Class 9 Science_clone

Chapter 9DOHQF\ DQG Antoine Lavoisier
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8 )RUPXOD

Objectives He is known for studying the properties of
Hydrogen and oxygen. He is sometimes known
as the father of chemistry.

ƐƟŵĂƚĞĚ WĞƌŝŽĚƐ ͗ϲ

ƚ ƚŚĞ ĞŶĚ ŽĨ ƚŚĞ ůĞƐƐŽŶ͕ ƐƚƵĚĞŶƚƐ ǁŝůů ďĞ ĂďůĞ ƚŽ͗

• GHÀQH PDWWHU

• H[SODLQ HOHPHQW FRPSRXQG DQG WKHLU FRQVWLWXHQWV

• GHÀQH RFWHW DQG GXSOHW UXOH

• explain radicals, valency and molecular formula.

Mind Openers

• What is chemistry? Does chemistry have branches?
• What are the differences between atom and molecule?
• &DQ \RX GHÀQH V\PERO" :KDW LV LWV LPSRUWDQFH" 'LVFXVV

Introduction

Matter is anything that has mass and occupies space. The matter
exists in several forms and each form of matter has its own
properties. One form of matter can be changed into another form.
There is a separate branch of science to study about the properties
and composition of matter. It is called chemistry.

Chemistry is the branch of science which deals with the study of
properties, composition and transformation of matter.

Chemistry can be divided into three main branches. They are as follows:
1. Inorganic Chemistry
2. Organic Chemistry
3. Physical Chemistry

1. Inorganic Chemistry

The branch of chemistry that deals with the study of inorganic
compounds is called inorganic chemistry. The compounds such as
iHn2oOrg, aNnaicClc,oNmHpo3,uNndasOHca,nHb2SeOfo4,remtcedarbeyinaonrygaenleicmceonmtspobuuntddso. The
contain carbon and hydrogen together. not

147 Times' Crucial Science Book - 9

2. Organic Chemistry

The branch of chemistry in which we study about hydrocarbons
and their derivatives is called organic chemistry. The compounds
which contain direct covalent bond between carbon and hydrogen
are called hydrocarbons. For example, CH4, C2H6, C3H8, C2H5OH,
etc are the organic compounds.

3. Physical Chemistry

The branch of chemistry which deals with the energy changes in the
chemical reactions is called physical chemistry.

In this chapter, we will study about the fundamental terms of
chemistry and the ways to derive molecular formula.

Elements, compounds and their mixtures are regarded as matter.
The elements and compounds have definite composition and
particular properties. So, they are called pure substances. But a
mixture can be formed if two or more substances are placed together
in any ratio. So, a mixture does not have definite composition and
properties and is known as impure substance. A brief classification
of matter is as follows:

Matter

Pure Substances Impure Substances

Elements Compounds Heterogeneous Homogeneous

Mixture Mixture

A short description of pure substances is as follows:
1. Elements
An element is the simplest pure form of substance which is made
of similar kinds of atoms. There are 118 elements discovered so
far. Among them 92 elements occur naturally and 26 elements are
obtained in laboratories artificially. Hydrogen, carbon, oxygen,
sodium, iron, cobalt, nickel, uranium, etc are some examples of
elements.

Times' Crucial Science Book - 9 148

The elements are represented in short form with the help of their
symbols. Symbols are the short and abbreviated forms for the full
name of elements. Generally, the symbols are expressed by taking
first letter of the English or Latin name of elements. In case the
name of more than one element begins with the same letter, two or
three letters are also used as symbol.
Symbols of some common elements are as follows:

SN Element Symbol Atomic SN Element Symbol Atomic
Number Number

1 Hydrogen H 1 21. Chromium Cr 39

2 Helium He 2 22. Manganese Mn 25

3 Lithium Li 3 23. Iron (Ferrum) Fe 26

4 Beryllium Be 4 24. Copper (Cuprum) Cu 29

5 Boron B 5 25. Zinc Zn 30

6 Carbon C 6 26. Molybdenum Mo 42

7 Nitrogen N 7 27. Silver (Argentum) Ag 47

8 Oxygen O 8 28. Tin (Stannum) Sn 50

9 Fluorine F 9 29. Iodine I 53

10 Neon Ne 10 30. Platinum Pt 78

11 Sodium (Natrium) Na 11 31. Gold (Aurum) Au 79

12 Magnesium Mg 12 32. Mercury (Hydrargyrum) Hg 80

13 Aluminium Al 13 33. Lead (Plumbum) Pb 82

14 Silicon Si 14 34. Tungsten (Wolfam) W 74

15 Phosphorus P 15 35. Lanthanum La 57

16 Sulphur S 16 36. Actinium Ac 89

17 Chlorine Cl 17 37. Uranium U 92

18 Argon Ar 18 38. Curium Cm 96

19 Potassium (Kalium) K 19 39. Californium Cf 98

20 Calcium Ca 20 40. Einsteinium Es 99

2. Compounds
Compounds are the pure substances which are formed by the chemical
combination of two or more elements in a definite proportion by
weight. The properties of compounds are different from those of
elements from which they are formed. For example, water, common
salt, etc. are compounds.

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A compound is composed of molecules. A molecule is the smallest
particle of a compound or an element which has independent existence.
A molecule of a compound is different from that of an element.
Differences between the molecule of a compound and that of an
element

Molecule of a compound Molecule of an element

1. It is made up of two or more 1. It is made up of similar types of
different types of atoms. atoms.

2. The atoms of different 2. It contains the atoms of same
elements form a molecule of a element.
compound.

3. EooffxSwaumaltppehlreu,: rHHic22SOaOcii4ds,itsehtaec. molecule 3. Eohxyxydagrmeongpe,lene:t, cO.H2 2 is is molecule of
molecule the molecule of

Atom

Atom is the smallest particle of an element that can take part in
a chemical reaction. An atom is represented by the symbol of an
element. For example, H represents an atom of hydrogen, Na
represents an atom of sodium, etc. An atom is an electrically neutral
particle because it has equal number of positively charged protons
and negatively charged electrons.

Structure of atom

An atom is extremely small. The diameter of an atom is approximately
10ï m. But it is made up of several particles called sub-atomic
particles. Electron, proton and neutron are the sub-atomic particles
present in an atom. Protons and neutrons are located in the nucleus
of an atom but the electrons revolve round the nucleus. The electrons
revolve around the nucleus in a fixed path which is known as orbit
or shell.

Electron

Protons Nucleus
Neutrons

Shell

Structure of an atom

Times' Crucial Science Book - 9 150

The mass of an atom is concentrated in the nucleus. ln nucleus, protons
and neutrons are present. The weight of a proton is equal to the weight
of one hydrogen atom. The weight of neutron is nearly equal to the
weight of a proton, i.e. one atom of hydrogen. The weight of an electron
is equal to 1/1837th the weight of one hydrogen atom. Since the weight
of electron is very less as compared to that of proton or neutron, it is
often neglected while calculating atomic weight.

Each orbit or shell is composed of P-subshell
sub-shells. There are four kinds
of sub-shells. They are s,p,d and
f sub-shells. The s-subshells are
spherical in shape where as the
p-subshells have dumb-bell shape. S-subshell

Each cell has a particular number (spherical shape) (dumb-bell shape)
of sub-shells. Each sub-shell can accommodate a definite number of
electrons. It is given in the following table:

Shells KL M N
Sub-shells
No. of electrons s sp s p d s p d f
14
2 2 6 2 6 10 2 6 10

The table shows that,

The s-sub-shell can hold a maximum of 2 electrons.

The p-sub-shell can hold a maximum of 6 electrons.

The d-sub-shell can hold a maximum of 10 electrons.

The f-sub-shell can hold a maximum of 14 electrons.

Fact Reason

Why is an atom electrically neutral?
Atom is electrically neutral because the number of positively
charged protons is equal to the number of negatively charged
electrons.

Atomic number
The total number of protons present in the nucleus of an atom is
called atomic number. In an electrically neutral atom, the number
of protons is equal to the number of electrons. Thus, atomic number
may also be defined as the total number of protons or electrons
present in an electrically neutral atom. The atomic number is
denoted by Z.

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Atomic number = No. of protons
= No. of electrons in an electrically neutral atom

? = = No. of p+ or eï
Atomic weight
The sum of number of protons and neutrons in the nucleus of an
atom is called atomic weight. It is also known as atomic mass. The
atomic weight of an element is denoted by A.

A = No. of protons + No. of neutrons
? A = p+ + n°
Electronic configuration of atoms
The systematic distribution of electrons in different energy levels in
an atom is called electronic configuration. Electrons are distributed
in different shells or orbits. A shell or orbit is a well defined fine
circular path in which the electrons move around the nucleus. The
shells can be represented as K, L, M, N,........... or 1, 2, 3, 4,........ etc.

Electronic configuration of some atoms is given below:

SN Element Symbol No. of No. of No. of At. Electronic
electrons protons neutrons Mass FRQÀJXUDWLRQ

1. Hydrogen H 1 1 01 K L MN

2. Helium He 2 2 2 4 1 - --

3. Lithium Li 3 3 47 2 - --

4. Beryllium Be 4 4 59 21--

5. Boron B 5 5 6 11 22--

6. Carbon C 6 6 6 12 23--

7. Nitrogen N 7 7 7 14 24--

8. Oxygen O 8 8 8 16 25--

9. Fluorine F 9 9 10 19 26--

10. Neon Ne 10 10 10 20 27--

11. Sodium Na 11 11 12 23 28--

12. Magnesium Mg 12 12 12 24 2 81-
2 82-
13. Aluminium Al 13 13 14 27 2 83-
2 84-
14. Silicon Si 14 14 14 28 2 85-
2 86-
15. Phosphorus P 15 15 16 31 2 87-
2 88-
16. Sulphur S 16 16 16 32 2 881
2 882
17. Chlorine Cl 17 17 18 35

18. Argon Ar 18 18 22 40

19. Potassium K 19 19 20 39

20. Calcium Ca 20 20 20 40

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The distribution of electrons in different shells is governed by
following rules.
1. The maximum number
of electrons present in Shell Number Maximum number of elec-trons

each shell is given by K n = 1 2n2 = 2 × 12 = 2
2n2 rule. The rule that
predicts the maximum L n = 2 2n2 = 2 × 22 = 8

number of electrons that M n=3 2n2 = 2 × 32 = 18
can be accommodated n=4 2n2 = 2 × 42 = 32 and so on.
in different shells of an N
atom is called 2n2 rule.
For example,
2. The outermost shell of an atom cannot have more than 8 electrons
and the shell just inner to it cannot contain more than 18 electrons.
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p=1 p=2 p=3 p=4
n=0 n=2 n=4 n=5

Hydrogen Helium Lithium Beryllium

p=5 p=6 p=7 p=8
n=6 n=6 n=7 n=8

Boron Carbon Nitrogen Oxygen

p=9 p = 10 p = 11 p = 12
n = 10 n = 10 n = 12 n = 12

Fluorine Neon Sodium Magnesium

p = 13 p = 14 p = 15 p = 16
n = 14 n = 14 n = 16 n = 16

Aluminium Silicon Phosphorus Sulphur

p = 17 p = 18 p = 19 p = 20
n = 18 n = 22 n = 20 n = 20

Chlorine Argon Potassium Calcium

153 Times' Crucial Science Book - 9

Duplet and octet

The element helium has two electrons in K-shell. The K-shell is
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electrons. The state in which the outermost shell of an atom has the
capacity to accommodate only two electrons is called duplet. Helium
is only one element in the periodic table which has duplet. It is an
inert gas because it does not undergo any chemical reaction due to
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Some elements have 8 electrons in their valence shell. A stable set
of eight electrons in the outermost shell of an atom is called octet.
The elements like neon, argon, krypton, xenon, etc have octet state.
They are also the inert gases.

