NCERT Textbook - Some Basic Concepts of Chemistry Class 11 Notes | EduRev

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Class 11 : NCERT Textbook - Some Basic Concepts of Chemistry Class 11 Notes | EduRev

 Page 1


1 SOME BASIC CONCEPTS OF CHEMISTRY
UNIT 1
After studying this unit, you will be
able to
• • • • • understand and appreciate the
role of chemistry in different
spheres of life;
• • • • • explain the characteristics of
three states of matter;
• • • • • classify different substances into
elements, compounds and
mixtures;
••••• define SI base units and list some
commonly used prefixes;
• • • • • use scientific notations and
perform simple mathematical
operations on numbers;
• • • • • differentiate between precision and
accuracy;
• • • • • determine significant figures;
• • • • • convert physical quantities from
one system of units to another;
• • • • • explain various laws of chemical
combination;
• • • • • appreciate significance of atomic
mass, average atomic mass,
molecular mass and formula
mass;
• • • • • describe the terms – mole and
molar mass;
• • • • • calculate the mass per cent of
different elements constituting a
compound;
• • • • • determine empirical formula and
molecular formula for a compound
from the given experimental data;
• • • • • perform the stoichiometric
calculations.
SOME BASIC CONCEPTS OF CHEMISTRY
Chemistry is the science of molecules and their
transformations. It is the science not so much of the one
hundred elements but of the infinite variety of molecules that
may be built from them ...
Roald Hoffmann
Chemistry deals with the composition, structure and
properties of matter. These aspects can be best described
and understood in terms of basic constituents of matter:
atoms and molecules.  That is why chemistry is called
the science of atoms and molecules.  Can we see, weigh
and perceive these entities? Is it possible to count the
number of atoms and molecules in a given mass of matter
and have a quantitative relationship between the mass and
number of these particles (atoms and molecules)? We will
like to answer some of these questions in this Unit. We
would further describe how physical properties of matter
can be quantitatively described using numerical values
with suitable units.
1.1 IMPORTANCE OF CHEMISTRY
Science can be viewed as a continuing human effort to
systematize knowledge for describing and understanding
nature.  For the sake of convenience science is sub-divided
into various disciplines: chemistry, physics, biology,
geology etc. Chemistry is the branch of science that studies
the composition, properties and interaction of matter.
Chemists are interested in knowing how chemical
transformations occur.  Chemistry plays a central role in
science and is often intertwined with other branches of
science like physics, biology, geology etc.  Chemistry also
plays an important role in daily life.
Chemical principles are important in diverse areas, such
as: weather patterns, functioning of brain and operation
© NCERT
not to be republished
Page 2


1 SOME BASIC CONCEPTS OF CHEMISTRY
UNIT 1
After studying this unit, you will be
able to
• • • • • understand and appreciate the
role of chemistry in different
spheres of life;
• • • • • explain the characteristics of
three states of matter;
• • • • • classify different substances into
elements, compounds and
mixtures;
••••• define SI base units and list some
commonly used prefixes;
• • • • • use scientific notations and
perform simple mathematical
operations on numbers;
• • • • • differentiate between precision and
accuracy;
• • • • • determine significant figures;
• • • • • convert physical quantities from
one system of units to another;
• • • • • explain various laws of chemical
combination;
• • • • • appreciate significance of atomic
mass, average atomic mass,
molecular mass and formula
mass;
• • • • • describe the terms – mole and
molar mass;
• • • • • calculate the mass per cent of
different elements constituting a
compound;
• • • • • determine empirical formula and
molecular formula for a compound
from the given experimental data;
• • • • • perform the stoichiometric
calculations.
SOME BASIC CONCEPTS OF CHEMISTRY
Chemistry is the science of molecules and their
transformations. It is the science not so much of the one
hundred elements but of the infinite variety of molecules that
may be built from them ...
Roald Hoffmann
Chemistry deals with the composition, structure and
properties of matter. These aspects can be best described
and understood in terms of basic constituents of matter:
atoms and molecules.  That is why chemistry is called
the science of atoms and molecules.  Can we see, weigh
and perceive these entities? Is it possible to count the
number of atoms and molecules in a given mass of matter
and have a quantitative relationship between the mass and
number of these particles (atoms and molecules)? We will
like to answer some of these questions in this Unit. We
would further describe how physical properties of matter
can be quantitatively described using numerical values
with suitable units.
1.1 IMPORTANCE OF CHEMISTRY
Science can be viewed as a continuing human effort to
systematize knowledge for describing and understanding
nature.  For the sake of convenience science is sub-divided
into various disciplines: chemistry, physics, biology,
geology etc. Chemistry is the branch of science that studies
the composition, properties and interaction of matter.
Chemists are interested in knowing how chemical
transformations occur.  Chemistry plays a central role in
science and is often intertwined with other branches of
science like physics, biology, geology etc.  Chemistry also
plays an important role in daily life.
