH HH
'H C C Br Alcoholic KOH C C/ + HBr
'H H
H/ H

Example: 3

When bromoethene (CH2=CHBr) reacts with alcoholic potassium hydroxide, it loses one
hydrogen atom and one bromine atom. As a result, ethyne (C2H2 ) molecule is obtained.
In this reaction double bonded reactant molecule is converted into triple bonded product.

CH3= CHBr Alcoholic KOH CH ≡ CH + HBr

H Br

'C C/ Alcoholic KOHCH ≡ CH + HBr
'H/ H

S.N. Addition reaction S.N. Substitution reaction
Those chemical reactions in which atoms
1. Those chemical reactions in which 1. or groups of atoms are eliminated from
atoms or groups of atoms are added the organic molecules to give unsaturated
to the organic molecule which has products are called elimination reactions.
one or more multiple covalent
bonds are called addition reactions. In this reaction, saturated reactant
molecules become unsaturated.
2. In this reaction, unsaturated reactant 2.
molecules become saturated.

Electrophiles and Nucleophiles

On the basis of excess and deficiency of electrons,

there are two types of chemical species. They are Nucleophile Electrophile

called nucleophiles (electron rich) and electrophiles

(electron poor). During the chemical reaction, these

species move towards the specific direction. The movement of these species depends upon the

availability of electrons. We know that opposite charges attract each other and like charges repel

each other. Therefore, electrophile and nucleophile combine together whereas nucleophile

and nucleophile or electrophile and electrophile repel each other. During chemical reaction,

electrons flow from “electron rich” area to the “electron poor” area. In another word, electrons

always flow from the nucleophile to the electrophile. The flow of electrons is represented by

overhead arrow.

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Nucleophiles

Nucleophile is a Greek word. It has two parts. Nucleon means “nucleus” and philos means
“lover”. Therefore, Nucleophiles means “nucleus loving” species. Since, nucleus is
positively charged area, therefore nucleophiles are also known as “positive-charge loving”
species. Nucleophile has unshared pair electrons on it. It means it is electron rich species. The
direction of movement of nucleophiles is always towards the electron poor area. As a result,
nucleophile makes a covalent bond with electron poor species. Thus, nucleophile is an atom or
group of atoms which provides a pair of electrons to form a new covalent bond. Nucleophiles
are either negatively charged species or neutral molecules with one pair of available electrons.
They are generally denoted by the symbol “Nu”.

Memory Plus

Nucleophiles are rich of electrons, so they attack electrophiles.
The property of a nucleophile due to which it donates electrons is called nucleophilicity.

i. Negatively charged nucleophiles
These nucleophiles have a negative charge on them. For example:

Name of negative nucleophiles Symbols

Hydroxide ion OH-

Chloride ion Cl-
Bromide ion Br-
Iodide ion I-
Cyanide ion CN-

Oxide ion O2-
Sulphide ion S2-
Nitride ion N3-

Alkoxide ion RO-

ii. Neutral nucleophiles
These are neutral molecules with one pair electron for donation. For example:

Name of neutral nucleophiles Symbols

Water H2O
Ammonia NH3
Alcohol R-OH

Ether R-O-R

Amines R-NH2

0226 Optional Science - 10 ( CHEMISTRY l

Electrophiles

Like nucleophiles, electrophile is a Greek word. It has two parts. Electron means “electron”
and philos means “lover”. So, electrophiles are“electron loving” species. As electrophiles are
electron poor species, they receive electrons from the electron rich species (nucleophiles). So,
the movement of electrophiles is always towards electron rich or negatively charged species.
Therefore, electrophiles are also known as “negative-charge loving” species. They have the
electron deficient centre. They accept electrons and form covalent bond. Thus, electrophiles are
atoms or groups of atoms which accept a pair of electrons to form a new covalent bond.
Electrophiles are reactive species and undergo reaction in suitable conditions. They can accept
an electron pair from the donor compound because they are either positively charged or
neutral molecules with electrons deficient centres. The chemical reaction which involves the
electrophiles is known as electrophilic reaction. Electrophiles are generally denoted by the
symbol “E”. There are two types of electrophiles. They are:

Memory Plus

Electrophiles are poor of electrons, so they attack nucleophiles
The property of an electrophile due to which it accepts electrons is called electrophilicity.

i. Positively charged electrophiles: These electrophiles have a positive charge on them. For
example:

Name of positively charged electrophiles Symbols

Hydronium ion H3O+
Hydrogen ion H+
Ammonium ion
Nitronium ion NH4+
Carbonium ion or carbocation NO2+
R3C+

ii. Neutral electrophiles: These electrophiles have electron deficient centers. For example:

Name of neutral electrophiles Symbols
Sulphur trioxide
Boron trifluoride SO3
Boron trichloride BF3
BCl3

Markonikov's rule

Markonikov’s rule is very important experiential rule. It is used to find out the final products

when hydrogen halides are added to alkenes. This rule states that "when an unsymmetrical

alkene reacts with hydrogen halide to give an alkyl halide, the hydrogen adds to the carbon of

the alkene that has the greater number of hydrogen atoms and the halogen to the carbon of the

alkene with the fewer number of hydrogen atoms". Alternatively, this rule can be stated that

the hydrogen atom is added to the carbon with the greatest number of hydrogen atoms while

halogen is added to the carbon with the least number of hydrogen atoms.

CHEMISTRY 0Optional Science - 10 227

Memory Plus

Markonikov's rule says that while adding hydrogen halide to the unsymmetrical alkene,
hydrogen always adds to the carbon which has more number of hydrogen atoms.

Example: 1

Reaction of propene with hydrogen bromide is explained with the help of Markonikov’s
rule. During this reaction, first of all, hydrogen bromide breaks down into hydrogen ion and
bromide ion. Hydrogen ion goes to the carbon atom where the numbers of hydrogen atoms are
two. At the same time bromide ion goes to the carbon atom where there is only one hydrogen
atom. Therefore, the product of this reaction is 2-bromo propane, but not 1-bromo propane.