Duplet and octet rule

The tendency of an element to maintain two electrons in outermost
shell is called duplet rule. Only hydrogen, lithium, beryllium and
boron have tendency to gain duplet by gaining or losing electrons.

For example, hydrogen attains duplet by gaining one electron. But
lithium attains duplet by losing one electron.

H +1eï o Hï
/L ï Hï o Li+

The tendency of an element to acquire eight electrons in its outermost
orbit is called octet rule. The atoms can acquire octet state by gaining,
losing or mutual sharing of electrons. The tendency of elements to
acquire octet or duplet is the cause of chemical reactions.
1D ï Hï o Na+ (octet state)
(2, 8, 1) (2, 8)
&O Hïo Clï (octet state)
(2, 8, 7) (2, 8, 8)

Valency

The shells of an atom can be shown as:

Nucleus + Valence shell
Penultimate shell
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Times' Crucial Science Book - 9 154

Valency of an element depends upon the number of electrons
present in its outermost shell.

The outermost shell or orbit of an atom is called valence shell. The
electrons present in the valence shell are called valence electrons.
The valency of an element is determined by the number of its
valence electrons. The combining capacity of an element is called
valency.

All the inert gases (Helium, Neon, Argon, Krypton, Xenon and
Radon) have zero valency because their outermost shells are
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Fact Reason

Why does Argon atom remain in atomic form?
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electrons, it neither gains nor looses electrons. So, Argon atom
remain in atomic form.

Valencies of some common elements are as follows:

SN Name of Symbol Valency SN Name of Symbol Valency
element element

1. Hydrogen H 1 1. Zinc Zn 2

2. Helium He 0 2. Chromium Cr 3

3. Lithium Li 1 3. Bromine Br 1

4. Beryllium Be 2 4. Gold Au 1, 3

5. Boron B 3 5. Iodine I1

6. Carbon C 4 6. Lanthanum La 3

7. Nitrogen N 3 7. Radon Ra 0

8. Oxygen O 2 8. Uranium U 4

9. Fluorine F 1 9. Manganese Mn 2, 4

10. Neon Ne 0 10. Arsenic As 3,5

11. Sodium Na 1 11. Krypton Kr 0

12. Magnesium Mg 2 12. Silver Ag 1

13. Aluminium Al 3 13. Xenon Xe 0

14. Silicon Si 4 14. Tungsten W 2,4

15. Phosphorus P 3 15. Actinium Ac 3

16. Sulphur S 2 16. Barium Ba 2

17. Chlorine Cl 1 17. Vanadium V 4

18. Argon Ar 0 18. Nickel Ni 2,3

19. Potassium K 1 19. Cobalt Co 2

20. Calcium Ca 2 20. Iron Fe 2, 3

155 Times' Crucial Science Book - 9

The number of electrons gained, lost or shared during a chemical
reaction gives the valency of an element. For example, sodium loses
one electron from its valence shell and attains octet. It is the stable
state of sodium. Hence, its valency is 1. Similarly, the valency of
chlorine is also 1 because it gains one electron to be in stable form.

Fact Reason

Why is the valency of Sodium 1?
The valency of Sodium is 1 because it looses 1 electron to other
atom to become stable.

Variable valency

Some of the elements may have two or more valencies. For example,
the valencies of iron are 2 and 3, the valencies of copper are 1 and
2 and so on. This property of elements is called variable valency.
7KH YDULDEOH YDOHQF\ RI HOHPHQWV LV GXH WR WKH LQFRPSOHWHO\ ÀOOHG
penultimate shell.

Variable valency of some common elements are as follows:

The valency of elements of group I, II, Element Variable
III are 1, 2, 3 respectively. Likewise,
the valency of element of group IV is Sulphur (S) valency
either 4 or –4. Similarly, the valency of Nitrogen (N) 2, 4, 6
group O is 0. The valency of elements Phosphorus (P) 3, 5
of group V, VI, VII are 3, 2, and 1 Manganese (Mn) 3, 5
respectively. This rule is not applicable 2, 4
in every element.

Radicals Copper (Cu) 1, 2

An atom or group of atoms carrying Mercury (Hg) 1, 2
either positive or negative charge Gold (Au) 1, 3
which acts as a single unit in chemical Iron (Fe) 2, 3
reactions is called radical. Radicals Lead (Pb) 2, 4
do not exist freely in nature because Tin (Sn) 2, 4
they react readily with other radicals
or elements of their surroundings and
form compounds.

On the basis of type of electric charge contained by radicals, they
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Times' Crucial Science Book - 9 156

1. Electropositive radicals
Electropositive radicals are those radicals which possess positive
charge. They possess positive charge because the number of positive
charge exceeds the number of electrons. Electropositive radicals are
also known as basic radicals or metallic radicals. Some examples of
electropositive radicals are as follows:

Valency'1' Valency'2' Valency'3' Valency'4'
Monovalent radical Divalent radical Trivalent radical Tetravalent radical
Hydrogen (H+) Magnesium (Mg++) Boron (B+++) Plumbic (Pb++++)
Sodium (Na+) Calcium (Ca++) Aluminium (Al+++) Stannic (Sn++++)
Potassium (K+) Zinc (Zn++) Chromium (Cr+++) Manganic (Mn++++)
Silver (Ag+) Cupric (Cu++) Auric (Au+++)
Cuprous (Cu+) Barium (Ba++) Arsenous (As+++) -
Mercurous (Hg+) Mercuric (Hg++) Ferric (Fe+++) -
Aurous (Au+) Stannous (Sn++) -
Ammonium(NH4+) Ferrous (Fe++) - -
- -

2. Electronegative radicals

Electronegative radicals are those radicals which carry negative
charge. They possess negative charge because the number of
electrons exceeds the number of protons present in the atoms.
Electronegative radicals are also known as acidic or non-metallic
radicals. Some examples of electronegative radicals are as follows:

Monovalent (’1’) Bivalent (’2’) Trivalent (’3’) Tetravalent (’4’)

Fluoride (F—) Oxide (O – –) Nitride (N– – –) Carbide (C– – – –)

Chloride (Cl—) Sulphide (S – –) Phosphate(PO4– – –) -
Phosphide (P – – –) -
Bromide (Br—) Carbonate (CO3 – –)
Silicate (SiO3 – –) --
Iodide (I—) Peroxide (O2 – –)
Sulphite (SO3 – –) --
Hydroxide (OH—) Sulphate (SO4 – –)
Zincate (ZnO2 – –) --
Chlorate (ClO3—)
Nitrite (NO2—) - --
Nitrate (NO3—)
Bicarbonate (HCO3—) - --
Cyanide (CN—)
--

--

Permanganate (MnO4—) - --
Bisulphate (HSO4—) -
--

157 Times' Crucial Science Book - 9

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SN Radicals Number of atoms and Residual
1. Hydroxide (OH—) their valencies valency
2. Carbonate (CO3— —)
9DOHQF\ RI 2 ï ï ï
3. Bicarbonate(HCO3—) Valency of H = +1
4. Ammonium (NH4+)
5. Phosphate (PO4— — —) Valency of C = +4 ï ï
Valency of O3 ï ð ï

Valency of H = +1 ï ï
Valency of C = +4
Valency of O3 ï

9DOHQF\ RI 1 ï ï
Valency of H4 ð

Valency of P = +5 ï ï
Valency of O4 ï ð ï

Ions
The electrically charged atoms or particles which carry positive or
negative charge are called ions. On the basis of charge carried by
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a. Cations: The ions which carry positive charge are called cations.
An atom becomes cation by losing electron during a chemical
reaction. For example, H+ , Na+, Mg++, K+, Ca++, etc.
b. Anions: The ions which carry negative charge are called anions.
An atom becomes anion when it gains electrons during chemical
reaction. For example, Clï, ClO3ï, SO4ïï, NO3ï, PO4ïïï etc are some
examples of anion.
Difference betweeen cations and anions

Cations Anions
1. They carry positive charge. 1. They carry negative charge.
2. They are formed by losing 2. They are formed by gaining

electrons. electrons.
3. Mostly metals form cations. 3. Mostly non-metals form

4. They are discharged at anions.
cathode during electrolysis. 4. They are discharged at

anode during the process of
electrolysis.

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Chemical bond
All elements except inert gases try to become stable by gaining,
losing or sharing electrons. This is the main cause of chemical
reaction. Metals lose electrons and become positively charged
while non-metals gain electrons and become negatively charged.
When positively charged particles come near to negatively charged
particles, they combine to form compound. In such compounds,
the atoms are held together by a force called chemical bond. In
other words, the force of attraction between two or more atoms in a
compound is called chemical bond. A chemical bond can be formed
by gaining, losing or mutual sharing of electrons.
There are three types of bonds. They are electrovalent bond,
covalent bond and co-ordinate covalent bond. In this chapter, we
shall discuss about electrovalent and covalent bond.
1. Electrovalent bond
The bond formed by the complete transfer of electrons from one atom
to another during a chemical reaction is called electrovalent bond. It
is also known as ionic bond. The compounds which have electrovalent
bond are called electrovalent compounds. Electrovalent compounds
are generally solid in nature. They have high melting and boiling
point. They undergo electrolysis. Calcium oxide, sodium chloride,
magnesium chloride, magnesium oxide, calcium chloride, etc are
some examples of electrovalent compounds.
Formation of sodium chloride
7KH HOHFWURQLF FRQÀJXUDWLRQ RI VRGLXP LV DQG WKDW RI FKORULQH
is (2, 8, 7). One atom of sodium and an atom of chlorine combine
with each other to form sodium chloride (NaCl). This combination
is called chemical reaction in which sodium atom loses one electron
and chlorine atom gains that electron. This ‘loss and gain’ of electron
results in a force of attraction between these two atoms. This force
of attraction is called electrovalent bond.

+ –

p = 11 p = 17 p = 11 p = 17
n = 12 n = 18 n = 12 n = 18

(Na:2,8,1) (Cl:2,8,7) (Na+:2,8) (Cl–:2,8,8)

Attractive force
(electrovalent bond)

Formation of sodium chloride (NaCl)

159 Times' Crucial Science Book - 9

Fact Reason

Magnesium chloride is called electrovalent compound, why?
Magnesium chloride is formed by the transfer of two electrons
of a magnesium atom to two chlorine atoms. Hence, magnesium
chloride is called electrovalent compound.

