CHEMISTRY FOR 9
TH
CLASS (UNIT # 1)
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SHORT QUESTIONS
QUESTIONS
ANSWERS
Define Science
It
is
knowledge
that
provides
understanding of this world and how it
works.
Define
Chemistry
The branch of science that deals with the
composition,
structure,
properties
and
reactions of matter, is called chemistry.
What was the concept of Greek
A Greek philosopher, Empedocles thought
philosophers?
that all the matter was
made up of only
four elements. Earth, Air, Water and Fire.
This concept exist for 2000 years.
What was Dolton’s Atomic Theory?
Matter is made up of small invisible
particles called atoms. The atoms of any
one element are identical in all
respects
but different from those of other
elements.
What is the modern concept about atom?
Atom is further composed of sub
-
atomic
particles such as electron, proton and
neutron.
Define Physical Chemistry
It is the study of
the laws and theories
that describe the structure and changes
of matter.
Define Organic Chemistry
It is the study of covalent compounds of
carbon and hydrogen (hydrocarbons) and
their derivatives.
Define Inorganic Chemistry
It is the
study of all elements &
compounds except organic compounds.
Define Biochemistry
It is the study of physical and chemical
changes that occur in living things.
Industrial Chemistry
It deals with preparation of chemical
substances on
commercial scale.
Define Nuclear Chemistry
It is the study of changes that occur in
atomic nuclei.
Define Environmental chemistry
It is the study of chemicals that pollute
the environment and their effects on
human beings.
Define
Analytical Chemistry
It deals with the methods and
instruments
that
determine
the
composition of matter.
Define Classification of Matter
•
Mixture.
Further
classified
into
Homogenous & Heterogeneous
•
Substance.
Further
classified
into
Elements & Compounds.
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CHEMISTRY FOR 9
TH
CLASS (UNIT # 1)
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Define substance
It is a matter that has a
particular set of
characteristics which differ from the
character of any other kind of matter. For
example oxygen, water, urea, glucose
etc.
Define Element
The substance that cannot be converted
into other simpler substance is
called an
element.
Modern Definition:
It is a substance made up of same type of
atoms, having same atomic number.
Define Compound
A compound is a pure substance that is
ade upof two or more elements held
together in fixed ratios
by natural forces
called chemical
bonds.
For
example
water, carbon dioxide etc.
Define mixture
An impure substance that contains two or
more pure substances which retain their
individual
chemical characteristics
is
called a mixture. For
example air, salt
dissolved in water etc.
What are the types of mixture?
Homogenous mixture:
A mixture that consists of only one phase
is called
homogenous
mixture.
For
example salt or sugar dissolved in water.
Heterogeneous mixture:
A mixture that consists of two or more
visibly different components is called
heterogeneous
mixture.
For
example
sand+salt, oil floating on water etc.
Define Atomic Number
The number of protons in the nucleus of
an atom is called atomic
number. It is
denoted by Z.
Define Mass Number
The total number of protons and neutrons
is called mass number.
What is relative atomic mass. Why it is
The mass of an atom of an element
needed?
relative to the mass of an atom of C
-
12 is
called relative atomic mass.
It is needed because atom is so small that
we cannot determine its mass. Therefore
we can determine its mass relative to
another experimentally.
Define atomic mass unit (amu)
One atomic mass unit is one
twelfth the
mass of one atom of carbon
-
12.
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CHEMISTRY FOR 9
TH
CLASS (UNIT # 1)
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Define Chemical Formula
A
chemical formula tell us:
i)
Which elements are present in
compound?
ii)
The whole number ratio of their
atoms.
Write types of chemical formula
Empirical Formula:
It gives the simplest whole number ratio
of atoms present in a
compound. For
example empirical formula of glucose is
CH
2
O.
Molecular Formula:
It gives the actual whole number ratio of
atoms of each element present in a
molecule of that compound. Molecular
Formula of Glucose is C
6
H
12
O
6
.
Define
Molecular Mass
It is the sum of atomic masses of all the
atoms present in the molecule.
Define Formula Unit
It indicates the simplest ratio between
cations (positive ions) and anions
(negative ions) in ionic compounds.
Define Formula Mass
The sum of the atomic masses of all the
atoms in the formula unit of a substance
is called Formula Mass.
Define Chemical species
Matter is composed of chemical species.
These species are atoms, ions, free
radicals, molecules and formula
units.
Define Atom
Atom is the smallest particle of an
element that cannot exist in free state. It
is electrically neutral.