H CI CI - H + HBr H Br H
H
H CI H I I Ii H C C C H
H III

HHH

Markonikov's Product

H H Br

ii I I I
HCCCH
III
HHH

Anti Markonikov's Product

Saytjeff’s rule

Saytzeff rule is pronounced as Zaitsev Rule. This rule looks like opposite of Markonikov’s
rule. This rule is used to predict the alkene products after elimination reaction. It can be used
in different elimination reactions. It states that, in an elimination reaction, the less hindrances
and more stable products are favoured over less stable and more hindrance products. Thus,
Saytzeff rule states that "the most substituted product is more stable and is favoured over
other products."

The stability of the compounds having multiple bonds is increased by electron withdrawing

groups. But the electron donating groups always decrease the stability. The examples of

electron donating groups are alkyl groups such as methyl, ethyl, propyl, etc. As these groups

are larger in size, they create more hindrance by making the alkene less stable. So, elimination

always occurs to give less bulky group towards the double bonds. CH3

i H3C CH2 CH C

HHHHH H

HCCCCCH

H H H Br H Satyzeff's Product

_/ H

"iiH3C CH2 CH2 CH C

H

Anti Satyzeff's Product

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Memory Plus

When hydrogen is removed from alkane than a group is formed called alkyl group.

Example: 1

Reaction (i) CH3-CH2-CH2 –CH2-CHBr–CH3  CH3-CH2-CH=CH-CH3 (2-Penene) It is a major
product.
Reaction (ii) CH3-CH2-CH2 –CH2-CHBr–CH 3  CH3-CH2-CH2-CH=CH2 (1-Penene) It is a
minor product
In the given example, when hydrogen and bromine are removed from 2-bromopentane,
two products are possible. They are 2-Pentene and 1-Pentene. According to Saytzeff Rule,
2-Pentene (Satyzeff’s product) is more possible than 1-Pentene (anti Satyzeff’s product). This
is because 2-Pentene has less hindered as compared to 1-Pentene.

Answer writing skill

1. Define organic chemistry.

Ans: The branch of chemistry in which we study about structure, properties, composition,
reactions and preparation of carbon containing compounds is called organic chemistry.

2. What are organic compounds? Write down the necessary conditions to be the organic
compouns?

Ans:The chemical compounds which contain carbon as one of the primary elements are called
organic compounds. For examples: protein, fat, vitamins, carbohydrates, methane,
ethane, etc.

Two conditions which are necessary to be the organic compounds are:

i. Organic compounds must have carbon atom as one of the elements.

ii. There must have covalent bond between the carbon atoms.

3. What is the main difference between addition and elimination reaction?

Ans: In addition reaction, atoms or groups of atoms are added to the molecule while in
elimination reaction, atoms or groups of atoms are eliminated from the molecule.
After addition reaction, the compound becomes more saturated while after elimination
reaction the compound becomes less saturated.

4. What are nucleophiles and electrophiles? Write down any two examples of each.

Ans: Nucleophile are atoms or groups of atoms which provide a pair of electrons to form a
new covalent bond. For example: OH- , Cl- , CN- , H2O, NH3, etc.
Electrophiles are atoms or groups of atoms which accept a pair of electrons to form a
new covalent bond. For example: H3O+, NH4+, SO3, BF3, etc.

CHEMISTRY 0Optional Science - 10 229

5. Write two differences between the Markonikov’s rule and the Satyzef’s Rule.
Ans: The differences between the Markonikov’s rule and the Satyzef’s Rule are given below.

S.N. Markonikov’s rule S.N. Satyzef’s Rule

1. According to this rule "when an 1. Saytzeff rule states that "the most

unsymmetrical alkene reacts with substituted product is more stable

hydrogen halide to give an alkyl halide, and is favoured over other products."

the hydrogen adds to the carbon of the

alkene that has the greater number of

hydrogen atoms and the halogen to

the carbon of the alkene with a fewer

number of hydrogen atoms".

2. It is used to predict the product of the 2. It is used to predict the product and

addition reaction. stability of the elimination reactions.

6. Write down any three uses of organic compounds.

Ans: The uses of organic compounds are given below.

i. Industrial use of organic compound: Organic compounds are used for
manufacturing different kinds of organic materials like dynes, detergent, soap,
fuel, plastic, rubber, cosmetics, pharmaceutical goods, pesticides, fertilizers, etc.

ii. Domestic use of organic compounds: Large numbers of organic compounds are
also used in our home. For example: sugar, oil, ghee, etc.

iii. Medicinal use of organic compounds: Various organic compounds are used for

the treatment of diseases. i

8. Complete the given chemical HHHHH

reaction. Which product is more
stable and why? Give reason. H C C C C C H

Ans: In the given example, when H H H Br H

hydrogen and bromine are

removed from 2-bromopentane, ii

two products are possible. They

are 2-Pentene and 1-Pentene. According to Saytzeff Rule, 2-Pentene (Satyzeff’s product)

is more possible than 1-Pentene (anti Satyzeff’s product).This is because 2-Pentene is less

sterically hindered as compared to 1-Pentene. As we know that according to Satyzeff’s

rule the product with less sterically hindered is major and more stable.

CH3

HHHHH i H3C CH2 CH C

H

HCCCCCH Satyzeff's Product
H H H Br H

H

_/

ii H3C CH2 CH2 CH C~ H

Anti Satyzeff's Product

230 Optional Science - 10

CHEMISTRY

- - - -~Exercise -----

1Step

1. Define the following

a. Organic chemistry b. Organic compounds

c. Nucleophiles d. Electrophyles

e. Addition reaction f. Substitution reaction.

g. Elimination reaction.
2. What are negative nucleophiles? Write any two examples.

3. What are neutral eletrophiles? Write any two examples.

4. What is Markonikov’s rule?

5. State Satyzef’s rule.

2Step

1. Differentiate between:
a. Organic compounds and inorganic compounds.
b. Nucleophiles and electrophiles.
c. Addition reaction and elimination reaction.
d. Markonikov’s rule and Satyzef’s rule.