2. Covalent bond

The bond which is formed by the mutual sharing of electrons
between the combining atoms during chemical reaction is called
covalent bond. The compounds which have covalent bond are called
covalent compounds. Ammonia, water, methane and carbon dioxide
are some examples of covalent compounds. The covalent bond is
also present in the molecule of some elements such as hydrogen,
oxygen, nitrogen, chlorine etc. Covalent compounds are generally
found in liquid or gaseous state. The solid covalent compounds have
low melting and boiling points. They cannot work as electrolyte.
They can easily dissolve in organic solvents.

Formation of water molecule

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is 1. Oxygen and hydrogen atoms share 2 pairs of electrons and
form a cmonotlercibuuleteofonweateelerc(tHro2On )e. aIcnhthfoirs combination, two hydrogen
atoms sharing with two electrons
from oxygen.

+ p=1 p=1

p=8 H p=8 H
n=8 n=8
p=1
+ p=1
H
OH O

Formation of Water (H2O)

Fact Reason

Why is water called covalent compound?
Water is formed by the sharing of electrons between two
hydrogen atom and one oxygen atom. Hence, water is called
covalent compound.

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Differences between ionic and covalent compounds

Electrovalent compounds Covalent compounds

1. They are formed by the complete 1. They are formed due to the mutual
transfer of electrons from one sharing of electrons between the
atom to another. combining atoms.

2. They are generally solids. 2. These compounds may be gas, liquid
or solid. The solids are soft and brittle.

3. They have low melting and 3. They have high melting and
boiling points. boiling points.

4. They conduct electricity in their 4. They generally do not conduct
molten or aqueous solution state. electricity.

5. They are usually insoluble in 5. They are usually soluble in water.
water.

Molecular formula

The symbolic representation of a molecule of a compound or an
element which shows the actual number of atoms in a molecule is
called molecular formula. For example, the molecular formula of
awmhimchoncioansisistNsHo3f. fHouerrea,toNmHs3. represents a molecule of ammonia
and three are hydrogen atoms. Among four atoms, one is nitrogen

Information from molecular formula
1. Molecular formula represents the ratio of different elements present in
the given compound.
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3. It gives the actual number of atoms present in a molecule of a compound.
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Molecular weight

The sum of atomic weights of all the atoms present in a molecule is
called molecular weight. lt is also known as molecular mass.

For example: =2×H+1×O
Molecular weight of H2O = 2 × 1 + 1 × 16
Molecular weight of CO2 = 18
=1×C+2×O
= 12 + 32
= 44

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Molecular weight of NH3 =1×N+3×H
= 1 × 14 + 3 × 1
=17

Method to write molecular formula of compounds

1. Write the symbol of elements or radicals side by side. Write
PHWDO ÀUVW DQG WKHQ QRQ PHWDO. For example, to write molecular
formula of calcium oxide.

Calcium Oxygen

Ca O

2. Write the valency of each element or radical on the top of symbol.

Calcium Oxygen

22

Ca O

3. Interchange the valency of the elements or radicals.

2 2 = Ca2O2
Ca O

4. Remove the common valency if present.

Ca2O2 = CaO (Here, 2 is common and is removed)
2. Ammonia
Examples:
1. Water

Hydrogen Oxygen Nitrogen Hydrogen
1 3
2 = H2O 1 = NH3
H O N H

3. Common salt (sodium chloride) 4. Calcium chloride
Calcium
Sodium Chloride 2 Chloride
11 Ca
= NaCl 1 =CaCl2
Na Cl Cl

5. Magnesium Oxide 6. Sodium hydroxide

Magnesium Oxygen Sodium Hydroxide
2
Mg 2 = Mg2O2 1 1 =NaOH
O = MgO Na OH

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7. Copper sulphate 8. Ammonium Sulphate

Copper Sulphate Ammonium Sulphate
2 2
SO4 = CCuu2S(SOO4 4)2 1 2 =(NH4)2SO4
Cu = NH4 SO4

9. Ferric oxide 10. Potassium Cyanide

Ferric Oxide Potassium Cyanide
3 1
Fe 2 = Fe2O3 K 1 =KCN
O CN

SN Name of Compound Chemical Name Molecular
Formula
1. Water Hydrogen Oxide
H2O
2. Common salt Sodium Chloride NaCl
CuNCSHHaaNO(H2NOS4H.COO5H3OH34)232O
3. Sulphuric acid Hydrogen Sulphate HCl
CaSO4.2H2O
4. Ammonia Hydrogen Nitride
CaCO3
5. Lime water Calcium Hydroxide CaSO4.1/2H2O
Na2CO3.10H2O
6. Baking soda Sodium Bicarbonate
CaO
7. Blue vitriol Copper Sulphate CH4
CaCl2
8. Nitric acid Hydrogen Nitrate MgCO3
K2SO4
9. Hydrochloric acid Hydrogen Chloride K2CO3
AgNO3
10. Gypsum Calcium Sulpahte

11. Chalk, limestone or marble Calcium Carbonate

12. Plaster of paris Calcium Sulphate

13. Washing soda Sodium Carbonate

14. Quick lime Calcium Oxide

15. Marsh gas Methane

16. Calcium chloride Calcium chloride

17. Magnesium carbonate Magnesium carbonate

18. Potassium sulphate Potassium sulphate

19. Potassium carbonate Potassium carbonate

20. Lunar caustic Silver nitrate

Main points to remember
7KH VLPSOHVW DQG SXUH IRUP RI VXEVWDQFH ZKLFK LV PDGH RI VLPLODU NLQGV

of atoms is called element.
2. Symbols are the short and abbreviated form for the full name of elements.
7KH SXUH VXEVWDQFHV ZKLFK DUH IRUPHG E\ WKH FKHPLFDO FRPELQDWLRQ RI WZR

RU PRUH HOHPHQWV LQ D GHÀQLWH SURSRUWLRQ E\ ZHLJKW DUH FDOOHG FRPSRXQGV

163 Times' Crucial Science Book - 9

$Q DWRP LV WKH VPDOOHVW SDUWLFOH RI DQ HOHPHQW WKDW FDQ WDNH SDUW LQ D
chemical reaction.

$ PROHFXOH LV WKH VPDOOHVW SDUWLFOH RI D FRPSRXQG RU DQ HOHPHQW ZKLFK
has independent existence.

7KH VWDWH LQ ZKLFK WKH RXWHUPRVW VKHOO RI DQ DWRP KDV WKH FDSDFLW\ WR
DFFRPPRGDWH QR PRUH WKDQ WZR HOHFWURQV LV FDOOHG GXSOHW

7. A stable set of eight electrons in the outermost shell of an atom is called octet.
8. The number of electrons in the valence shell of an atom that determines

the combining capacity of an element is called valency.
9. An atom or group of atoms carrying either positive or negative charge

that behaves as a single unit in a chemical reaction is called radical.
10. The radicals that carry positive charge due to the loss of electron are

called electropositive radicals.
(OHFWURQHJDWLYH UDGLFDOV DUH WKH UDGLFDOV ZKLFK FDUU\ QHJDWLYH FKDUJH

due to the gain of electrons.
7KH DWWUDFWLYH IRUFH ZKLFK KROGV WZR RU PRUH DWRPV LQ D PROHFXOH RI D

compound is called chemical bond.
7KH ERQG ZKLFK LV IRUPHG E\ WKH WUDQVIHU RI HOHFWURQV IURP RQH DWRP WR

another during chemical reaction is called electrovalent bond.
7KH ERQG ZKLFK LV IRUPHG E\ WKH PXWXDO VKDULQJ RI HOHFWURQV DPRQJ

atoms during chemical reaction is called covalent bond.
15. The symbolic representation of the molecule of a compound or element,

ZKLFK VKRZV WKH DFWXDO QXPEHU RI DWRPV SUHVHQW LQ D PROHFXOH LV FDOOHG
molecular formula.

Learn and Write

1. WtHw2horaaetptoirsmestsehnoeftsdhyaifdfmreorogeleennccu. eleboefthwyedernogHen2 and 2H? represents
whereas 2H

2. What is the difference between Co and CO?
Co represents the element Cobalt but CO represents the
compound carbon monoxide.
3. Neon is an inert gas. Why?
Neon atom has 8 electrons in the outermost orbit and thus it is in
octet state. Hence, its outermost orbit is already complete and it
does not take part in chemical reactions and is an inert gas.
4. Valency of chlorine is 1. Why?
Electronic configuration of chlorine is 2, 8, 7. Therefore, it needs
one electron to make its outermost orbit complete. During its
combination with other elements, it takes one electron from
them. Thus, its valency is 1.

Times' Crucial Science Book - 9 164

5. Sodium is a reactive metal. Why?
Sodium has only one electron in the outermost orbit. Therefore,
it always tries to lose that one electron to make the outermost
orbit complete. Therefore, it is a reactive metal.

6. What is variable valency? Why do some elements have
variable valency?
The property of an element to have more than one kind of
valency is called variable valency. An element has a single
kind of valency if only the valence electrons take part in bond
formation. If the valence electrons as well as the electrons of
penultimate shell of an element take part in bond formation,
such element has variable valency.

7. What changes occur in the atoms if they combine to
form a compound?
Each atom attains a stable configuration of the nearest inert
gas after it undergoes chemical combination. The combining
atoms completely lose their individual property and a molecule
with different properties is formed.

8. What are the characteristics of covalent compounds?
The characteristics of covalent compounds are as follows:
a. Covalent compounds are generally found in liquid or gas state.
b. The solid covalent compounds have low melting and boiling points.
c. They are non-electrolyte and hence they do not undergo electrolysis.
d. Most of them are insoluble in water. They are soluble in organic solvents.

9. Why don’t the covalent compounds undergo electrolysis?
The covalent compounds contain covalent bonds. The covalent
bonds are formed by the mutual sharing of electrons. Hence,
these compounds do not dissociate into ions in their aqueous
solution or molten state. As these compounds have no ions,
they cannot undergo electrolysis

Glossary : related to the parts of organisms, containing carbon and
hydrogen
Organic
: state of being present
Existence : nearly, roughly, about
Approximately : hold, suit, have room for
Accommodate : changing, not fixed
Variable : just inner to the outermost
Penultimate : to lose charge, to remove charge
Discharge : ability to break down into pieces
Brittle

165 Times' Crucial Science Book - 9

Exercise

1. Choose the best alternative in each case.

a. The mass of hydrogen nucleus is equivalent to

i. Electron ii. Proton iii. Neutron iv. All of these

b. An element having three shells consists of one electron in its

valence shell; the element is

i. Fluorine ii. Magnesium iii. Sodium iv. Potassium

c. Which of the following elements has a variable valency?

i. Calcium ii. Silver iii. Chlorine iv. Copper

d. What is the molecular formula of sulphuric acid?

i. HCl ii. H2SO4 iii. HNO3 iv. NaOH

e. Which of the following is the most reactive non-metal?

i. Fluorine ii. Chlorine iii. Bromine iv. Iodine

2. Answer these questions in very short.
a What is chemistry?
b. How many elements are discovered so far?
c. What is a molecule? Which is a molecule of hydrogen either
d. DHe2 foinr e2Hoc?tet with examples.
e. What is a valence shell?
f. What do you mean by Co and CO?
g. What is duplet rule?
h. Calculate the molecular owf eeilgehcttroofnCegaaCtOiv3e.
i. Give any four examples radicals.