Define Ion
Ion is an atom or group of atoms having
a charge on it.
Define Types of Ions
Cations:
An atom or
group of atoms having
positive charge on it is called cation.
Anions:
An atom or group of atoms having
negative charge on it is called anion.
Why an ion acquires a net positive or
Ion acquires a net positive or negative
negative charge?
charge due to difference in number of
protons and electrons in it.
Define Molecular ions
When a molecule loses or gains electrons,
the resulting species is called a molecular
ion. These ions are short lived and only
exists at high temperature.
They do not
form ionic compounds.
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CHEMISTRY FOR 9
TH
CLASS (UNIT # 1)
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Differentiate
between
molecule
&
Molecule:
molecular ion
It is smallest particle that can exist
independently.
It is always neutral
It is formed by combination of atoms
It is a stable unit
Molecular Ion:
It is formed by loss or gain of electrons
It can
have positive or negative charge.
It is formed by ionization of molecule.
It is reactive specie.
Define Free Radicals.
Free radicals are atoms or group of atoms
having odd number of electrons. They are
neutral specie.
Differentiate
between ion and free
Ion:
radicals
An atom or group of atoms having charge
They exist in solution or in crystal lattice
Ion cannot be produced with the help of
light
Free Radical:
An atom or group of atoms having
unpaired electrons.
They exist in solution as well in air.
Light may produce free radicals.
Define representative particles of an
The representatives particles of a
element or compound
substance are atoms, molecules or
formula unit.
Define molecule and its types
Smallest particle of a covalent compound
or an element which can exist
independently is called molecule. Its
types are as under:
Monoatomic:
A molecule that contains only
one atom is
called monoatomic molecule. i.e. He, Ne,
Ar, Kr etc.
Polyatomic:
A molecule that contains two or more
similar or different atoms called
polyatomic is molecule. i.e. H
2
, O
2
, N
2
, Cl
2
etc.
Define
Avogadro’s Number. How it is
The number of atoms, ions or molecules
related to mole of a substance.
present in one mole of a substance is
called Avogadro’s number. It is denoted
by N
A
and is equal to 6.02x10
23
.
Relation to a mole:
1 mole of substance = Avogadro’s No.
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CHEMISTRY FOR 9
TH
CLASS (UNIT # 1)
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Define molar mass
The mass of one mole of
a substance is
called its molar mass.
Define mole
The atomic mass, molecular mass or
formula mass expressed in grams is
called mole.
Define Gram Atomic Mass
Atomic mass of an element expressed in
grams is called gram atomic mass.
Define Gram Molecular Mass
Molecular mass of an element or a
compound expressed in grams is called
gram molecular mass.
Define Gram Formula Mass
Formula mass of an ionic compound
expressed in grams is called gram
molecular mass or
gram molecule.
Write empirical and molecular formula of
Molecular = C
9
H
8
O
4
aspirin.
Empirical = C
9
H
8
O
4
Write empirical and molecular formula of
Molecular = C
2
H
4
O
2
vinegar.
Empirical = CH
2
O
Write Atomic Masses of
Potassium = K = 39
Carbon = C = 12
Chlorine = Cl = 35.5
Oxygen = O = 16
Hydrogen = H = 1
Sodium = Na = 23
Nitrogen = N = 14
What do you know about corpuscular
As per this concept, matter is supposed to
nature of matter?
be composed of minute particles. An
ancient Greek philosopher,
Empedocles
thought that all materials were made up
four things called elements. Earth, Air,
Water and Fire
Classify into elements, mixture and
Element = Iron
compounds
Mixture = Milk, Smoke, Tea
Compound = Water,
Sugar
LONG QUESTIONS
Question: What is the difference between Compound & Mixture?
Compound:
-
•
It is formed by a chemical combination of atoms of the elements.
•
The constituents lose their identity and form a new substance having entirely
different properties from them.
•
Compounds always have fixed composition by mass.
•
The components cannot be separated by physical means.
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CHEMISTRY FOR 9
TH
CLASS (UNIT # 1)
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•
Every compound is represented by a chemical formula.
•
Compounds have homogeneous composition.
•
Compounds have sharp and fixed melting points.
Mixture:
-
•
Mixture is formed by the simple mixing up of the substances.
•
Mixture shows the properties of the constituents.
•
Mixtures do not have fixed composition.
•
The components can be separated by simple physical methods.
•
It consists of two or more components and does not have any chemical formula.
•
They may be homogeneous of heterogeneous in composition.
•
Mixture does not have sharp fixed melting points.
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