2. Give reason.
a. Carbides, carbonates, cyanides, etc. are called inorganic compounds.
b. Wohler synthesis is considered as a milestone in the history of organic chemistry.
c. The saturation of hydrocarbon increases after addition reaction.
d. BF3 is an electrophile. Why?

3. Give an example of the chemical reaction to explain Markonikov’s rule.

4. Give an example of the chemical reaction to explain Satyzef’s rule.

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3Step

1. What are the uses of organic compounds in our daily life?

2. Explain the importance of markonikov’s rule.

3. Describe the importance of Satyzef’s rule.

4. According to which rules do the given reactions belong to? State the rules.

(i) CH3-CH2-CH2 –CH2-CHBr–CH  CH3-CH2-CH=CH-CH3

3

(ii) CH3-CH=CH2 +HBr CH3-CHBr-CH3

5. Identify the types of reactions with reason.

i) CH2 = CH2 + H2  CH3 - CH3 ii) CH3-CH2-Br + H2O  CH3-CH2-OH + HBr

4Step

1. What is carbocation? In which reaction does it form? Explain this reaction with one example.
2. What types of reaction are called substitution reactions? Describe with the help of reactions.
3. Explain Markovnikov's and Saytjeff’s rules with the help of chemical reactions.

Multiple choice questions (MCQ):
1. A nucleophile is a reactant that………a pair of electrons to form a new covalent bond.

a. Shares b. Donates
d. Accepts
c. Bonds
2. Which is an electrophile? b. Cl
d. None of the above
a. H2O
c. R3C+ b. SCN-
3. Which is a neutral nucleophile? d. H2O

a. BF3
c. OH-
4. Identify neutral eletrophile?

a. NH4+ b. NO2+
c. BCl3 d. All above

5. Addition reaction

a. Increases saturation of the products b. Decreases saturation of the products

c. Saturation does not change d. None of the them

6. Addition of …………to alkene explains Markonikov’s rule.

a. NH3 b. NaCl
c. HBr d. CH4

0232 Optional Science - 10 CHEMISTRY

UNIT METALS AND
METALLURGY
12

Georgius Agricola born on 1495 AD and died on 1555 AD. His real name is George Bauer.
Georgius Agricola is his Latin version name. He was the pioneer chemist who studied
about the Earth, rocks, minerals and fossils. He was the first person who studied about
metallurgy and mineralogy. He also studied structural geology and paleontology. He
wrote the first book on physical geology named De Ortu et Causis Subterraneorum in 1946.

Syllabus issued by CDC Learning objectives:

• Introduction to aluminium, silver After completing the study of this unit, students

and gold will be able to:

• General metallurgy of aluminium, • Describe the metallurgical process of

silver and gold aluminium, silver and gold.

• Physical properties, chemical • Explain the physical and chemical properties

properties and uses of aluminium, of aluminium, silver and gold.

silver and gold • Explain the uses of aluminium, silver and gold.

• Alloy and amalgum • Describe the alloys and amalgam.

Key terms and terminologies of the unit

1. Ore: ore is a mineral which contains large amount of the particular metal.

2. Metallurgy: metallurgy is a process of different steps by which pure metal can be
extracted from the particular ore.

3. Aquaregia: Aquaregia is a homogenous mixture of three part of concentrated
hydrochloric acid and one part of concentrated nitric acid.

4. Alloy: Alloy is a homogeneous mixture of two or more metals or sometimes metals
and non-metals.

5. Amalgam: Amalgam is an alloy of mercury with other metals like sodium, potassium,
silver, tin, copper, zinc, etc.

CHEMISTRY 0Optional Science - 10 233

Introduction

Metals are present on the earth's crust. Some metals are also found in the form of compounds
in seawater. Examples: sodium chloride, potassium chloride, magnesium chloride, etc. The
elements or compounds which occur naturally in the earth's crust are known as minerals.
Some minerals contain high percentage of particular metal are called ores. Thus, ore is a
mineral which contains large amount of the particular metal. From the respective ores metals
can be extracted by applying several steps. Collectively these steps are called metallurgical
steps. Thus, metallurgy is a process of different steps by which pure metal can be extracted
from the particular ore. After getting the pure metal, it is used for different purposes. In this
unit, we will discuss about the metallurgy of aluminium, silver and gold along with their
physical and chemical properties.

Aluminium

Introduction

Name Aluminium
Symbol Al
Atomic weight 27
Atomic number 13
Electronic configuration 1s2 ,2s22p6,3s23p1
Position in periodic table Period-3 and group-IIIA (13)
Valency 3

Occurrence

Aluminum is the most abundant metal on the earth's crust. It occurs about 68.1% in natu re.
It is a most reactive metal. So, it does not occur in Free State in the nature. In combined state,
aluminium is found in different types of compounds like sulphides, oxides, fluorides, etc. The
main ores of aluminium are bauxite, cryolite, felspar, etc. Aluminium is a light metal, so it is
used to make different kinds of household utensils, parts of aircraft, etc.

Ores of aluminium

Name of ores Molecular formula

Bauxite Al2O3.2H2O
Felspar K(AlSiO3.O8)
Cryolite Na3AlF6

Extraction of aluminium

Aluminium is one of the abundant elements on the earth. It is mainly found in the bauxite

ore. This ore contains different kinds of impurities like silica (SiO2), ferric oxide, etc. So, we
can extract aluminium mainly from the bauxite ore. The extraction of aluminium involves

following steps.

i. Collection of ore: The chief ore of aluminium is bauxite. It is found in the different places

on the earth. So, bauxite ore is collected from the respective places using different kinds

of tools and techniques. After collection, the ore is transported to the site of extraction.