Chemical principles are important in diverse areas, such
as: weather patterns, functioning of brain and operation
© NCERT
not to be republished
2 CHEMISTRY
of a computer. Chemical industries
manufacturing fertilizers, alkalis, acids, salts,
dyes, polymers, drugs, soaps, detergents,
metals, alloys and other inorganic and organic
chemicals, including new materials, contribute
in a big way to the national economy.
Chemistry plays an important role in meeting
human needs for food, health care products
and other materials aimed at improving the
quality of life.  This is exemplified by the large
scale production of a variety of fertilizers,
improved varieties of pesticides and
insecticides.  Similarly many life saving drugs
such as cisplatin and taxol, are effective in
cancer therapy and AZT (Azidothymidine)
used for helping AIDS victims, have been
isolated from plant and animal sources or
prepared by synthetic methods.
With a better understanding of chemical
principles it has now become possible to
design and synthesize new materials having
specific magnetic, electric and optical
properties.  This has lead to the production of
superconducting ceramics, conducting
polymers, optical fibres and large scale
miniaturization of solid state devices. In recent
years chemistry has tackled with a fair degree
of success some of the pressing aspects of
environmental degradation. Safer alternatives
to environmentally hazardous refrigerants like
CFCs (chlorofluorocarbons), responsible for
ozone depletion in the stratosphere, have been
successfully synthesised.  However, many big
environmental problems continue to be
matters of grave concern to the chemists. One
such problem is the management of the Green
House gases like methane, carbon dioxide etc.
Understanding of bio-chemical processes, use
of enzymes for large-scale production of
chemicals and synthesis of new exotic
materials are some of the intellectual challenges
for the future generation of chemists. A
developing country like India needs talented
and creative chemists for accepting such
challenges.
1.2 NATURE OF MATTER
You are already familiar with the term matter
from your earlier classes. Anything which has
mass and occupies space is called matter.
Fig. 1.1 Arrangement of particles in solid, liquid
and gaseous state
Everything around us, for example, book, pen,
pencil, water, air, all living beings etc. are
composed of matter. You know that they have
mass and they occupy space.
You are also aware that matter can exist in
three physical states viz. solid, liquid and gas.
The constituent particles of matter in these
three states can be represented as shown in
Fig. 1.1. In solids, these particles are held very
close to each other in an orderly fashion and
there is not much freedom of movement. In
liquids, the particles are close to each other
but they can move around. However, in gases,
the particles are far apart as compared to those
present in solid or liquid states and their
movement is easy and fast. Because of such
arrangement of particles, different states of
matter exhibit the following characteristics:
(i) Solids have definite volume and definite
shape.
(ii) Liquids have definite volume but not the
definite shape. They take the shape of the
container in which they are placed.
(iii) Gases have neither definite volume nor
definite shape. They completely occupy the
container in which they are placed.
These three states of matter are
interconvertible by changing the conditions of
temperature and pressure.
Solid 
heat
cool
? 
?  liquid 
heat
cool
? 
?  Gas
On heating a solid usually changes to a
liquid and the liquid on further heating
© NCERT
not to be republished
Page 3


1 SOME BASIC CONCEPTS OF CHEMISTRY
UNIT 1
After studying this unit, you will be
able to
• • • • • understand and appreciate the
role of chemistry in different
spheres of life;
• • • • • explain the characteristics of
three states of matter;
• • • • • classify different substances into
elements, compounds and
mixtures;
••••• define SI base units and list some
commonly used prefixes;
• • • • • use scientific notations and
perform simple mathematical
operations on numbers;
• • • • • differentiate between precision and
accuracy;
• • • • • determine significant figures;
• • • • • convert physical quantities from
one system of units to another;
• • • • • explain various laws of chemical
combination;
• • • • • appreciate significance of atomic
mass, average atomic mass,
molecular mass and formula
mass;
• • • • • describe the terms – mole and
molar mass;
• • • • • calculate the mass per cent of
different elements constituting a
compound;
• • • • • determine empirical formula and
molecular formula for a compound
from the given experimental data;
• • • • • perform the stoichiometric
calculations.
SOME BASIC CONCEPTS OF CHEMISTRY
Chemistry is the science of molecules and their
transformations. It is the science not so much of the one
hundred elements but of the infinite variety of molecules that
may be built from them ...
Roald Hoffmann
Chemistry deals with the composition, structure and
properties of matter. These aspects can be best described
and understood in terms of basic constituents of matter:
atoms and molecules.  That is why chemistry is called
the science of atoms and molecules.  Can we see, weigh
and perceive these entities? Is it possible to count the
number of atoms and molecules in a given mass of matter
and have a quantitative relationship between the mass and
number of these particles (atoms and molecules)? We will
like to answer some of these questions in this Unit. We
would further describe how physical properties of matter
can be quantitatively described using numerical values
with suitable units.