3. Define:
a. Atom b. Duplet c. Octet rule d. Symbol
e. Electronic configuration f. Chemical bond

4. Give reason:
a. The valency of sodium is 1.
b. The valency of argon is 0.
c. Sodium chloride is an electrovalent compound.
d. Carbon dioxide is a covalent compound.
e. An atom is electrically neutral.

5. Differentiate between:
a. Elements and compounds
b. Electrovalent and covalent bond
c. Molecule and atom
d. Symbol and molecular formula

Times' Crucial Science Book - 9 166

e. Molecule of a compound and molecule of an element
f. Atomic number and atomic weight

6. Write the molecular formulae of following:
a. Water b. Ammonia
c. Sodium chloride d. Calcium oxide
e. Carbon dioxide f. Magnesium hydroxide
g. Potassium oxide h. Methane
i. Potassium chloride j. Nitric acid
k. Sulphuric acid l. Hydrochloric acid
m. Limestone n. Sulphur trioxide
o. Nitrous acid p. Ferrous chloride
q. Hydrogen peroxide s. Auric chloride
r. Sodium peroxide t. Phosphoric acid
u. Potassium bicarbonate v. Ammonium sulphate
w. Potassium chlorate x. Silver chloride
y. Washing soda z. Ammonium nitrate

7. Write down the names of following compounds.

a. Ca(OH)2 b. H2SO4 c. CH4 d. MgCO3
e. Ca(HCO3)2
i. KCN f. Ca3(PO4)2 g. HNO3 h. KClO3

m. HCl j. Na2O k. HgO l. CuSO4

n. (NH4)2SO4

Project Work

Observe different elements or compounds that are being used
in your home and school. Write down their names, symbols or
molecular formulae. Also explain their uses.

167 Times' Crucial Science Book - 9

Chapter

9 &KHPLFDO
5HDFWLRQ
Fritz Haber

He discovered Haber's process, Born –
Haber Cycle, Fertilizer, Chemical warfare,
explosive, etc.

ƐƟŵĂƚĞĚ WĞƌŝŽĚƐ ͗ϲ

Objectives

ƚ ƚŚĞ ĞŶĚ ŽĨ ƚŚĞ ůĞƐƐŽŶ͕ ƐƚƵĚĞŶƚƐ ǁŝůů ďĞ ĂďůĞ ƚŽ͗
• GHÀQH SK\VLFDO DQG FKHPLFDO FKDQJHV ZLWK H[DPSOHV
• GLIIHUHQWLDWH EHWZHHQ SK\VLFDO DQG FKHPLFDO FKDQJH
• GHÀQH FKHPLFDO FKDQJH DQG UHSUHVHQW FKHPLFDO FKDQJH LQ WHUPV RI ZRUG DQG IRUPXOD

HTXDWLRQV
• GHÀQH UHYHUVLEOH DQG LUUHYHUVLEOH UHDFWLRQ

Mind Openers

• What is a change?
• What are physical and chemical changes? Can you tell their examples?
• What is a chemical reaction? Can you represent chemical reaction in terms of symbols

and molecular formula? Discuss.

Introduction

Several types of changes occur in our surrounding continuously.
Some of these changes take place slowly while some take place
faster. The conversion of one form of things into another is called
change. The change can be categorized into two types: physical
change and chemical change.

1. Physical change

A physical change is a temporary change in which no new substance
is formed. The shape, size, volume, state, etc of the given substance
may change in this change. For example, if ice is heated, it changes
into water. If water is again heated, it changes into vapour. In
these cases, there is change in the physical state of matter. If water
vapour is cooled, it changes into water and the further cooling of
water produces ice. Hence, such changes can be reversed. It means
that the physical change is temporary and reversible.

Some common examples of physical change are as follows:

0HOWLQJ RI LFH LQWR ZDWHU (YDSRUDWLRQ RI ZDWHU LQWR YDSRXU

Times' Crucial Science Book - 9 168

'LVVROYLQJ VDOW LQWR ZDWHU 7HDULQJ SDSHU LQWR SLHFHV
0DNLQJ VLFNOH IURP LURQ URG 0DJQHWL]LQJ DQ LURQ QDLO
0DNLQJ VKLUW IURP UDZ FORWK 0HOWLQJ JKHH LQWR OLTXLG VWDWH

In physical change, there is no change in the molecular level of
substances.

Fact Reason

Melting of ice is a physical change, why?
When ice melts into water only the physical properties change
but the chemical structure remains same as both of them contain
water molecules. Hence, melting of ice is a physical change.

2. Chemical change

A chemical change is a permanent change in which an entirely new
substance with new properties is formed. There is change in the
molecular level of matter. Chemical change cannot be reversed
HDVLO\ )RU H[DPSOH LI ERLOHG PLON LV FRROHG DQG DFWHG E\ D VSHFLÀF
type of bacteria for some hours, it gets converted to curd. But the
curd cannot be reversed back to milk. Hence, chemical change is a
permanent and irreversible change.

Some common examples of chemical change are as follows:

%XUQLQJ RI ÀUHZRRG 5XVWLQJ RI LURQ
7XUQLQJ RI PLON LQWR FXUG 'LJHVWLRQ RI IRRG LQ RXU ERG\
*URZWK RI EDE\ LQWR DGXOW (OHFWURO\VLV RI ZDWHU

A molecule of water is made by the chemical combination of hydrogen
and oxygen atoms. By electrolysis, the water molecule can be
decomposed into totally new substances, i.e. oxygen and hydrogen.

Fact Reason

Why is rusting of iron chemical change?
Rusting of iron is formed by the formation of the layer of new
substance i.e. iron oxide and this change is irreversible. So,
rusting of iron is chemical change.

Differences between physical and chemical change

Physical change Chemical change
1. Physical change is a temporary 1. Chemical change is a permanent

change in which no new change in which a new substance is
substance is formed. formed.

169 Times' Crucial Science Book - 9

2. There is change in physical 2. There is change in both physical and
properties only. chemical properties.

3. There is no change in energy. 3. There is change in energy and mass.
4. It is usually reversible. Energy is either absorbed or evolved.

4. It is usually irreversible

5. It involves change in shape, size, 5. It involves change in molecular level
volume, state of matter, etc. of matter.

Chemical reaction

A chemical reaction is a chemical change in which combination,
decomposition or exchange of molecules of matter takes place. The
chemical reaction can be represented by using symbols of atoms
and molecular formulae of molecules.

Chemical equation

The symbolic representation of actual chemical reaction in terms of
symbols and formulae is called chemical equation. There are two
types of chemical equations on the basis of the way of representation.
They are:

L :RUG HTXDWLRQ
LL 6\PEROLF HTXDWLRQ RU IRUPXOD HTXDWLRQ

i. Word equation

Word equation is a form of chemical equation which is represented
by writing the full names of reactants and products. The substances
which undergo the chemical change are called reactants. They are
written on the left hand side of chemical equation. Similarly, the
substances which are produced as a result of chemical change in the
reactants are called products. They are written on the right hand
side of chemical equation. Some examples of word equations are as
follows:
+\GURJHQ R[\JHQ ń :DWHU
&DUERQ K\GURJHQ ń 0HWKDQH
&DUERQ R[\JHQ ń &DUERQ GLR[LGH
&DOFLXP FDUERQDWH ń &DOFLXP R[LGH &DUERQ GLR[LGH
(Reactant) (Products)

ii. Symbolic equation or formula equation

Symbolic equation is a form of chemical equation which is represented
by writing the symbols and formulae of reactants and products.

Times' Crucial Science Book - 9 170

Some examples of symbolic equation are as follows:
R2Hea2c+taOn2t s ń P+ro2dOuct
RCe+ac2tHan2 ts ń &Pr+o4duct
R2KeaCcltOan3 t ń P.ro&dOu c ts22

The physical states of the reactants and products can be represented
by using symbols.

For example:
Calcium carbonate (s) oh e a t Calcium oxide (s) + Carbon dioxide (g)
CaCO3 (s)o¨ CaO(s) + CO2(g)
Here, s = solid, g = gas

+HDW UHTXLUHG IRU WKH UHDFWLRQ LV UHSUHVHQWHG E\ ¨ (delta) above the
arrow. When calcium carbonate is heated, it decomposes to give
calcium oxide and carbon dioxide.

Endothermic and exothermic reactions

A chemical reaction which needs heat is called endothermic reaction.

For example,
2KClO3o¨ 2KCl + 3O2

A chemical reaction in which heat is produced is called exothermic
reaction.

CH4 + 2O2 o CO2 + 2H2O + heat
Reversible and irreversible reactions

A chemical reaction in which reactants are changed into products
and the products are changed back into reactants is called reversible
reaction. Reversible reaction is denoted by arrows that point forward
and backward (O).

For example:

N2 + 3H2 O 2NH3
A chemical reaction which occurs in only one direction, i.e., reactants
to product is called irreversible reaction. Irreversible reactions are
represented by ń .

171 Times' Crucial Science Book - 9

2H2 + O2 ń +2O
CaCO3 o¨ CaO + CO2

Unbalanced and balanced chemical equations
The chemical equation which has unequal number of atoms in the
reactant and product side is called unbalanced chemical equation.
Such equations are also called Skeleton chemical equations.

For example:

H2 + O2 ń +2O
The cheNmaic+aCl el2q u ńat i1onD&wOhich has equal number of atoms in reactant
and product side is called balanced chemical equation.

For example
2H2 + O2o¨ 2H2O

CH4 + 2O2 ń &22 + 2H2O

Diatomic elements
The elements whose molecules are made up of two atoms are
called diatomic elements. When diatomic elements take part in
reaction, they are always represented in molecular form. Hydrogen,
QLWURJHQ R[\JHQ ÁXRULQH FKORULQH LRGLQH DQG EURPLQH DUH WKH
diatomic elements. They are written as H2, N2, O2, F2, Cl2, I2 and Br2
respectively.

Examples:

2H2 + O2 o¨ 2H2 O
H2 + F2 o¨ 2HF

2Na + Cl2o¨ 2NaCl

Methods of writing chemical equations
1. Write the word equation. Write down the names of reactants

on left side of arrow and that of products on the right side.
+\GURJHQ &KORULQH ń +\GURFKORULF DFLG
2. Rewrite the equation in symbolic form.
H2 + Cl2 ń +&O

Times' Crucial Science Book - 9 172

3. If the equation obtained is not balanced equation, balance
it by counting the number of atoms in each side and writing
QXPHULFDO FRHIÀFLHQW ZKHUH LW LV QHFHVVDU\

H2 + Cl2 ń +&O
4. If any conditions are necessary for the reaction, write it

above the arrow. Thus, a required equation is obtained.