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CHEMISTRY

ii. Grinding of ore: While mining, the bauxite ore is present in the form of big lumps. These
big lumps of ore are crushed into smaller pieces with the help of crusher machine. After
crushing the pieces of bauxite ore are ground in the stamp mill or pulverizing machine to
make fine powder.

iii. Heating of ore with sodium hydroxide: After grinding, we get powder ore. This powder
ore is mixed with sodium hydroxide to make solution. Now, the solution is heated to
form a complex compound called sodium metal aluminate (NaAlO2).This sodium metal
aluminate is separated and treated with water and dilute hydrochloric acid. As a result,
we get precipitate of aluminium hydroxide [Al(OH)3].This aluminium hydroxide is
dehydrated to remove moisture. The anhydrous aluminium hydroxide is heated strongly
to get aluminium oxide. This aluminium oxide is commonly called alumina (Al2O3).

iv. Purification of aluminium: After getting alumina, we apply refining process. It is the last
step of the metallurgical process. In this step, the molten alumina is purified by using
electrolysis process. In electrolysis process, the impure alumina is kept at anode and pure
alumina is kept at cathode. As a result, about 99% pure aluminium is obtained.

Properties of aluminium

A) Physical properties

i. Aluminium is a white coloured metal with bluish shine.

ii. Its melting point is 6600C and boiling point is 18000C.

iii. It is a light metal having specific gravity 2.7.

iv. It is malleable, ductile and good conductor of heat and electricity.

B) Chemical Properties

i. Reaction of aluminium with air: Aluminium does not react with dry air but in

presence of moisture, it reacts with oxygen to form aluminium oxide (Al2O3).

4Al + 3O2 ∆ 2Al2O3

ii. Reaction with water: Aluminium is passive with cold water but it reacts with steam
to produce aluminium hydroxide {Al(OH)3} and hydrogen gas (H2).

2Al + 6H2O → 2Al(OH)3 + 3H2↑
iii. Reaction with Acids

a. Reaction with hydrochloric acid: Aluminium reacts with dilute and concentrated

hydrochloric acid to give aluminium chloride (AlCl3) and hydrogen gas (H2).

2Al + 6HCl → 2AlCl3 + 3H2↑
b. Reaction with sulphuric acid

i. Reaction of aluminium with dilute sulphuric acid

Aluminium reacts withdilutesulphuric acidtogivealuminiumsulphate{(Al2(SO4)3}
and hydrogen gas (H2).

2Al + 2H2SO4 → Al2(SO4)3 + 3H2↑

CHEMISTRY 0Optional Science - 10 235

ii. Reaction of aluminium with concentrated sulphuric acid

Aluminium reacts with concentrated sulphuric acid to give aluminium sulphate
Al2(SO4)3, sulphur dioxide(SO2) and water(H2O).
2Al + 6H2SO4 → Al2(SO4)3 + 3SO2+ 6H2O
c. Reaction with nitric acid

Aluminium does not react with dilute as well as concentrated nitric acid. It is because
aluminium makes a protective oxide layer over its surface.

Fact with reason

Aluminium does not react with dilute as well as concentrated nitric acid. Why?
Aluminium does not react with dilute as well as concentrated nitric acid because in
presence of oxygen, aluminium makes a thin protective oxide layer over its surface.
This oxide layer prevents the reaction of aluminium with nitric acid.

3. Reaction with halogens
Aluminium reacts with halogens to form respective halides.
2Al + 3Cl2 → 2AlCl3 (Aluminium chloride)
2Al + 3Br2 → 2AlBr3 (Aluminium bromide)
2Al + 3I2 → 2AlI3 (Aluminium iodide)

Memory Plus

The elements which are kept in VIIA group of the modern periodic table are called
halogens. Examples: F2, Cl2, Br2, I2, etc. They are called halogens because they are
present in the sea water in the form of salt.

4. Reaction with nitrogen
Aluminium reacts with nitrogen to form aluminium nitride.
2Al + N2 → 2AlN (Aluminium nitride)

Memory Plus

Aluminium nitride (AlN) can be used as a chemical fertilizer.

5. Reaction with alkalis
Aluminium reacts with sodium hydroxide to give sodium metal aluminate. This reaction
takes place in presence of water.
2Al + 2NaOH + 2H2O → 2NaAlO2 + 2H2↑

Fact with reason

Aluminium is used for making aluminium foils for wrapping foods, pharmaceutical
products, biscuits, chocolates, cigarettes, etc. Why?
Aluminium is used for making aluminium foils for wrapping foods, pharmaceutical
products, biscuits, chocolates, cigarettes, etc. because it is highly malleable and light
metal. It does not get rust.

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Uses of aluminium
1. As aluminium is good conductor of electricity, it is used to make electric transmission wires.
2. It is used to make different types of household utensils, pictures, frames, etc.
3. It is used for making aluminium foils for wrapping foods, pharmaceutical products,

biscuits, chocolates, cigarettes, etc.
4. It is used to make different parts of aircrafts, ships, bushes, cars, etc. as it is a light metal.
5. It is used to make alloys and coins.
6. It is used to make reducing agent in different metallurgical process.

Fact with reason

Aluminium is used to make parts of aircrafts, ships, bushes, cars, etc. Why?
Aluminium is used to make parts of aircrafts, ships, bushes, cars, etc. because
i. Aluminium is a light metal.
ii. It is highly malleable metal.
iii. It does not get rust continuously.

Silver

Introduction Silver -
Argentum -
[Name I Ag

Latin name

I

I Symbol

Atomic weight 107.9 -

~ Atomic number I 47 -
I [Kr]4d10 5s1
Electronic configuration Period-5 and group-IB (11) -

Position in periodic table 1

I

I Valency

Occurrence

Silver is an example of d-block element. It is white coloured shiny metal. It is comparatively
less reactive metal than iron, copper and aluminium. So, it occurs in Free State in the earth.
Compare to gold, silver is more reactive metal. So, it also occurs in in the different compounds.
The compounds of silver which are present in the earth's crust are called ores of silver.
Examples: argentite, horn solver, ruby silver, etc.