1.1 IMPORTANCE OF CHEMISTRY
Science can be viewed as a continuing human effort to
systematize knowledge for describing and understanding
nature.  For the sake of convenience science is sub-divided
into various disciplines: chemistry, physics, biology,
geology etc. Chemistry is the branch of science that studies
the composition, properties and interaction of matter.
Chemists are interested in knowing how chemical
transformations occur.  Chemistry plays a central role in
science and is often intertwined with other branches of
science like physics, biology, geology etc.  Chemistry also
plays an important role in daily life.
Chemical principles are important in diverse areas, such
as: weather patterns, functioning of brain and operation
© NCERT
not to be republished
2 CHEMISTRY
of a computer. Chemical industries
manufacturing fertilizers, alkalis, acids, salts,
dyes, polymers, drugs, soaps, detergents,
metals, alloys and other inorganic and organic
chemicals, including new materials, contribute
in a big way to the national economy.
Chemistry plays an important role in meeting
human needs for food, health care products
and other materials aimed at improving the
quality of life.  This is exemplified by the large
scale production of a variety of fertilizers,
improved varieties of pesticides and
insecticides.  Similarly many life saving drugs
such as cisplatin and taxol, are effective in
cancer therapy and AZT (Azidothymidine)
used for helping AIDS victims, have been
isolated from plant and animal sources or
prepared by synthetic methods.
With a better understanding of chemical
principles it has now become possible to
design and synthesize new materials having
specific magnetic, electric and optical
properties.  This has lead to the production of
superconducting ceramics, conducting
polymers, optical fibres and large scale
miniaturization of solid state devices. In recent
years chemistry has tackled with a fair degree
of success some of the pressing aspects of
environmental degradation. Safer alternatives
to environmentally hazardous refrigerants like
CFCs (chlorofluorocarbons), responsible for
ozone depletion in the stratosphere, have been
successfully synthesised.  However, many big
environmental problems continue to be
matters of grave concern to the chemists. One
such problem is the management of the Green
House gases like methane, carbon dioxide etc.
Understanding of bio-chemical processes, use
of enzymes for large-scale production of
chemicals and synthesis of new exotic
materials are some of the intellectual challenges
for the future generation of chemists. A
developing country like India needs talented
and creative chemists for accepting such
challenges.
1.2 NATURE OF MATTER
You are already familiar with the term matter
from your earlier classes. Anything which has
mass and occupies space is called matter.
Fig. 1.1 Arrangement of particles in solid, liquid
and gaseous state
Everything around us, for example, book, pen,
pencil, water, air, all living beings etc. are
composed of matter. You know that they have
mass and they occupy space.
You are also aware that matter can exist in
three physical states viz. solid, liquid and gas.
The constituent particles of matter in these
three states can be represented as shown in
Fig. 1.1. In solids, these particles are held very
close to each other in an orderly fashion and
there is not much freedom of movement. In
liquids, the particles are close to each other
but they can move around. However, in gases,
the particles are far apart as compared to those
present in solid or liquid states and their
movement is easy and fast. Because of such
arrangement of particles, different states of
matter exhibit the following characteristics:
(i) Solids have definite volume and definite
shape.
(ii) Liquids have definite volume but not the
definite shape. They take the shape of the
container in which they are placed.
(iii) Gases have neither definite volume nor
definite shape. They completely occupy the
container in which they are placed.
These three states of matter are
interconvertible by changing the conditions of
temperature and pressure.
Solid 
heat
cool
? 
?  liquid 
heat
cool
? 
?  Gas
On heating a solid usually changes to a
liquid and the liquid on further heating
© NCERT
not to be republished
3 SOME BASIC CONCEPTS OF CHEMISTRY
Many of the substances present around
you are mixtures. For example, sugar solution
in water, air, tea  etc., are all mixtures. A mixture
contains two or more substances present in it
(in any ratio) which are called its components.
A mixture may be homogeneous or
heterogeneous. In a homogeneous mixture,
the components completely mix with each other
and its composition is uniform throughout.
Sugar solution, and air are thus, the examples
of homogeneous mixtures. In contrast to this,
in heterogeneous mixtures, the composition
is not uniform throughout and sometimes the
different components can be observed. For
example, the mixtures of salt and sugar, grains
and pulses along with some dirt (often stone)
pieces, are heterogeneous mixtures. You can
think of many more examples of mixtures
which you come across in the daily life. It is
worthwhile to mention here that the
components of a mixture can be separated by
using physical methods such as simple hand
picking, filtration, crystallisation, distillation
etc.
Pure substances have characteristics
different from the mixtures. They have fixed
composition, whereas mixtures may contain
the components in any ratio and their
composition is variable. Copper, silver, gold,
water, glucose are some examples of pure
substances. Glucose contains carbon,
hydrogen and oxygen in a fixed ratio and thus,
like all other pure substances has a fixed
composition. Also, the constituents of pure
substances cannot be separated by simple
physical methods.