H + Cl Sunlight 2HCl
22

Some other examples:

1. Word equation:

=LQF GLO VXOSKXULF DFLGń =LQF VXOSKDWH +\GURJHQ

Chemical equation: Zn + dil. H2SO4 ń =Q624 + H2 Ń
2. Word equation:

3RWDVVLXP FKORUDWH ń 3RWDVVLXP FKORULGH 2[\JHQ

Skeleton equation: KClO3 ń .&O 22 Ń
Balanced equation: 2KClO3 ń .&O 22 Ń
3. Word equation:
Hydrogen peroxide MnO2 Water + Oxygen

Skeleton equation: H2O2 MnO2 H2O + O2Ń
Balanced equation: 2H2O2 MnO2 2H2O + O2Ń
4. Word equation:

6RGLXP FKORULGH 6LOYHU QLWUDWH ń 6RGLXP QLWUDWH 6LOYHU FKORULGH

Chemical equation: NaCl + AgNO3 ń 1D123 + AgCl
5. Word equation:

Hydrogen + Nitrogen O Ammonia
Skeleton equation: H2 + N2 O NH3
Balanced equation: 3H2 + N2 O 2NH3
6. Word equation:
Balanced equation: ZnO + C o¨ Zn + CO

7. Word equation:

3KRVSKRUXV 2[\JHQ ń 3KRVSKRUXV SHQWR[LGH

Skeleton equation: P + O2 ń 32O5
Balanced equation: 4P + 5O2 ń 32O5
8. Word equation:
BSkaelalentcoendeeqquuaattiioonn::M3Mg+gN+2No2¨ oM¨ gM3Ng23N2

173 Times' Crucial Science Book - 9

9. Word equation:
/LPH ZDWHU &DUERQ GLR[LGH ń &DOFLXP FDUERQDWH ZDWHU

Chemical equation: Ca(OH)2 + CO2 ń &D&23 +H2O
10. Word equation:

6RGLXP K\GUR[LGH +\GURFKORULF DFLG ń 6RGLXP FKORULGH ZDWHU

Chemical equation: 1D2+ +&O ń 1D&O +2O
Information conveyed by chemical equation

A chemical equation gives full meaning if it is balanced. A balanced
chemical equation gives the following information:

1. It represents the name of the reactants and products.
,W VKRZV WKH H[DFW DPRXQW RI UHDFWDQWV DQG SURGXFWV
,W UHSUHVHQWV WKH UDWLR RI PROHFXODU ZHLJKW RI WKH UHDFWDQW DQG SURGXFW

separately.
,W FDQ VKRZ WKH FRQGLWLRQV UHTXLUHG IRU D FKHPLFDO UHDFWLRQ OLNH KHDW OLJKW

catalyst, etc.
,W FDQ VKRZ WKH W\SH RI FKHPLFDO UHDFWLRQ

Catalyst

A catalyst is a chemical substance which increases or decreases the
rate of chemical reaction without being consumed itself. The process
of changing the rate of chemical reaction by the use of catalyst is
called catalysis. Many chemical reactions take place in the presence
RI FDWDO\VW 7KH FDWDO\VWV DUH FODVVLÀHG LQWR WZR W\SHV

a. Positive catalyst b. Negative catalyst

a. Positive Catalyst: The catalyst which increases the rate of chemical
reaction without being consumed itself is called positive catalyst.
For an example, when potassium chlorate is heated strongly,it
decomposes to produce potassium chloride and oxygen.

2KClO3 360°C 2KCl + 3O2
(Without catalyst high temperature is required)

But when manganese dioxide (MnO2) is added, the reaction takes
place at lower temperature.

2KClO3 240°C 2KCl + 3O2
MnO2

(If we use catalyst, reaction occurs at reduced temperature)

pMenroOx2iadleso(Ha2cOts2)a.s positive catalysts in the decomposition of hydrogen
2H2O2 MnO2 2H2O + O2Ń

Times' Crucial Science Book - 9 174

Fact Reason

Why is manganese dioxide called positive catalyst?
Manganese dioxide is called positive catalyst because it
increases the rate of chemical reaction.

b. Negative catalyst: A catalyst which decreases the rate of chemical
reaction without being consumed itself is called negative catalyst.
For example, glycerine or phosphoric acid acts as a negative catalyst
in the decomposition of hydrogen peroxide.

2H O 2H O + O ŃGlycerine2 2
2 2 (Slowed rate)

Fact Reason

Why is glycerine called negative catalyst?

Glycerine is called negative catalyst because it decreases the
rate of chemical reaction.

Learn and Write

1. What happens when magnesium burns in oxygen?
When magnesium burns in oxygen, it gives magnesium oxide.
2Mg + O2 o 2MgO
Magnesium + Oxygen o Magnesium oxide

2. Melting of ice is a physical change. Why?
When ice is heated, it melts. During this change, there occurs
only the change in its state. There is no change in the molecular
level. It means molecular structure of ice and water is not
different. Therefore, melting of ice is a physical change.

3. Dissociation of water into hydrogen and oxygen on the
passage of current is a chemical change. Why?
When current passes through acidified water, the water
dissociates into oxygen and hydrogen. In this change, the
change occurs in the molecular level. In one molecule of water,
there are three atoms in which two are hydrogen atoms and
one is oxygen atom. When water dissociates, the hydrogen
and oxygen atoms get separated forming separate hydrogen
and oxygen molecules. Therefore, dissociation of water into
hydrogen and oxygen is a chemical change.

175 Times' Crucial Science Book - 9

Glossary : that can be reversed
: lasting for a short time
Reversible : break down
Temporary
Dissociation

Main points to remember

$ SK\VLFDO FKDQJH LV D WHPSRUDU\ FKDQJH LQ ZKLFK QR QHZ VXEVWDQFH LV
formed.

$ FKHPLFDO FKDQJH LV D SHUPDQHQW FKDQJH LQ ZKLFK D QHZ VXEVWDQFH
ZLWK HQWLUHO\ QHZ SURSHUWLHV LV IRUPHG

$ FKHPLFDO UHDFWLRQ LV D FKHPLFDO FKDQJH LQ ZKLFK FRPELQDWLRQ
GHFRPSRVLWLRQ RU H[FKDQJH RI PROHFXOHV RI PDWWHU WDNHV SODFH

$ FKHPLFDO HTXDWLRQ LV WKH V\PEROLF UHSUHVHQWDWLRQ RI DFWXDO FKHPLFDO
reaction in terms of symbols and formulae.

7KH VXEVWDQFHV ZKLFK XQGHUJR FKHPLFDO FKDQJH DUH FDOOHG UHDFWDQWV
ZKHUHDV WKH VXEVWDQFHV ZKLFK DUH IRUPHG DV D UHVXOW RI FKHPLFDO
change in reactants are called products.

7KH FKHPLFDO UHDFWLRQ ZKLFK KDV XQHTXDO QXPEHU RI DWRPV LQ UHDFWDQW
and product side is called unbalanced chemical reaction.

7KH FKHPLFDO UHDFWLRQ ZKLFK KDV HTXDO QXPEHU RI DWRPV LQ UHDFWDQW
and product side is called balanced chemical reaction.

7KH HOHPHQWV ZKRVH PROHFXOHV DUH PDGH XS RI WZR DWRPV DUH FDOOHG
diatomic elements.

Exercise

1. Define:
a. Chemical equation b. Reversible reaction
c. Balanced chemical equation d. Reactant
e. Unbalanced chemical equation f. Product
g. Irreversible reaction h. Endothermic reaction

2. Answer these questions:
a. What is physical change? Give examples.
b. What is chemical equation? What are the ways to write
chemical equations?
c. What do you mean by word equation and symbolic equation?
Give two examples of each.
d. Define diatomic elements with examples.

Times' Crucial Science Book - 9 176

3. Differentiate between:
a. Physical and chemical change
b. Endothermic and exothermic reaction
c. Reversible and irreversible reaction
d. Balanced and Skeleton chemical equation

4. Give reasons:
a. Melting of ice into water is a physical change.
b. Change of milk into curd is a chemical change.
c. The reaction between hydrogen and nitrogen to give
ammonia is a reversible reaction.

5. Change the following word equations into balanced chemical
equations.
D +\GURJHQ 2[\JHQ ń:DWHU
E +\GURJHQ 1LWURJHQ ń $PPRQLD
F +\GURJHQ &KORULQH ń +\GURFKORULF DFLG
G &DUERQ 2[\JHQ ń &DUERQ GLR[LGH
H 6RGLXP 2[\JHQ ń6RGLXP R[LGH
I 6RGLXP &KORULQH ń 6RGLXP FKORULGH
J 0DJQHVLXP 1LWURJHQ ń 0DJQHVLXP QLWULGH
K /LPH ZDWHU &DUERQ GLR[LGH ń &DOFLXP FDUERQDWH :DWHU
L 3RWDVVLXP FKORUDWH ń 3RWDVVLXP FKORULGH 2[\JHQ
M &DOFLXP K\GUR[LGH &DUERQ GLR[LGH ń &DOFLXP FDUERQDWH
+ Water.
N &DOFLXP K\GUR[LGH K\GURFKORULF DFLG ń &DOFLXP FKORULGH
+ Water.
O &DOFLXP FDUERQDWH +\GURFKORULF DFLG ń &DOFLXP FKORULGH
+ Water +Carbon dioxide.
P &RSSHU FDUERQDWH ń &RSSHU R[LGH &DUERQ GLR[LGH
Q &RSSHU 6XOSKXULF DFLG ń&RSSHU VXOSKDWH ZDWHU
Sulphur dioxide.

6. Balance the following skeleton equations.
Deb.. KC$uOC C lO O+33 & ńńO ń .&&X $O2 O & O2 3&2+2H2 2 d. cfH.. 2ASPOg+4+ O B 21 rń2D ń2 3+ 2$O ńJ5% 1U D2SO4 + H2O

Project Work

Several changes occur in your home, school or surrounding. Notice the
changes that occur at your home, especially in the kitchen. Classify these
FKDQJHV DV SK\VLFDO FKDQJH DQG FKHPLFDO FKDQJH ZLWK MXVWLÀFDWLRQ

177 Times' Crucial Science Book - 9

Chapter

10 6ROXELOLW\

Mikhail Tswett

Objectives He is also known by the name Mikhail Tsvet.
He is known for the discovery of adsorption
chromatography.