Memory Plus

The symbol of silver "Ag" is derived from the Latin word "argentum" meaning white.
Thus, silver is white coloured shiny metal.

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Ores of Silver Molecular formula

S.N. Ores of silver Ag2S
1. Argentite (silver glance) AgCl
2. Horn silver 3Ag2S.Sb2S3
3. Ruby silver (Ag.Cu)2S
4. Silver copper glance

Extraction of Silver

The main ore of silver is argentite. So, silver is mainly extracted from the argentite ore by using
cyanide process. The cyanide process involves following steps.

1. Mining: Argentite ore is found in the different parts of the earth. From these respective
places, argentite ore is taken out using different methods. The ore thus obtained contains
large amount of impurities. Now, the ore is concentrated to remove these unwanted
impurities.

2. Crushing and pulverization: The ore is crushed in jaw crushers and then powdered to
make fine particles in ball mills.

3. Concentration of the argentite ore: Since argentite is a sulphide ore, we use froth

floatation process to remove its impurities. During cyanide process, we make a mixture

of argentite ore, water and pine oil. This Froth bubbles Compressed
mixture is kept in a container. In the container carrying sulphide _,.... air
we pass strong current of air. The sulphide
ore particles

ore now becomes wet in pine oil. The density I
of wetted sulphide ore is less than water. So, it
floats on the container in the form of froth. Sulphide ore
particles

The impurities present in the ore have more Water
density than water. So, they settle down at the containing

pine oil

bottom of the container. Now, the froth i:::;.::::.::;.:.:::::.:.=.::.:s.::!.::!:~:=.:Jc!::r- - Gangue
containing argentite ore is separated like
Froth floatation process for the concentration of sulphide ores.

cream from the milk.

Memory Plus

The impurities present in sulphide ore are separated by using froth floatation process.

4. Treatment with sodium cyanide: The concentrated ore is treated with dilute aqueous
solution of sodium cyanide (about 0.4 to 0.7%) and agitated with a strong blow of air.
Now, sodium cyanide reacts with argentite ore to give a complex compound called
sodium argentocyanide Na [Ag(CN)2] and sodium sulphide (Na2S).The sodium sulphide
then converted into sodium sulphate due to the reaction with oxygen. Sodium sulphate
is removed to proceed the reaction.

Ag2S + 4NaCN → 2Na[Ag(CN)2] + Na2S
Na2S + 2O2 → Na2SO4

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5. Silver precipitation: The solution containing sodium argentocyanide is filtered to remove
insoluble impurities. The clear solution of sodium argentocyanide is then treated with
zinc. As zinc is more electropositive than silver, it displaces silver from the solution. Now,
silver gets precipitated on the bottom of the container. Silver obtained in this process is
called spongy silver. Finally, the separated silver is fused with borax or potassium nitrate
to get compact silver.

Zn + 2Na[Ag(CN)2] → Na2[Zn(CN)4] + 2Ag
6. Silver Refining: Silver obtained in this process contains some impurities like zinc,

copper, gold, etc. So, purification of silver is done by using electro-refining process.
During electro-refining process, impure silver is connected at cathode and pure silver is
connected at anode. In this process, silver nitrate solution is used as an electrolyte.

Properties of Silver

A) Physical Properties
1. Silver is a white lustrous metal.
2. It is highly malleable, ductile and good conductor of heat and electricity.
3. Its melting point is 9560C and boiling point is 19550C.
4. Its specific gravity is 10.52

B) Chemical Properties

1. Reaction of silver with halogens (Cl2, Br2, I2)

Silver reacts with halogens to give corresponding halides.

2Ag + Cl2 → 2AgCl (Silver chloride)
2Ag + Br2 → 2AgBr (Silver bromide)

2Ag + I2 → 2AgI (Silver iodide)

2. Reaction of silver with sulphur

Silver reacts with sulphur to give silver sulphide.
2Ag + S ∆ Ag2S
3. Reaction of silver with acids

a. Reaction of silver with hydrochloric acid:

Silver does not react with dilute hydrochloric acid but in presence of oxygen it
reacts with concentrated hydrochloric acid to give silver chloride and water.

4Ag + 4HCl + O2 → 4AgCl + 2H2O
b. Reaction of silver with sulphuric acid

Silver does not react with dilute sulphuric acid but it reacts with concentrated
H2SO4 to give silver sulphate, sulphur dioxide and water.
2Ag + 2H2SO4 → Ag2SO4 + SO2 + 2H2O

Memory Plus

Silver does not react with dilute hydrochloric acid and dilute sulphuric acid.

CHEMISTRY 0Optional Science - 10 239

c. Reaction of silver with nitric acid
i. Reaction of silver with dilute nitric acid
Silver reacts with dilute nitric acid to give silver nitrate, nitric oxide and water.
3Ag + 4HNO3 → 3AgNO3 + NO + 2H2O
ii. Reaction of silver with concentrated nitric acid
Silver reacts with concentrated nitric acid to give silver nitrate, nitrogen dioxide
and water.
Ag + 2HNO3 → AgNO3 + NO2 + H2O

Memory Plus

Silver nitrate (AgNO3) is sensitive to light and organic compounds. So, it reacts easily
with our skin to make black stain. So, this property is applied during election for staining
fingers of voters who have cast their votes.

Uses of silver
1. Silver is used to make different types of ornaments, utensils, coins, medals, etc.
2. Silver bromide (AgBr) is used in photography.
3. It is used in silver plating.
4. It is used for making silvering mirror.
5. It is used for filling the teeth.
6. It is also used to make medicines.

Memory Plus

Dental alloy is made from silver and tin. It is used for filling teeth cavity.