Pure substances can be further classified
into elements and compounds. An element
consists of only one type of particles. These
particles may be atoms or molecules. You may
be familiar with atoms and molecules from the
previous classes; however, you will be studying
about them in detail in Unit 2. Sodium, copper,
silver, hydrogen, oxygen etc. are some
examples of elements. They all contain atoms
of one type. However, the atoms of different
elements are different in nature. Some elements
such as sodium or copper, contain single
atoms held together as their constituent
particles whereas in some others, two or more
atoms combine to give molecules of the
element. Thus, hydrogen, nitrogen and oxygen
gases consist of molecules in which two atoms
combine to give their respective molecules. This
is illustrated in Fig. 1.3.
When two or more atoms of  different
elements combine, the molecule of a
compound is obtained. The examples of some
compounds are water, ammonia, carbon
Fig. 1.3  A representation of atoms and molecules
Fig. 1.2  Classification of matter
changes to the gaseous ( or vapour) state. In
the reverse process, a gas on cooling liquifies
to the liquid and the liquid on further cooling
freezes to the solid.
At the macroscopic or bulk level, matter
can be classified as mixtures or pure
substances. These can be further sub-divided
as shown in Fig. 1.2.
© NCERT
not to be republished
Page 4


1 SOME BASIC CONCEPTS OF CHEMISTRY
UNIT 1
After studying this unit, you will be
able to
• • • • • understand and appreciate the
role of chemistry in different
spheres of life;
• • • • • explain the characteristics of
three states of matter;
• • • • • classify different substances into
elements, compounds and
mixtures;
••••• define SI base units and list some
commonly used prefixes;
• • • • • use scientific notations and
perform simple mathematical
operations on numbers;
• • • • • differentiate between precision and
accuracy;
• • • • • determine significant figures;
• • • • • convert physical quantities from
one system of units to another;
• • • • • explain various laws of chemical
combination;
• • • • • appreciate significance of atomic
mass, average atomic mass,
molecular mass and formula
mass;
• • • • • describe the terms – mole and
molar mass;
• • • • • calculate the mass per cent of
different elements constituting a
compound;
• • • • • determine empirical formula and
molecular formula for a compound
from the given experimental data;
• • • • • perform the stoichiometric
calculations.
SOME BASIC CONCEPTS OF CHEMISTRY
Chemistry is the science of molecules and their
transformations. It is the science not so much of the one
hundred elements but of the infinite variety of molecules that
may be built from them ...
Roald Hoffmann
Chemistry deals with the composition, structure and
properties of matter. These aspects can be best described
and understood in terms of basic constituents of matter:
atoms and molecules.  That is why chemistry is called
the science of atoms and molecules.  Can we see, weigh
and perceive these entities? Is it possible to count the
number of atoms and molecules in a given mass of matter
and have a quantitative relationship between the mass and
number of these particles (atoms and molecules)? We will
like to answer some of these questions in this Unit. We
would further describe how physical properties of matter
can be quantitatively described using numerical values
with suitable units.
1.1 IMPORTANCE OF CHEMISTRY
Science can be viewed as a continuing human effort to
systematize knowledge for describing and understanding
nature.  For the sake of convenience science is sub-divided
into various disciplines: chemistry, physics, biology,
geology etc. Chemistry is the branch of science that studies
the composition, properties and interaction of matter.
Chemists are interested in knowing how chemical
transformations occur.  Chemistry plays a central role in
science and is often intertwined with other branches of
science like physics, biology, geology etc.  Chemistry also
plays an important role in daily life.
Chemical principles are important in diverse areas, such
as: weather patterns, functioning of brain and operation
© NCERT
not to be republished
2 CHEMISTRY
of a computer. Chemical industries
manufacturing fertilizers, alkalis, acids, salts,
dyes, polymers, drugs, soaps, detergents,
metals, alloys and other inorganic and organic
chemicals, including new materials, contribute
in a big way to the national economy.
Chemistry plays an important role in meeting
human needs for food, health care products
and other materials aimed at improving the
quality of life.  This is exemplified by the large
scale production of a variety of fertilizers,
improved varieties of pesticides and
insecticides.  Similarly many life saving drugs
such as cisplatin and taxol, are effective in
cancer therapy and AZT (Azidothymidine)
used for helping AIDS victims, have been
isolated from plant and animal sources or
prepared by synthetic methods.
With a better understanding of chemical
principles it has now become possible to
design and synthesize new materials having
specific magnetic, electric and optical
properties.  This has lead to the production of
superconducting ceramics, conducting
polymers, optical fibres and large scale
miniaturization of solid state devices. In recent
years chemistry has tackled with a fair degree
of success some of the pressing aspects of
environmental degradation. Safer alternatives
to environmentally hazardous refrigerants like
CFCs (chlorofluorocarbons), responsible for
ozone depletion in the stratosphere, have been
successfully synthesised.  However, many big
environmental problems continue to be
matters of grave concern to the chemists. One
such problem is the management of the Green
House gases like methane, carbon dioxide etc.