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ƚ ƚŚĞ ĞŶĚ ŽĨ ƚŚĞ ůĞƐƐŽŶ͕ ƐƚƵĚĞŶƚƐ ǁŝůů ďĞ ĂďůĞ ƚŽ͗

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• H[SODLQ GLIIHUHQW W\SHV RI VROXWLRQV
• GLIIHUHQWLDWH EHWZHHQ VDWXUDWHG DQG XQVDWXUDWHG VROXWLRQ
• explain solubility and solubility curve.

Mind Openers

• What is a mixture?
• What do you mean by homogeneous and heterogeneous mixtures?
• Are mixtures useful? Discuss.

Introduction
The substances are generally found in the form of mixtures. The
mixture can be formed among all three states of matter, i.e. solid,
liquid and gas.
A mixture is a mass formed by mixing two or more substances in
any proportion by weight without any change in the individual
property of the components. So, each component of mixture retains
its own identity and properties. The mass obtained by mixing salt
and water, water and sand, sand and salt, sugar and water, etc are
some examples of mixture.
&ODVVLÀFDWLRQ RI PL[WXUHV
On the basis of the distribution of components, mixture can be
FODVVLÀHG LQWR WZR W\SHV 7KH\ DUH KHWHURJHQHRXV PL[WXUH DQG
homogenous mixture.
1. Heterogeneous mixture
A mixture in which the components are not distributed uniformly
DQG WKH\ FDQ EH LGHQWLÀHG E\ QDNHG H\HV LV FDOOHG KHWHURJHQHRXV
mixture. Mixture of sand and water, sugar and sand, smoke, oily
water, etc are some examples of heterogeneous mixture.

Times' Crucial Science Book - 9 178

2. Homogeneous mixture
A mixture in which the components are uniformly distributed and
FDQQRW EH LGHQWLÀHG E\ QDNHG H\HV LV FDOOHG KRPRJHQHRXV PL[WXUH
Mixture of salt and water, sugar and water, alcohol and water, etc
are some examples of homogeneous mixture.

Fact Reason

Why is sugar solution called homogenous mixture?
Sugar solution is called homogeneous mixture because sugar
particles are uniformly distributed in the solution to get
dissolved. Hence, sugar solution is called homogeneous mixture.

Differences between homogeneous mixture and
heterogeneous mixture

Homogeneous mixture Heterogeneous mixutre
1. The components are distributed 1. The components are not distributed

uniformly. uniformly.
2. Each component of the mixture 2. The components of the mixture can be

FDQQRW EH LGHQWLÀHG E\ QDNHG H\HV LGHQWLÀHG E\ QDNHG H\HV

3. A solution is a homogeneous mixture. 3. A suspension is a heterogeneous mixture.

On the basis of size of particles
0L[WXUH FDQ EH FODVVLÀHG LQWR WKUHH W\SHV RQ WKH EDVLV RI VL]H RI
particles: suspension, colloids and solution.
1. Suspension
Suspension is a heterogeneous mixture in which the size of the
particles is 10ï cm or larger. The size of particles means the diameter
RI WKH SDUWLFOHV 7KH SDUWLFOHV RI VXVSHQVLRQ FDQ HDVLO\ EH LGHQWLÀHG
with the help of simple microscope as well as naked eye. Muddy
water, sandy water, etc are some examples of suspension.
The components of a suspension can be separated by, sedimentation
GHFDQWDWLRQ DQG ÀOWUDWLRQ SURFHVVHV
2. Colloid
A homogeneous mixture in which the size of the particles ranges
between 10ï to 10ï cm is called colloid. Blood, urine, wax, milk, etc
are some examples of colloid. The components of a colloid can be
VHSDUDWHG E\ XVLQJ XOWUDÀOWUDWLRQ DQG FHQWULIXJDWLRQ PHWKRGV
3. Solution
A solution is a homogeneous mixture in which the size of particles
is 10ï cm or smaller. The components which are needed to make
solution are solute and solvent.

Solvent + Solute = Solution

179 Times' Crucial Science Book - 9

A component of a solution which dissolves solute to form a solution
is called solvent. Solvent is present in relatively more amount than
solute. Water, alcohol, kerosene, petrol, benzene, etc are some
examples of solvent.

A solute is a component of a solution which gets dissolved in solvent.
The amount of solute present in solution is relatively less than that
of solvent. Sugar, salt, alum, etc are some examples of solute.
Concentration of solution
The amount of solute present in given amount of solvent is called
concentration of solution.
On the basis of relative amount of solute present in solution, it can
EH FODVVLÀHG LQWR WZR W\SHV GLOXWH DQG FRQFHQWUDWHG VROXWLRQ
Dilute and concentrated solution
A solution which contains relatively less amount of solute in the
given amount of solvent is called dilute solution. It is represented
by ‘dil.’ in chemical equation.
A solution which contains relatively more amount of solute in
the given amount of solvent is called concentrated solution. It is
represented by ‘conc’ in chemical equation.
The terms dilute and concentrated are relative terms and they do
not tell the exact concentration of the solution.
Saturated, Unsaturated and Supersaturated Solution
$ VROXWLRQ FDQ EH FODVVLÀHG LQWR WKUHH W\SHV RQ WKH EDVLV RI LWV DELOLW\
to dissolve more amount of solute. They are: unsaturated, saturated
and super saturated solution.

1. Unsaturated and saturated solution
The solution which can dissolve more amount of solute at a particular
temperature is called an unsaturated solution.
Take a glass of water, two spoonfuls of sugar and a glass rod. Add
a little amount of sugar in the water and stir it with the glass rod.
The sugar dissolves easily in it due to stirring. Again add some more
sugar to the glass and stir it. The sugar dissolves again. Since the
solution dissolves more amount of solute, it is unsaturated solution.
After adding sugar in the solution for some time, the solution at
that temperature is unable to dissolve any more amount of sugar.
This solution is called saturated solution.
Thus, the solution which cannot dissolve any more amount of solute

Times' Crucial Science Book - 9 180

at a given temperature is called a saturated solution. A saturated
solution becomes unsaturated solution if temperature or the amount
of solvent is increased.

Differences between unsaturated and saturated solution

Unsaturated solution Saturated solution
1. The solution which can dissolve 1. The solution which cannot
more amount of solute at a dissolve any more amount of
particular temperature is called solute at a particular temperature
unsaturated solution. is called saturated solution.
2. It contains less solute than it can 2. It contains solute as much as it
dissolve. can dissolve.
3. It becomes saturated when 3. It becomes unsaturated on
solute is added. heating.

2. Supersaturated solution
The solution which holds more solute than that is required to
make a saturated solution at a particular temperature is called
supersaturated solution. On heating, the saturated solution becomes
unsaturated and hence it can dissolve more amount of solute in it.
On cooling, the solution throws out excess amount of solute in the
form of crystals. Thus, the solution turns to be supersaturated
solution. This solution contains more solute dissolved than a
saturated solution at that temperature.
Activity 10 .1 To identify the unsaturated, saturated and suer saturated solution

Materials required:

Three beakers each containing saturated, unsaturated and supersaturated
solution of sugar, glass rods, some crystals of sugar, sugar tongs, etc.

AB C

Procedure:
1. Place the beakers with solution on the table and name them as A, B andC.
2. Drop little amount of sugar in each beaker and stir it.
Observation:
Suppose you will observe that:
1. Beaker ’A’ easily dissolves the added sugar. Since this solution can

dissolve more solute, it is unsaturated solution.
2. Beaker ’B’ does not dissolve the added sugar even while stirring. So

it is saturaled solution.

181 Times' Crucial Science Book - 9

3. Beaker ’C’ also does not dissolve the sugar. Instead of dissolving, the
size of crystal increases. So, it is supersaturated solution.

Conclusion:
8QVDWXUDWHG VDWXUDWHG DQG VXSHU VDWXUDWHG VROXWLRQ FDQ EH LGHQWLÀHG ZLWK
the help of some crystals of the solute.

Importance of solution
1. The plants obtain minerals from soil in the form of solution
2. Digested foods circulate in our body in the form of solution.
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2[\JHQ LV SUHVHQW DV VROXWH LQ ZDWHU $TXDWLF DQLPDOV EUHDWKH LQ
dissolved oxygen.
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Solubility
Solubility of a solute at a given temperature is the amount of solute
required to make a saturated solution in 100gm of solvent at that
temperature.
Solubility of some substances are as follows:

SN Solute Solubility at 20°C

1. Sodium chloride 35

2. Copper sulphate 21

3. Sodium nitrate 88

4. Potassium nitrate 21

5. Calcium sulphate 0.20

Solved Numerical Problem 10.1

Calculate the solubility of sodium nitrate if 85g of sodium nitrate can be dissolved
in 100g of water at 20°C.

Given,

Weight of solute (W1) = 85g
Weight of solvent (W2) = 100g
Solubility (S) = ?

Now,

S= W1 ð or, = 85
W2 100 ð = 85

Therefore, the solubility of sodium nitrate at 20°C is 85.

Times' Crucial Science Book - 9 182

Solved Numerical Problem 10.2

The solubility of sodium chloride at 20°C is 35. What amount of salt is required to
make a saturated solution in 2 kg water at the same temperature?

Given,

Solubility (S) = 35

Weight of water (W2) = 2 kg = 2000g
Weight of sodium chloride (W1) = ?

Now,

S= W1 ð or, 35 = W1 ð
or, W2 2000

W1 = ð or, W1 700g
100

Therefore, the required amount of sodium chloride is 700g.

Factors affecting solubility
The solubility of a substance is affected by the following factors:
1. Temperature
The solubility of a substance increases with the increase in
temperature. The increase in temperature of a solution increases
the kinetic energy of solute and solvent molecules. The increase in
kinetic energy makes the solvent molecules move apart creating
a large intermolecular space as well as it breaks up the solute
particles. Hence, the increased intermolecular space of the solvent
can hold more solute particles and the solubility of solute increases.
When the temperature is decreased, the kinetic energy of the solute
and solvent molecules decreases. As a result, the solvent molecules
come closer decreasing the intermolecular space. Also, the solute
particles do not break down as rapidly as in higher temperature.
Hence, less amount of solute is dissolved.

Fact Reason

Saturated solution can dissolve more solute on heating, why?
When the saturated solution is heated, the solvent molecules
expand and create more intermolecular space due to which
more solute dissolves in it. Hence, saturated solution can
dissolve more solute on heating.

183 Times' Crucial Science Book - 9

2. Size of solute particles

7KH ÀQHO\ SRZGHUHG VROXWH GLVVROYHV IDVWHU WKDQ LWV ELJ OXPS ,W
is because small size of solute particles can be held easily by the
intermolecular space of the solvent.

3. Shaking or stirring

A solute dissolves faster if the mixture is shaken or stirred. Stirring
or shaking increases the kinetic energy of the molecules. It also
helps in the uniform distribution of solute particles in the solvent
molecules. Thus, the solute dissolves faster.