Gold

Introduction

Name Gold
Latin name Aurum
Symbol Au
Atomic weight 197
Atomic number 79
Electronic configuration [Xe] 4f14 5d10 6s1
Position in periodic table Period-6 and group-IB (11)
Valency 1 and 3

Occurrence

Gold is a passive metal. In general condition, it does not react with most of the substances like
air, water, acid, base, etc. So, it is also called a noble metal and occurs in Free State. In native
state gold is found in alluvial soil. In general conditions, it is found in reef gold mixed with

240 Optional Science - 10

CHEMISTRY

quartz or alluvial soil. In Nepal, alluvial gold is found in Karnali and Sunkoshi Rivers. But, the
commercial extraction of gold from these places has been found uneconomical.

Fact with reason

Gold occurs freely in nature. Why?
Gold occurs freely in nature because in general conditions, it does not react with most of
the substances like air, water, individual acid, base, etc.

Ores of Gold Molecular formula
It is a mixture
S.N. Name of Ores AuTe2
1 Alluvial soil or quartz veins (Ag.Au)2
2 Calverite
2 Petzite

Fact with reason

Gold is also called a noble metal. Why?
Gold is also called a noble metal because in general conditions, it does not react with
most of the substances like air, water, individual acid, base, etc.

Extraction of Gold
Gold is mainly extracted by using two methods. They are:

1. Sluicing Method 2. Cyanide Method

1. Sluicing

Sluicing is a physical method of extraction of gold. In this method we use a pan or long
channel called sluice. During this process, the alluvial soil is washed using high pressure
water. The mixture of alluvial soil and water runs downward through the sluice. While
running the mixture, water and impurities continuously flow down along the sluice while
the heavier gold particles settle down in the cavities of sluice. Finally, gold particles are
taken out from the sluice cavities and refined for the further use.

2. Cyanide process

Cyanide process is a chemical method of extraction of gold. In this process, we use
solution of sodium cyanide (NaCN) for the extraction of gold. During this process, we
make a mixture of impure gold and sodium cyanide. A strong jet of air is blown to this
mixture. While passing air jet, the gold gets dissolved in the sodium cyanide solution
to form sodium aurocyanide and sodium hydroxide. Now, gold is extracted by adding
zinc dust in the solution of sodium aurocyanide. As zinc is more reactive than gold, it
displaces gold from the aurocyanide in the form of precipitate. The necessary reactions
for this process are given below.

4Au + 8NaCN + 2H2O → 4 Na[Au(CN)2] + 4NaOH
Zn + 2Na[Au(CN)2] → Na2[Zn(CN)4] + 2Au

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Gold obtained from the above process is not completely pure. It contains impurities
of zinc and lead. So, it is purified by using several refining processes to get pure gold.
Among the several refining methods, electro-refining is the best one. In this method, we
take impure gold at anode and pure gold at cathode. For this process, acidified gold
chloride solution is used as an electrolyte.

Properties of Gold
A. Physical properties of gold

1. Gold is a soft heavy metal with shining yellow lutstre.

2. It is good conductor of heat and electricity.

3. It is highly malleable and ductile in natrue.

4. Its melting point is 10630C and boiling point is 25300C.

5. Its specific gravity is 19.3.

Memory Plus

The relative density or specific gravity of gold is 19.3. It means gold is 19.3 times heavier
than pure water at 40C.

B. Chemical properties of gold
i. Reaction of gold with halogens
Gold reacts with halogens to give gold halides.

2Au + 3Cl2 → 2AuCl3 (Gold chloride)
ii. Reaction of gold with aquaregia

As we have already discussed that gold is very less reactive metal. In ordinary
condition, It does not react with acids. But, it reacts with aquaregia. Aquaregia is a
homogenous mixture of three part of concentrated hydrochloric acid and one part of
concentrated nitric acid. Gold reacts with aquaregia to give auric chloride, nitrosyl
chloride and water.

{3HCl + HNO3 → NOCl + 2H2O + 2Cl } x 3

9HCl + 3HNO3 → 3NOCl + 6H2O + 6Cl
2Au + 6Cl → 2AuCl3

2Au + 9HCl + 3HNO3 → 2AuCl3 + 3NOCl + 6H2O

Memory Plus

Aquaregia produces nascent chlorine (Cl) that reacts with gold to give gold chloride.

0242 Optional Science - 10

CHEMISTRY

Uses of Gold
1. Gold is widely used for making ornaments and jewelleries.
2. It is used to coat precious metals, idols, statues, roof tops, temples, etc.
3. It is used for making gold coins.
4. It is used in photography.
5. It is used for making medicines
6. It is used for filling teeth cavities.
7. It is used in gold-leaf electroscope.
8. It is used for making important gold compounds like gold chloride.

Alloy

When two or more non-reacting substances are mixed together, we get a resultant mass called
mixture. In mixture, the mixing substances do not lose their identity. While making mixture,
when two or more metals are mixed together, then the resultant substance is called an alloy.
Thus, alloy is a homogeneous mixture of two or more metals or sometimes metals and non-
metals. If an alloy is made from two components, it is called binary alloy. If an alloy is made
from three components, it is called a ternary alloy. Similarly, if an alloy is made from four
components, it is called a quaternary alloy.

While making alloys we always desire certain common properties that are different from
their components. For example, steel is an alloy of iron and carbon. In steel, iron is a main
component but it is stronger than iron.

As an alloy is a mixture of metals, it does not have fixed melting point. Its melting point
depends upon the mixing components and their percentage. Therefore, alloys have a melting
range. For specific purposes, we design some alloys that have single melting point. Such alloys
are called eutectic alloys.

Properties of alloys

Alloys have the following properties.
1. Generally alloys are harder than their components.
2. Generally, alloys are brittle than their components.
3. They are good conductors of heat and electricity.
4. Strength of alloys is more than their components.
5. They are malleable and ductile.
6. They have low melting point.

Memory Plus

Sometimes an alloy is just named according to the base metal. For example, 14 karat
(58%) gold is an alloy of gold with other elements.