Understanding of bio-chemical processes, use
of enzymes for large-scale production of
chemicals and synthesis of new exotic
materials are some of the intellectual challenges
for the future generation of chemists. A
developing country like India needs talented
and creative chemists for accepting such
challenges.
1.2 NATURE OF MATTER
You are already familiar with the term matter
from your earlier classes. Anything which has
mass and occupies space is called matter.
Fig. 1.1 Arrangement of particles in solid, liquid
and gaseous state
Everything around us, for example, book, pen,
pencil, water, air, all living beings etc. are
composed of matter. You know that they have
mass and they occupy space.
You are also aware that matter can exist in
three physical states viz. solid, liquid and gas.
The constituent particles of matter in these
three states can be represented as shown in
Fig. 1.1. In solids, these particles are held very
close to each other in an orderly fashion and
there is not much freedom of movement. In
liquids, the particles are close to each other
but they can move around. However, in gases,
the particles are far apart as compared to those
present in solid or liquid states and their
movement is easy and fast. Because of such
arrangement of particles, different states of
matter exhibit the following characteristics:
(i) Solids have definite volume and definite
shape.
(ii) Liquids have definite volume but not the
definite shape. They take the shape of the
container in which they are placed.
(iii) Gases have neither definite volume nor
definite shape. They completely occupy the
container in which they are placed.
These three states of matter are
interconvertible by changing the conditions of
temperature and pressure.
Solid 
heat
cool
? 
?  liquid 
heat
cool
? 
?  Gas
On heating a solid usually changes to a
liquid and the liquid on further heating
© NCERT
not to be republished
3 SOME BASIC CONCEPTS OF CHEMISTRY
Many of the substances present around
you are mixtures. For example, sugar solution
in water, air, tea  etc., are all mixtures. A mixture
contains two or more substances present in it
(in any ratio) which are called its components.
A mixture may be homogeneous or
heterogeneous. In a homogeneous mixture,
the components completely mix with each other
and its composition is uniform throughout.
Sugar solution, and air are thus, the examples
of homogeneous mixtures. In contrast to this,
in heterogeneous mixtures, the composition
is not uniform throughout and sometimes the
different components can be observed. For
example, the mixtures of salt and sugar, grains
and pulses along with some dirt (often stone)
pieces, are heterogeneous mixtures. You can
think of many more examples of mixtures
which you come across in the daily life. It is
worthwhile to mention here that the
components of a mixture can be separated by
using physical methods such as simple hand
picking, filtration, crystallisation, distillation
etc.
Pure substances have characteristics
different from the mixtures. They have fixed
composition, whereas mixtures may contain
the components in any ratio and their
composition is variable. Copper, silver, gold,
water, glucose are some examples of pure
substances. Glucose contains carbon,
hydrogen and oxygen in a fixed ratio and thus,
like all other pure substances has a fixed
composition. Also, the constituents of pure
substances cannot be separated by simple
physical methods.
Pure substances can be further classified
into elements and compounds. An element
consists of only one type of particles. These
particles may be atoms or molecules. You may
be familiar with atoms and molecules from the
previous classes; however, you will be studying
about them in detail in Unit 2. Sodium, copper,
silver, hydrogen, oxygen etc. are some
examples of elements. They all contain atoms
of one type. However, the atoms of different
elements are different in nature. Some elements
such as sodium or copper, contain single
atoms held together as their constituent
particles whereas in some others, two or more
atoms combine to give molecules of the
element. Thus, hydrogen, nitrogen and oxygen
gases consist of molecules in which two atoms
combine to give their respective molecules. This
is illustrated in Fig. 1.3.
When two or more atoms of  different
elements combine, the molecule of a
compound is obtained. The examples of some
compounds are water, ammonia, carbon
Fig. 1.3  A representation of atoms and molecules
Fig. 1.2  Classification of matter
changes to the gaseous ( or vapour) state. In
the reverse process, a gas on cooling liquifies
to the liquid and the liquid on further cooling
freezes to the solid.
At the macroscopic or bulk level, matter
can be classified as mixtures or pure
substances. These can be further sub-divided
as shown in Fig. 1.2.
© NCERT
not to be republished
4 CHEMISTRY
dioxide, sugar etc. The molecules of water and
carbon dioxide are represented in Fig 1.4.
You have seen above that a water molecule
comprises two hydrogen atoms and one
oxygen atom. Similarly, a molecule of carbon
dioxide contains two oxygen atoms combined
with one carbon atom. Thus,  the atoms  of
different elements are present in a compound
in a fixed and definite ratio and this ratio is
characteristic of a particular compound. Also,
the properties of a compound are different
from those of its constituent elements. For
example, hydrogen and oxygen are gases
whereas the compound formed by their
combination i.e., water is a liquid. It is
interesting to note that hydrogen burns with
a pop sound and oxygen is a supporter of
combustion, but water is used as a fire
extinguisher.
Moreover, the constituents of a compound
cannot be separated into simpler substances
by physical methods. They can be separated
by chemical methods.