Solubility curve 130
The solubility of a substance increases with the 120
rise in temperature and decreases with the fall 110Solubility (100 gm of water)
of temperature. This variation of solubility can 100
be plotted on a graph. The curved line obtained 90 Potassium nitrate
on a graph paper by plotting the solubility of 80
a substance at different temperatures is called 70
solubility curve. In the solubility curve, the 60 MagSnoedsiiuumm chloride
temperature is plotted on X-axis and solubility 50
is plotted on Y-axis. 40 chloride

30
20
10
0
10 20 30 40 50 60 70 80 90 100
Temperature°C

Solved Numerical Problem 10.3

If the solubility of copper sulphate at 70°C and 20°C is 85 and 25 respectively,
what amount of copper sulphate will be crystallized out when its 50g saturated
solution is cooled from 70°C to 20°C?

Given,

At 70°C,

Solubility = 85

Amount of solution = 50g

Let the weight of solute = x g

Weight of solvent = (50 —x)g

Now, we have

Solubility weight of solute
ð
weight of solevnt

or, 100x ï [ RU [ ï

or, 185x 4250 ?x = 22.97

Times' Crucial Science Book - 9 184

+HQFH DPRXQW RI VROYHQW ï [ ï J

At 20°C,

Solubility = 25

Amount of solvent = 27.03g

Let, the weight of solute = y

Again,

solubility weight of solute ð y
or, 25 = 27.03 ð
weight of solevnt

or, 100y 27.03 x 25 ?y = 6.75g
Now, the amount of solute is 6.75g.

Therefore, the amount of copper sulphate which gets crystallized out
ï J

g

Information from a solubility curve
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2. Solubility of different substances at a particular temperature can be
compared.
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4. Solubility curve helps to indicate the temperature of formation of
hydrate.
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are present in a solution.
6. It also helps to calculate the amount of solute precipitated during the
cooling of saturated solution.

Fact Reason

Temperature is always mentioned with a solubility of a
substance, why?
The solubility of a substance is different in different temperature
as solubility is directly proportional to the temperature. Hence,
temperature is always mentioned with a solubility of a substance.

Crystallization
The process of formation of crystals from a hot saturated solution of
a solute while cooling is called crystallization.

185 Times' Crucial Science Book - 9

$ FU\VWDO LV D VROLG VXEVWDQFH ZKLFK KDV D GHÀQLWH JHRPHWULFDO VKDSH
with smooth surface and sharp edges. Crystals are pure substances.
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VSHFLÀF WKUHH GLPHQVLRQDO SDWWHUQ

Activity 10.2 To obtain the crystals of copper sulphate

Materials required:
Solution of copper sulphate, burner, copper sulphate crystals, tripod
stand, porcelain basin, glass rod etc.
Procedure:
1. Take a solution of copper

sulphate in a porcelain basin.
2. Heat the solution and dissolve

more crystals of copper sulphate.
3. Keep on heating to evaporate the

excess of solvent.
4. After heating for some minutes, insert a glass rod into the boiling

solution and draw it back. If tiny rough crystals are seen on the
glass rod, the solution is ready for crystallization. Now, remove
the solution from the burner and place it in a cool and dry place.
/HDYH WKH VROXWLRQ XQGLVWXUEHG IRU ï KRXUV ,I \RX QHHG FU\VWDOV
fast, place the hot solution over a trough containing cold water.
But this process does not produce big crystals.
Observation:
Attractive blue crystals of copper sulphate are seen in the procelain
basin after cooling the solution.
Conclusion:
When a hot saturated solution of a solute is cooled down slowly,
crystals are formed. This process is called crystallization.

Types of solids
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categories. They are: amorphous solid and crystalline solid.
1. Amorphous solids
7KH VROLG VXEVWDQFHV ZKLFK GR QRW KDYH À[HG JHRPHWULFDO VKDSH DUH
called amorphous solids. Rubber, plastics, soil, glass etc are some
amorphous solids.

Times' Crucial Science Book - 9 186

2. Crystalline solids
7KH VROLG VXEVWDQFHV ZKLFK KDYH À[HG JHRPHWULFDO VKDSH ZLWK
smooth surface and sharp edges are called crystalline solids. They
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SDUWLFOHV 7KH\ KDYH GHÀQLWH PHOWLQJ SRLQWV 6RGLXP FKORULGH
(NaCl), alum, copper sulphate (CuSO4), etc

Learn and Write
1. Temperature is also mentioned while explaining solubility. Why?

Solubility of a solute changes with the change in temperature.
It increases with the rise in temperature and decreases with
the fall in temperature. Therefore, temperature should be
mentioned to specify the solubility.
2. Saturated solution becomes unsaturated on heating. Why?
When saturated solution is heated, there will be increase in
the intermolecular space of the solvent molecules. Due to it,
more solute particles can be accommodated. Thus, saturated
solution becomes unsaturated on heating.
3. Muddy water is a suspension. Why?
In muddy water, the size of particles is greater than 10ï cm.
The particles can be seen with naked eyes. Therefore, muddy
water is a suspension.
4. Salt solution is a homogeneous mixture. Why?
In salt solution, the components are mixed uniformly and
the particles cannot be seen with naked eyes. Therefore, salt
solution is a homogeneous mixture.
5. What happens when a supersaturated solution is cooled?
When a supersaturated solution is cooled, the solubility of the
solute decreases. Hence, some amount of solute gets separated
from the solution. The separated solute settles at the bottom of
the containing vessel.
6. Is it possible to prepare a saturated solution from an
unsaturated solution without adding solute?
It is possible to prepare a saturated solution from an unsaturated
solution without adding solute in the following ways:
a. An unsaturated solution at higher temperature becomes saturated

if it is cooled.
b. If the solution in unsaturated at low temperature, it can be made

saturated by evaporating excess solvent from it.

187 Times' Crucial Science Book - 9

7. What is a crystal? Write any two characteristics of a crystal.
$ FU\VWDO LV D VROLG VXEVWDQFH ZKLFK KDV D GHÀQLWH JHRPHWULFDO

shape with smooth surface and sharp edges. A crystal has the
following characteristics:

a. A crystal has a sharp melting and boiling point.
b. 7KH FU\VWDOV DUH DUUDQJHG LQ D VSHFL¿F WKUHH GLPHQVLRQDO SDWWHUQ

Glossary : ratio
: eyes without spectacles or contact lens
Proportion : filtration using powerful filter
Naked eyes : change, difference
Ultrafiltration : a solid particle that appears from solution
Variation
Precipitate

Main points to remember

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SURSRUWLRQ E\ ZHLJKW ZLWKRXW DQ\ FKDQJH LQ WKH LQGLYLGXDO SURSHUW\ RI
the components.

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WKH\ FDQ EH LGHQWLÀHG E\ QDNHG H\HV LV FDOOHG KHWHURJHQHRXV PL[WXUH

$ PL[WXUH LQ ZKLFK WKH FRPSRQHQWV DUH XQLIRUPO\ GLVWULEXWHG DQG
FDQQRW EH LGHQWLÀHG E\ QDNHG H\HV LV FDOOHG KRPRJHQHRXV PL[WXUH

6XVSHQVLRQ LV D KHWHURJHQHRXV PL[WXUH LQ ZKLFK WKH VL]H RI SDUWLFOHV LV
10ï cm or larger.

&ROORLG LV D KRPRJHQHRXV PL[WXUH LQ ZKLFK WKH VL]H RI XQLW SDUWLFOHV
ranges from 10ï cm to 10ï cm.

6ROXWLRQ LV D KRPRJHQHRXV PL[WXUH LQ ZKLFK VL]H RI XQLW SDUWLFOHV LV
10ï cm or smaller.

$ VROXWLRQ ZKLFK KDV UHODWLYHO\ OHVV DPRXQW RI VROXWH LQ WKH JLYHQ
amount of solvent is called dilute solution.

$ VROXWLRQ ZKLFK FRQWDLQV UHODWLYHO\ PRUH DPRXQW RI VROXWH LQ WKH JLYHQ
amount of solvent is called concentrated solution.

7KH VROXWLRQ ZKLFK FDQ GLVVROYH RWKHU PRUH DPRXQW RI VROXWH DW D JLYHQ
temperature is called an unsaturated solution.

7KH VROXWLRQ ZKLFK FDQQRW GLVVROYH DQ\ PRUH DPRXQW RI VROXWH DW D
given temperature is called a saturated solution.

7KH VROXWLRQ ZKLFK KROGV PRUH VROXWH WKDQ UHTXLUHG WR PDNH VDWXUDWHG
solution at that temperature is called supersaturated solution.

7KH DPRXQW RI VROXWH UHTXLUHG WR PDNH D VDWXUDWHG VROXWLRQ LQ J RI D
solvent at a particular temperature is called solubility.

Times' Crucial Science Book - 9 188

13. The curved line obtained on a graph paper by plotting the solubility of
a substance at different temperature is called solubility curve.

14. The process of formation of crystal from a hot saturated solution of a
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ZLWK VPRRWK VXUIDFH DQG VKDUS HGJHV

Exercise

1. Choose the best alternative in each case.

a. Which of the following is a pure substance?

i. Air ii. Tea iii. Syrup iv. Oxygen

b. The size of particles in a solution is

i. Greater than 10-7cm in diameter

ii. Equal to 10-7cm in diameter or less

iii. Equal to 10-7 cm in diameter

iv. All of these

c. Which of the following is an amorphous substance?

i. Sodium Chloride ii. Plastics

iii. Copper sulphate iv. Alum

d. Which of the following is a homogeneous mixture?

i. Muddy water ii. Oily water

ii. Sea water iv. Sandy water

e. Which of the following is a colloid?

i. Blood ii. Milk iii. Urine iv. All of these

2. Answer these questions in very short:
a. What is a mixture?
b. What is the size of the particles in a colloid?
c. Give any two examples of a colloid.
d. What do you understand by saturated and unsaturated
solution?
e. What is a supersaturated solution?
f. What do you mean by solute?
g. What is solubility?
h. What is the size of particles in a solution?
i. Define (with examples)
(i) Amorphous solids (ii) Crystalline solids
(iii) Crystals (iv) Solvent
(v) Colloid (vi) Suspension

3. Differentiate between:
a. Heterogeneous and homogeneous mixture.

189 Times' Crucial Science Book - 9

b. Dilute and concentrated solution.
c. Unsaturated and saturated solution.
d. Solute and solvent
e. Crystalline and amorphous solids.

4. Give reasons:
a. Solubility of a substance increases with rise in temperature.
b. When a saturated solution is heated, it becomes unsaturated.
c. A solute dissolves faster if the mixture is stirred.

5. Answer these questions:
a. Explain homogeneous and heterogeneous mixtures with
examples.
b. Define dilute and concentrated solution with examples.
c. What is solubility curve? Write down the information that
can be obtained from a solubility curve.
d. What is a crystal? Write down its properties.
e. How do you obtain crystals from a solution of copper
sulphate? Explain the process with diagram.