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Some examples of alloys

S.N Name of alloy Composition
1. Steel Iron (80–98%), carbon (0.2–2%), other metals such as chromium,
manganese, and vanadium
2. Stainless steel Iron (more than 50%), chromium (10–30%), small amount of
carbon, nickel, manganese, molybdenum and other metals.
3. Brass Copper (65–90%) and zinc (10–35%)
4. Amalgam Mercury (45–55%), silver, tin, copper and zinc
5. Bronze Copper (78–95%), tin (5–22%), manganese, phosphorus,
aluminum, or silicon
6. Bell Metal Copper (78%) and tin (22%),
7. White gold Gold (75%), palladium (17%), silver (4%) and copper (4%)

(18 carat gold) Nickel (80%) and chromium (20%)
8. Nichrome

Amalgam

Amalgam is an alloy of mercury with other metals like sodium, potassium, silver, tin, copper,
zinc, etc. Almost all metals can form amalgams with mercury. In amalgam, mercury is present
about 45–55%. The metals which form amalgam with mercury are silver, tin, copper, zinc,
etc. There are many amalgams for example, zinc amalgam, potassium amalgam, sodium
amalgam, aluminium amalgam, tin amalgam, silver amalgam, etc. Among them, the silver-
mercury amalgam is used in dentistry to fill teeth and gold-mercury amalgam is used in the
extraction process of gold.

Memory Plus

The silver-mercury amalgam is used in dentistry to fill teeth cavity.

Answer writing skill

1. What is metallurgy? Why is it important?

Ans: Metallurgy is a process of different steps by which pure metal can be extracted from the
particular ore. As ore contains different kinds of impurities, metallurgical process is very
important to take out the pure metal from the respective ores.

2. Write down two important ores of each aluminium, silver and gold.

Ans:

Metal Two ores
Aluminium Bauxite and Cryolite
Silver Argentite and Horn silver
Gold Alluvial soil and Calverite

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3. Aluminium is used for making aluminium foils for wrapping foods, pharmaceutical
products, biscuits, chocolates, cigarettes, etc. Why?

Ans: Aluminium is used for making aluminium foils for wrapping foods, pharmaceutical
products, biscuits, chocolates, cigarettes, etc. because

i. Aluminium is highly malleable and light metal.

ii. It does not get rust continuously.
4. Write down the reaction of silver with sulphuric acid.
Ans: Silver does not react with dilute sulphuric acid but it reacts with concentrated H2SO4 to

give silver sulphate, sulphur dioxide and water.

2Ag + 2H2SO4 → Ag2SO4 + SO2 + 2H2O
5. What is aquaregia? Write down the balance chemical reaction of aquaregia with gold.
Ans: Aquaregia is a homogenous mixture of three parts of concentrated hydrochloric acid and

one part of concentrated nitric acid. Gold reacts with aquaregia to give auric chloride,
nitrosyl chloride and water.

2Au + 9HCl + 3HNO3 → 2AuCl3 + 3NOCl + 6H2O
6. Write down any three properties of alloy.
Ans: The properties of alloys are:

1. Generally alloys are harder than their components.

2. Generally, alloys are brittle than their components.

3. They are good conductors of heat and electricity.
7. Write down the composition of stainless steel, brass and bronze.

i. Composition of stainless steel is Iron (more than 50%), chromium (10–30%), small
amount of carbon, nickel, manganese, molybdenum and other metals.

ii. Composition of brass is copper (65–90%) and zinc (10–35%)
iii. Composition of bronze is copper (78–95%), tin (5–22%), manganese, phosphorus,

aluminum, or silicon
8. Write down any three uses of Gold.

i. Gold is widely used for making ornaments, jewelleries, idols, statues, roof tops,
temples, etc.

ii. It is used for making gold coins, photography and gold-leaf electroscope.
iii. It is used for making medicines and filling the teeth cavities.

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Exercise

1Step

1. What is metallurgy?
2. Define ore.
3. Show the sub-shell electronic configuration of aluminium
4. What is aquaregia?
5. Where do we use sluice method?
6. Mention the period and group of aluminium, silver and gold.
7. Which element is present in aquaregia to react with gold?
8. Write down any two ores of each aluminium, silver and gold.
9. Define alloy and amalgam.
10. Write down any two alloys with their composition.
11. Which metal can be extracted from bauxite, argentite and alluvial soil.
12. What is amalgam? Which amalgam is used to fill the cavity of teeth?

2Step

1. Give reason.

a. Aluminium does not react with dilute as well as concentrated nitric acid.

b. Aluminium is used for making aluminium foils for wrapping foods, pharmaceutical
products, biscuits, chocolates, cigarettes, etc.

c. Aluminium is used to make parts of aircrafts, ships, bushes, cars, etc.

d. Gold occurs freely in nature.

e. Gold is also called a noble metal.
2. Write down the occurrence of the following metals.

(a) Aluminium (b) Silver (c) Gold

3. Write down the electronic configuration, period and group of the following metals.

(a) Aluminium (b) Silver (c) Gold

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3Step

1. Write down any three physical properties of the following metals.

(a) Aluminium (b) Silver (c) Gold

2. Write down any three uses of the following metals.

(a) Aluminium (b) Silver (c) Gold

3. Write down the balanced chemical reaction of the aluminium with

(a) Hydrochloric acid (b) Sulphuric acid

4. Write down the balanced chemical reaction of the silver with

(a) Hydrochloric acid (b) Sulphuric acid (c) Nitric acid

5. Write down the balanced chemical reaction of gold with aquaregia.

4Step

1. Describe the metallurgical steps of aluminium.
2. What is chief ore of silver? Describe the extraction of silver from its chief ore.
3. Describe sluice method of extraction of gold.
4. Describe cyanide method to extract gold.
5. What is alloy? Why do we make alloy? Write down three properties of alloy.