1.3 PROPERTIES OF MATTER AND
THEIR MEASUREMENT
Every substance has unique or characteristic
properties. These properties can be classified
into two categories – physical properties and
chemical properties.
Physical properties are those properties
which can be measured or observed without
changing the identity or the composition of the
substance. Some examples of physical
properties are colour, odour, melting point,
boiling point, density etc. The measurement
or observation of chemical properties require
a chemical change to occur. The examples of
Water molecule
(H
2
O)
 Carbon dioxide
molecule (CO
2
)
Fig. 1.4 A depiction of molecules of water and
carbon dioxide
chemical properties are characteristic
reactions of different substances; these include
acidity or basicity, combustibility etc.
Many properties of matter such as length,
area, volume, etc., are quantitative in nature.
Any quantitative observation or measurement
is represented by a number followed by units
in which it is measured. For example length of
a room can be represented as  6 m; here 6 is
the number and m denotes metre – the unit in
which the length is measured.
Two different systems of measurement, i.e.
the English System and the Metric System
were being used in different parts of the world.
The metric system which originated in France
in late eighteenth century, was more
convenient as it was based on the decimal
system. The need of a common standard
system was being felt by the scientific
community. Such a system was established
in 1960 and is discussed below in detail.
1.3.1 The International System of Units
(SI)
The International System of Units (in French
Le Systeme International d’Unités –
abbreviated as SI) was established by the 11th
General Conference on Weights and Measures
(CGPM from Conference Generale des Poids
et Measures). The CGPM is an inter
governmental treaty organization created by
a diplomatic treaty known as Metre Convention
which was signed in Paris in 1875.
The SI system has seven base units and
they are listed in Table 1.1. These units pertain
to the seven fundamental scientific quantities.
The other physical quantities such as speed,
volume, density etc. can be derived from these
quantities.
The definitions of the SI base units are given
in Table 1.2.
The SI system allows the use of prefixes to
indicate the multiples or submultiples of a unit.
These prefixes are listed in Table 1. 3.
Let us now quickly go through some of the
quantities which you will be often using in this
book.
© NCERT
not to be republished
Page 5


1 SOME BASIC CONCEPTS OF CHEMISTRY
UNIT 1
After studying this unit, you will be
able to
• • • • • understand and appreciate the
role of chemistry in different
spheres of life;
• • • • • explain the characteristics of
three states of matter;
• • • • • classify different substances into
elements, compounds and
mixtures;
••••• define SI base units and list some
commonly used prefixes;
• • • • • use scientific notations and
perform simple mathematical
operations on numbers;
• • • • • differentiate between precision and
accuracy;
• • • • • determine significant figures;
• • • • • convert physical quantities from
one system of units to another;
• • • • • explain various laws of chemical
combination;
• • • • • appreciate significance of atomic
mass, average atomic mass,
molecular mass and formula
mass;
• • • • • describe the terms – mole and
molar mass;
• • • • • calculate the mass per cent of
different elements constituting a
compound;
• • • • • determine empirical formula and
molecular formula for a compound
from the given experimental data;
• • • • • perform the stoichiometric
calculations.
SOME BASIC CONCEPTS OF CHEMISTRY
Chemistry is the science of molecules and their
transformations. It is the science not so much of the one
hundred elements but of the infinite variety of molecules that
may be built from them ...
Roald Hoffmann
Chemistry deals with the composition, structure and
properties of matter. These aspects can be best described
and understood in terms of basic constituents of matter:
atoms and molecules.  That is why chemistry is called
the science of atoms and molecules.  Can we see, weigh
and perceive these entities? Is it possible to count the
number of atoms and molecules in a given mass of matter
and have a quantitative relationship between the mass and
number of these particles (atoms and molecules)? We will
like to answer some of these questions in this Unit. We
would further describe how physical properties of matter
can be quantitatively described using numerical values
with suitable units.
1.1 IMPORTANCE OF CHEMISTRY
Science can be viewed as a continuing human effort to
systematize knowledge for describing and understanding
nature.  For the sake of convenience science is sub-divided
into various disciplines: chemistry, physics, biology,
geology etc. Chemistry is the branch of science that studies
the composition, properties and interaction of matter.
Chemists are interested in knowing how chemical
transformations occur.  Chemistry plays a central role in
science and is often intertwined with other branches of
science like physics, biology, geology etc.  Chemistry also
plays an important role in daily life.
Chemical principles are important in diverse areas, such
as: weather patterns, functioning of brain and operation
© NCERT
not to be republished
2 CHEMISTRY
of a computer. Chemical industries
manufacturing fertilizers, alkalis, acids, salts,
dyes, polymers, drugs, soaps, detergents,
metals, alloys and other inorganic and organic
chemicals, including new materials, contribute
in a big way to the national economy.