6. Solve the following numerical problems.
a. What is the solubility of sugar if 60 gram of sugar can be
dissolved in 200 gram of water at 50°C?
b. Calculate the solubility of common salt at 30°C if 80 gram
of salt dissolves in 250 gram of water to make saturated
solution.
c. If the solubility of copper sulphate at 50°C is 34, what
amount of copper sulphate will be required to make its
saturated solution in 80 gram water?
d. The solubility of sodium nitrate at 20°C and 30°C is 88
and 95 respectively. What amount of sodium nitrate will
crystallize out if 130 gram saturated solution is cooled from
30°C to 20°C?

Answers 6. a. 30 b. 32 c. 27.2g d. 4.67g

Project Work

Take suspension, colloid and solution in three different transparent
glasses. Find out differences among these three mixtures and write them
down.

Times' Crucial Science Book - 9 190

Chapter

11 6RPH *DVHV

Henry Cavendish

He is known for the discovery of hydrogen and
measuring the earth's density, etc.

ƐƟŵĂƚĞĚ WĞƌŝŽĚƐ ͗ ϭϮ

Objectives

ƚ ƚŚĞ ĞŶĚ ŽĨ ƚŚĞ ůĞƐƐŽŶ͕ ƐƚƵĚĞŶƚƐ ǁŝůů ďĞ ĂďůĞ ƚŽ͗
• H[SODLQ HOHFWURQLF FRQÀJXUDWLRQ ODERUDWRU\ SUHSDUDWLRQ DQG SURSHUWLHV RI VRPH JDVHV

K\GURJHQ R[\JHQ DQG QLWURJHQ
• explain the uses of different gases.

Mind Openers

• Can you say how hydrogen gas is prepared?
• How are hydrogen, oxygen and nitrogen gases useful?
• :KLFK JDV LV XVHG DV ÀUH H[WLQJXLVKHU" :K\" 'LVFXVV

Introduction
The earth is surrounded by atmosphere. The atmosphere consists
of a mixture of different gases such as nitrogen, oxygen, carbon
dioxide, etc. The gases can be elements or compounds. There
are 11 elements in the periodic table which are gases at normal
temperature and pressure. For example, hydrogen, helium, nitrogen,
R[\JHQ ÁXRULQH QHRQ FKORULQH DUJRQ NU\SWRQ [HQRQ DQG UDGRQ
are gaseous elements. Some compounds that are found in gas state
are carbon dioxide, methane, oxides of nitrogen, oxides of sulphur,
ammonia, etc. Among all gaseous state of matter, hydrogen is the
lightest and simplest gas.
Composition of air
Nitrogen and oxygen are the major components of air. They
constitute 99.03% of the total volume of air. Nitrogen alone occupies
78.08% of total volume of air and oxygen occupies 20.95% of total
volume of air. water vapour is pressent in variable amount in air.

SN Gases in air Percentage by volume
1. 78.08%
2. Nitrogen (N2) 20.95%
3. Oxygen (O2) 0.03%
Carbon dioxide (CO2)

191 Times' Crucial Science Book - 9

4. Argon (Ar) 0.93%
0.00005%
5. Hydrogen (H2) 0.000004%
6. Ozone (O3) 0.003945%
7. Other gases

Carbon dioxide is the fourth most abundant gas in the atmosphere.
It occupies about 0.03% to 0.04% of the total volume of air. The
amount of carbon dioxide in the atmosphere is roughly balanced by
the processes of respiration and photosynthesis by plants. But the
amount aocftiCvOiti2eiss. increasing day by day in the atmosphere due to
human The amount of other gases is somehow constant
in the atmosphere.

Hydrogen

Symbol: H Atomic number : 1

Molecular formula : H2 Atomic weight : 1.008 amu
Molecular weight : 2.016 amu Valency : 1

Position in the periodic table: Period - 1, Group - IA

p=1 p=1 p=1
n=0 n=0 n=0

Hydrogen atom Hydrogen molecule

Hydrogen was discovered by an English scientist Henry Cavendish
LQ $' +H QDPHG LW DV LQÁDPPDEOH JDV EHFDXVH RI LWV SURSHUW\
to burn in air. The name hydrogen was given by Lavoisier in 1783
AD after studying its properties.
Hydrogen is the simplest and lightest gas. It is very reactive gas.
So, it is mostly found in the form of compound. It is abundant in
the form of water in rivers, lakes, ponds, oceans, seas, etc. Free
hydrogen gas is very rare. It forms less than 0.01% of total volume
of air in free state and is mostly present in the uppermost layer of
atmosphere.

General methods of preparation of hydrogen

1. By the action of acids on metals

The metals such as zinc, magnesium, etc react with dilute acids to
produce hydrogen gas and metal salts.

Zn + H2SO4 ń =Q624 + H2Ń

Times' Crucial Science Book - 9 192

=Q +&O ń =Q&O2 + H2Ń
2. By the reaction of metals with alkalis

Zinc and aluminium react with alkalis such as sodium hydroxide to
produce hydrogen gas.

=Q 1D2+ ń 1D2ZnO2 + H2Ń
Sodium zincate

2Al + 2NaOH + 2H22 ń 1D$O22 + 3H2Ń
Sodium meta aluminate

3. From water

Hydrogen gas can be produced by electrolysis of water in the presence
of sulphuric acid as electrolyte. Oxygen gas is also produced in this
reaction.

2H2O electrolysis 2H2Ń 22Ń
4. Hydrogen gas can also be prepared by the action of reactive
metals like sodium, calcium, potassium, etc on water.

2K + 2H22 ń .2+ +2Ń
2Na + 2H22 ń 1D2+ +2Ń
Laboratory preparation of hydrogen gas

Principle:

Hydrogen gas is prepared in the laboratory by the action of dilute
sulphuric acid on granulated zinc. Granulated zinc is the impure
zinc in the form of small pieces.

Zn + H2SO4 ń =Q624 + H2Ń

In this process, impure zinc should be used instead of pure zinc
because the action of pure zinc on the dil. H2SO4 is very slow. The
impurity in zinc makes the reaction faster.

Apparatus required

Woulfe’s bottle, thistle funnel, delivery tube, gas jar, water trough,
beehive self etc.

Chemicals required:
1. Granulated zinc (Zn)

193 Times' Crucial Science Book - 9

2. Dilute sulphuric acid (dil. H2SO4)
3. Water (H2O)

Dilute sulphuric acid

Thistle funnel Delivery tube Hydrogen gas

Woulfe's bottle Gas jar

Water

Granulated zinc Trough
Beehive shelf

Laboratory preparation of hydrogen gas

Procedure
1. Put some amount of granulated zinc in Woulfe’s bottle.
2. Connect a thistle funnel and delivery tube to two openings of
Woulfe’s bottle with the help of cork.
3. Make the open end of the delivery tube lie below the gas jar
through the beehive shelf.
4. Pour dilute sulphuric acid into the Woulfe’s bottle through
thistle funnel.

Now, dilute sulphuric acid reacts with granulated zinc and
hydrogen gas is produced. Hydrogen gas travels through delivery
tube and is collected in the gas jar by downward displacement
of water.

Hydrogen gas is collected in the gas jar by downward displacement
of water because it is lighter than water and it does not react
with water. Also, it is the least soluble in water.

Precautions
1. The apparatus should be airtight.
2. Impure granulated form of zinc should be used.
3. The end of the thistle funnel should be dipped in the solution.
4. The end of the delivery tube should not touch the solution
within Woulfe’s bottle.
'LOXWH VXOSKXULF DFLG VKRXOG EH XVHG LQVWHDG RI FRQFHQWUDWHG
sulphuric acid because concentrated sulphuric acid produces
sulphur dioxide instead of hydrogen gas when it reacts with
zinc.

Times' Crucial Science Book - 9 194

Fact Reason

1. Impure granulated zinc is used during the laboratory
preparation of hydrogen gas, why?
Impure granulated zinc is used during the laboratory
preparation of hydrogen gas to increase the rate of chemical
reaction as pure zinc reacts slowly with dilute acids.

2. Concentrated sulphuric acid is not used during the
laboratory preparation of hydrogen gas, why?
Concentrated sulphuric acid is not used during the
laboratory preparation of hydrogen gas because sulphur
dioxide gas would be formed instead of hydrogen gas.

Test of hydrogen
+\GURJHQ EXUQV ZLWK D SDOH EOXH ÁDPH DW WKH RSHQLQJ RI WKH
gas jar.
2. When a lighted candle is taken to the mouth of the gas jar,
containing hydrogen gas, the candle gets extinguished
immediately producing a pop sound. This proves that the
gas in the jar is hydrogen.

Manufacture of hydrogen gas
Hydrogen gas is manufactured in industrial scale by using different
methods. Some of them are explained below:
1. By the reaction of methane gas with steam
Hydrogen gas is prepared in large scale by the reaction of methane
with steam at 1200°C and under 30 atmospheric pressure in the
presence of nickel as catalyst.
0HWKDQH :DWHU YDSRXU ń&DUERQ PRQR[LGH +\GURJHQ
The miCxtHu4re(go)f+cHar2b2o n Jm ńon &ox2id eJ a n d +hy2d(gro) gen is called water gas.
It can be used as fuel.
A catalyst is a chemical compound which increases or decreases
the rate of chemical reaction without being consumed itself. The
catalyst which increases the rate of chemical reaction is called
positive catalyst whereas the catalyst which decreases the rate of
chemical reaction is called negative catalyst. In this case, nickel is
a positive catalyst.

195 Times' Crucial Science Book - 9

2. By the electrolysis of water
The hydrogen gas can be produced in large scale by the electrolysis
RI ZDWHU :KHQ HOHFWULFLW\ LV SDVVHG WKURXJK DFLGLÀHG ZDWHU LW
dissociates into hydrogen and oxygen. Both gases can be collected
in separate vessels.

2H2O electrolysis 2H2Ń 22Ń
Mechanism of electrolysis

When dilute H2SO4 is added in water, it ionizes as follows:
At cathode; H2O electrolysis H+ + OHï

H+ +e H (Nascent hydrogen)

+ + ń +2Ń
Nascent hydrogen is the newly formed hydrogen. In other words, it is
the hydrogen at the moment of its origin. It is in atomic form and is
highly reactive. The nascent hydrogen immediately reacts with another
hydrogen atom to form molecular hydrogen which is less reactive.

Properties of hydrogen

Physical properties
1. It is colourless, odourless and tasteless gas.
2. It is slightly soluble in water.
,W LV KLJKO\ FRPEXVWLEOH JDV ,W EXUQV ZLWK SDOH EOXH ÁDPH
4. It is neutral to litmus paper.
,W LV WKH OLJKWHVW DQG VLPSOHVW JDV
,WV ERLOLQJ SRLQW LV ï ƒ& DQG PHOWLQJ SRLQW LV ï ƒ&

Chemical properties

1. Hydrogen burns with oxygen to produce water.

2H2 + O2 ¨ 2H2O
water

2. Active metals like sodium, potassium, calcium, barium, etc
react with hydrogen to form hydride.

2Na + H2 300°C 2NaH
Sodium hydride

Ca + H2 300°C CaH2
Calcium hydride (Hydrolith)

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