Multiple choice questions (MCQ)

1. The symbol of gold is

i. Ag ii. Au

iii. G iv. Al

2. What is added during the extraction of silver?

i. Potassium permanganate ii Sodium argento cyanide
iv. Potassium phosphate
iii. Sodium cyanide
3. Gold reacts with

i. Aquaregia ii Sulphuric acid
iv. Nitric acid
iii. Hydrochloric acid
4. Aluminium does not react with

i. Oxygen ii Sulphuric acid
iv. Nitric acid
iii. Hydrochloric acid
5. The main ore of silver is

i. Alluvial soil ii. Argentite

iii. Haematite iv. Bauxige

6. Which one is used to extract aluminium

i. Alluvial soil ii. Argentite
iii. Haematite iv. Bauxige

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UNIT

13 BIOMOLECULES

Carl Alexander Neuberg (1877–1956) was born in Germany and died in New York. He
was an early pioneer in biochemistry, and he is often referred to as the "Father of Modern
Biochemistry". One of his important contributions in science is the discovery of carboxylase
and the elucidation of alcoholic fermentation.

Syllabus issued by CDC Learning objectives:

Theory 3 At the end of this unit, the students will be able to:
Practical 1
• describe the importance of some biomolecules
like carbohydrates, proteins, lipids, enzymes

Atomic mass, molecular mass and and nucleic acids

mole concept • describe sources and uses in the human body
of carbohydrates, proteins, lipids, enzymes
Quantum numbers and nucleic acids.

Concentration (Normality, molarity,

gram per litre and percentage)

Key terms and terminologies of the unit

1. Micromolecule: A molecule containing a low number of atoms.

2. Macromolecule: A molecule containing a very large number of atoms, such as a
protein, nucleic acid or synthetic polymer.

3. Biomolecules: Biomolecules are defined as organic molecules present in living things
that are involved in the growth, repair and metabolic process of a living organism

4. Carbohydrates: Carbohydrate is a biomolecule consisting of carbon (C), hydrogen
(H) and oxygen (O) atoms which is used as energy source (calories) by the body of
living organisms.

5. Monosaccharide: A monosaccharide is the most basic form of carbohydrates with a
single sugar molecule.

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6. Disaccharide: A carbohydrate consisting of two sugar molecules is called a
disaccharide.

7. Oligosaccharide: Oligosaccharide is a type of sugar containing a chain of 2-10
monosaccharides.

8. Polysaccharide: A polysaccharide is a polymer formed by many monosaccharides
joined together by a glycosidic bond.

9. Lipid: Lipids are the naturally occuring organic compounds commonly known as
fats or oils and their derivatives.

10. Proteins: Proteins are large, complex nitrogen-containing organic compounds
composed of one or more long chains of amino acids.

11. Amino acids: Amino acids are organic biomolecules that are attached to each other
to form long chains of proteins.

12. Essential amino acids: Amino acids that cannot be synthesized in the body and
should be supplied through diet are called essential amino acids.

13. Non-essential amino acids: Amino acids that are synthesized in the human body
either from essential amino acids or from normal protein breakdowns are called non-
essential amino acids.

14. Semi-essential amino acids: Amino acids which can partly synthesized in adult
human body but are necessary to supply through diet in infants are called semi-
essential amino acids.

15. First class protein: Proteins that contain all the essential amino acids are called first
class proteins.

16. Second class protein: Proteins that are deficient in one or more essential amino acids
are called second class proteins.

17. Enzyme: Enzymes are large biomolecules that act as catalysts for biochemical
reactions within living cells in every plant and animal.

18. Nucleic acids: Nucleic acids are large organic compounds found in the chromosomes
of living cells and viruses.

19. Minerals: Minerals are essential inorganic nutrients for organisms to perform
functions necessary for life.

20. Deoxyribonucleic Acid (DNA): DNA is a double helical macromolecule. It contains
genetic information.

Introduction

All living matter is largely made of up biomolecules. They are the essential chemical
components of living organisms. The chemical components of a cell can be inorganic (salts,
minerals and water) and organic (carbohydrates, proteins, lipids, nucleic acids and vitamins).
Biomolecules are defined as organic molecules present in living things that are involved in the
growth, repair and metabolic process of a living organism.

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Carbohydrates

Carbohydrates are the most abundant biomolecules found in living organism on earth. The
carbohydrates are the major source of energy, both for plants and animals. It contains carbon,
hydrogen and oxygen as its main elements. Carbohydrate is a biomolecule consisting of
carbon (C), hydrogen (H) and oxygen (O) atoms which is used as energy source (calories) by
the body of living organisms. Carbohydrates are often referred to as sugars or saccharides.

Memory Plus

The word saccharide refers to sugar, or sweet substance, and is derived from the Greek
word sakcharon.

Classification of Carbohydrates

Carbohydrates

Monosaccharides Oligosaccharides Polysaccharides
(1 sugar molecule) (2-10 sugar molecule) (10+ sugar molecule)

glucose, fructose, sucrose, lactose, starch, glycogen,
galactose maltose, raffinose cellulose, fibers

On the basis of the number of sugar units, carbohydrates can be classified into three major
classes as monosaccharides, oligosaccharides and polysaccharides.

Monosaccharide (Gk: mono- one)

A monosaccharide is the most basic form of carbohydrates with a single sugar molecule. It is
also called a simple carbohydrage. Monosaccharides are composed of 3-7 carbon atoms. The
common monosaccharides are glucose, fructose, galactose, ribose and deoxyribose.
Monosaccharides are sweet in taste and soluble in water. The general formula of
monosaccharides is written as (CH2O)n; where, n= 3 to 7. For example, when n=6, (CH2O)n =
C6H12O6 (Glucose)

HOP ;O; - JH

H OH H H OH H'H'H OH OH
Fructose
Glucose Galactose OH H Ribose

Deoxyribose

Oligosaccharides

Oligosaccharide is a type of sugar containing a chain of 2-10 monosaccharides. Disaccharide is
one of the examples of oligosaccharides.

Disaccharides (Gk: di- two): A carbohydrate consisting of two sugar molecules is
called a diasaccharide. In a disaccharide, two monosaccharides are joined together by a
glycosidic bond. A glycosidic bond is a type of covalent bond that joins a carbohydrate
(sugar) molecule to another group.

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