Chemistry plays an important role in meeting
human needs for food, health care products
and other materials aimed at improving the
quality of life.  This is exemplified by the large
scale production of a variety of fertilizers,
improved varieties of pesticides and
insecticides.  Similarly many life saving drugs
such as cisplatin and taxol, are effective in
cancer therapy and AZT (Azidothymidine)
used for helping AIDS victims, have been
isolated from plant and animal sources or
prepared by synthetic methods.
With a better understanding of chemical
principles it has now become possible to
design and synthesize new materials having
specific magnetic, electric and optical
properties.  This has lead to the production of
superconducting ceramics, conducting
polymers, optical fibres and large scale
miniaturization of solid state devices. In recent
years chemistry has tackled with a fair degree
of success some of the pressing aspects of
environmental degradation. Safer alternatives
to environmentally hazardous refrigerants like
CFCs (chlorofluorocarbons), responsible for
ozone depletion in the stratosphere, have been
successfully synthesised.  However, many big
environmental problems continue to be
matters of grave concern to the chemists. One
such problem is the management of the Green
House gases like methane, carbon dioxide etc.
Understanding of bio-chemical processes, use
of enzymes for large-scale production of
chemicals and synthesis of new exotic
materials are some of the intellectual challenges
for the future generation of chemists. A
developing country like India needs talented
and creative chemists for accepting such
challenges.
1.2 NATURE OF MATTER
You are already familiar with the term matter
from your earlier classes. Anything which has
mass and occupies space is called matter.
Fig. 1.1 Arrangement of particles in solid, liquid
and gaseous state
Everything around us, for example, book, pen,
pencil, water, air, all living beings etc. are
composed of matter. You know that they have
mass and they occupy space.
You are also aware that matter can exist in
three physical states viz. solid, liquid and gas.
The constituent particles of matter in these
three states can be represented as shown in
Fig. 1.1. In solids, these particles are held very
close to each other in an orderly fashion and
there is not much freedom of movement. In
liquids, the particles are close to each other
but they can move around. However, in gases,
the particles are far apart as compared to those
present in solid or liquid states and their
movement is easy and fast. Because of such
arrangement of particles, different states of
matter exhibit the following characteristics:
(i) Solids have definite volume and definite
shape.
(ii) Liquids have definite volume but not the
definite shape. They take the shape of the
container in which they are placed.
(iii) Gases have neither definite volume nor
definite shape. They completely occupy the
container in which they are placed.
These three states of matter are
interconvertible by changing the conditions of
temperature and pressure.
Solid 
heat
cool
? 
?  liquid 
heat
cool
? 
?  Gas
On heating a solid usually changes to a
liquid and the liquid on further heating
© NCERT
not to be republished
3 SOME BASIC CONCEPTS OF CHEMISTRY
Many of the substances present around
you are mixtures. For example, sugar solution
in water, air, tea  etc., are all mixtures. A mixture
contains two or more substances present in it
(in any ratio) which are called its components.
A mixture may be homogeneous or
heterogeneous. In a homogeneous mixture,
the components completely mix with each other
and its composition is uniform throughout.
Sugar solution, and air are thus, the examples
of homogeneous mixtures. In contrast to this,
in heterogeneous mixtures, the composition
is not uniform throughout and sometimes the
different components can be observed. For
example, the mixtures of salt and sugar, grains
and pulses along with some dirt (often stone)
pieces, are heterogeneous mixtures. You can
think of many more examples of mixtures
which you come across in the daily life. It is
worthwhile to mention here that the
components of a mixture can be separated by
using physical methods such as simple hand
picking, filtration, crystallisation, distillation
etc.
Pure substances have characteristics
different from the mixtures. They have fixed
composition, whereas mixtures may contain
the components in any ratio and their
composition is variable. Copper, silver, gold,
water, glucose are some examples of pure
substances. Glucose contains carbon,
hydrogen and oxygen in a fixed ratio and thus,
like all other pure substances has a fixed
composition. Also, the constituents of pure
substances cannot be separated by simple
physical methods.
Pure substances can be further classified
into elements and compounds. An element
consists of only one type of particles. These
particles may be atoms or molecules. You may
be familiar with atoms and molecules from the
previous classes; however, you will be studying
about them in detail in Unit 2. Sodium, copper,
silver, hydrogen, oxygen etc. are some
examples of elements. They all contain atoms
of one type. However, the atoms of different
elements are different in nature. Some elements
such as sodium or copper, contain single
atoms held together as their constituent
particles whereas in some others, two or more
atoms combine to give molecules of the
element. Thus, hydrogen, nitrogen and oxygen
gases consist of molecules in which two atoms
combine to give their respective molecules. This
is illustrated in Fig. 1.3.
When two or more atoms of  different
elements combine, the molecule of a
compound is obtained. The examples of some
compounds are water, ammonia, carbon
Fig. 1.3  A representation of atoms and molecules
Fig. 1.2  Classification of matter
changes to the gaseous ( or vapour) state. In
the reverse process, a gas on cooling liquifies
to the liquid and the liquid on further cooling
freezes to the solid.
At the macroscopic or bulk level, matter
can be classified as mixtures or pure
substances. These can be further sub-divided
as shown in Fig. 1.2.
© NCERT
not to be republished
4 CHEMISTRY
dioxide, sugar etc. The molecules of water and
carbon dioxide are represented in Fig 1.4.
You have seen above that a water molecule
comprises two hydrogen atoms and one
oxygen atom. Similarly, a molecule of carbon
dioxide contains two oxygen atoms combined
with one carbon atom. Thus,  the atoms  of
different elements are present in a compound
in a fixed and definite ratio and this ratio is
characteristic of a particular compound. Also,
the properties of a compound are different
from those of its constituent elements. For
example, hydrogen and oxygen are gases
whereas the compound formed by their
combination i.e., water is a liquid. It is
interesting to note that hydrogen burns with
a pop sound and oxygen is a supporter of
combustion, but water is used as a fire
extinguisher.
Moreover, the constituents of a compound
cannot be separated into simpler substances
by physical methods. They can be separated
by chemical methods.
1.3 PROPERTIES OF MATTER AND
THEIR MEASUREMENT
Every substance has unique or characteristic
properties. These properties can be classified
into two categories – physical properties and
chemical properties.
Physical properties are those properties
which can be measured or observed without
changing the identity or the composition of the
substance. Some examples of physical
properties are colour, odour, melting point,
boiling point, density etc. The measurement
or observation of chemical properties require
a chemical change to occur. The examples of
Water molecule
(H
2
O)
 Carbon dioxide
molecule (CO
2
)
Fig. 1.4 A depiction of molecules of water and
carbon dioxide
chemical properties are characteristic
reactions of different substances; these include
acidity or basicity, combustibility etc.
Many properties of matter such as length,
area, volume, etc., are quantitative in nature.
Any quantitative observation or measurement
is represented by a number followed by units
in which it is measured. For example length of
a room can be represented as  6 m; here 6 is
the number and m denotes metre – the unit in
which the length is measured.
Two different systems of measurement, i.e.
the English System and the Metric System
were being used in different parts of the world.
The metric system which originated in France
in late eighteenth century, was more
convenient as it was based on the decimal
system. The need of a common standard
system was being felt by the scientific
community. Such a system was established
in 1960 and is discussed below in detail.
1.3.1 The International System of Units
(SI)
The International System of Units (in French
Le Systeme International d’Unités –
abbreviated as SI) was established by the 11th
General Conference on Weights and Measures
(CGPM from Conference Generale des Poids
et Measures). The CGPM is an inter
governmental treaty organization created by
a diplomatic treaty known as Metre Convention
which was signed in Paris in 1875.
The SI system has seven base units and
they are listed in Table 1.1. These units pertain
to the seven fundamental scientific quantities.
The other physical quantities such as speed,
volume, density etc. can be derived from these
quantities.
The definitions of the SI base units are given
in Table 1.2.
The SI system allows the use of prefixes to
indicate the multiples or submultiples of a unit.
These prefixes are listed in Table 1. 3.
Let us now quickly go through some of the
quantities which you will be often using in this
book.
© NCERT
not to be republished
5 SOME BASIC CONCEPTS OF CHEMISTRY
Table 1.1  Base Physical Quantities and their Units
Base Physical Symbol Name of Symbol
Quantity for SI Unit for SI
Quantity Unit
Length l metre m
Mass m kilogram kg
Time t second s
Electric current I ampere A
Thermodynamic T kelvin K
temperature
Amount of substance n mole mol
Luminous intensity I
v
candela cd
Table 1.2  Definitions of SI Base Units
Unit of length metre The metre is the length of the path travelled
by light in vacuum during a time interval of
1/299 792 458 of a second.
Unit of mass kilogram The kilogram is the unit of mass; it is equal
to the mass of the international prototype
of the kilogram.
Unit of time second The second is the duration of 9 192 631 770
periods of the radiation corresponding to the
transition between the two hyperfine levels
of the ground state of the caesium-133 atom.
Unit of electric current ampere The ampere is that constant current which,
if maintained in two straight parallel
conductors of infinite length, of negligible
circular cross-section, and placed 1 metre
apart in vacuum, would produce between
these conductors a force equal to 2 × 10
–7
newton per metre of length.
Unit of thermodynamic kelvin The kelvin, unit of thermodynamic
temperature temperature, is the fraction 1/273.16 of the
thermodynamic temperature of the triple
point of water.
Unit of amount of substance mole 1. The mole is the amount of substance of a
system which contains as many elementary
entities as there are atoms in 0.012
kilogram of carbon-12; its symbol is “mol.”
2. When the mole is used, the elementary
entities must be specified and may be atoms,
molecules, ions, electrons, other particles,
or specified groups of such particles.
Unit of luminous intensity candela The candela is the luminous intensity, in a
given direction, of a source that emits
monochromatic radiation of frequency
540 × 10
12
 hertz and that has a radiant
intensity in that direction of 1/683 watt per
steradian.
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