NCERT BOOK QUESTIONS
AND EXERCISES
Class - 9
Chapter 1 : Matter in
Our Surroundings
Question 1. Which of the
following are matter ?
Chair, air, love, smell, hate, almonds, thought, cold, cold-drink, smell of perfume
Answer - Chair, air,
smell, almonds, cold-drink, and smell of perfume are matter (because they
occupy space and have mass). It should be noted that ‘smell’ is a matter
because it is due to the presence of some volatile substances in air which
occupy space and have mass.
Question 2. Give reason for
the following observations :
The smell of hot sizzling food reaches you several metres
away but to get the smell from cold food, you have to go close.
Answer – The smell of food
reaches us by the process of diffusion of gases (Released by the food) in to
the air. The rate of diffusion of hot gases in to air is faster than that of
cold gases. So, the smell of hot sizzling food reaches us quickly even hen we
are several metres away because the rate of diffusion of hot gases (released by
the hot sizzling food) is much faster than the rate of diffusion of cold gases
released by the cold food.
Question 3. A diver is able to
cut through water in a swimming pool. Which property of matter does this
observation show?
Answer – Water is a liquid
form of matter. The observation that a diver is able to cut through water in a
swimming pool shows that through there is a quite strong force of attraction
between the particles of a liquid (like water) which holds them together but
the force is not strong enough to hold the particles of the liquid in fixed
position. So, by applying somewhat greater force, a diver is able to overcome
the forces of attraction present among the particles of water and hence cut
through water in the swimming pool.
Question 4. What are the
characteristics of the particle of matter?
Answer – The important
characteristics of particles of matter (like atoms or molecules) are the
following :
(i) The particles of
matter are very very small.
(ii) The particles of
matter have spaces between them.
(iii) The particles of
matter are constantly moving.
(iv) The particles of
attract one another.
Question
5. The mass per unit volume of a substance is called density (density=
mass/volume). Arrange the following in order of increasing density :
Air, Exhaust from chimneys,
Honey,Water, Chalk, Cotton and Iron.
Answer
. The order of increasing density of the given substance is :
Air<Exhaust
from chimneys<Cotton<Water<Honey<Chalk<Iron
Question
6. (a) Tabulate the difference in the characteristics of the three states of
matter.
(b)
Comment upon the following :
Rigidity,
Compressibility, Fluidity, Filling a gas container, Shape, Kinetic energy and
Density
Answer
. (a)
Solids |
Liquids |
Gases |
(i) Solids have a fixed shape
and a fixed volume |
(i) Liquids have a fixed volume
but they have no fixed shape. Liquids take the shape of the container in
which they are placed. |
(i) Gases have neither a fixed
shape nor a fixed volume. Gases acquire the shape and volume of the container
in which they are kept. |
(ii) Solids cannot be
compressed much. |
(ii) Like solids, liquids
cannot be compressed much. |
Gases can be compressed easily
(in to a small volume). |
(iii) Solids have high
densities. They are heavy. |
(iii) Liquids have moderate to
high densities. They are usually less dense than solids. |
(iii) Gases have very low
densities. They are very, very, light. A gas is much lighter than the same
volume of a solid or a liquid. |
(iv) Solids do not fill their
container completely. |
(iv) Liquids do not fill their container
completely. |
(iv) Gases fill their container
completely. |
(v) Solids do not flow. |
(v) Liquids generally flow
easily. |
(v) Gases flow easily. |
(b) (i) Rigidity – Rigidity refers to the property
of a solid to resist change in its shape ehwn an outside force is applied. In
most simple terms, rigidity means ‘stiffness’. The particle in a solid are very
closely packed and there are very strong forces of attraction between them, so
solids possess high rigidity. Liquids and gases are not rigid because the
positions of their particles are not fixed.
(ii) Compressibility – Compressibility is the
property of fluid due to which its volume decreases when pressure is applied.
The particles in a gases have large spaces between them due to which their volume
decrease too much when pressure is applied on them. So gases have high
compressibility. On the other hand, the particles in solids and liquids are
closely packed, so solids and liquids do not have much compressibility.
(iii) Fluidity – The property of flowing easily is
called fluidity. Gases and Liquids exhibit the property of fluidity, so they
are called fluids. Due to large interparticle distances and very weak forces of
attraction, gases can flow extremely easily. So the gases have very high fluidity.
And because of comparatively smaller interparticle distances and stronger
forces of attraction between their particles, the fluidity of liquids is less
than that of gases. Solids are not fluids, they have no fluidity.
(iv) Filling of gas container – A gas fills its
container completely because due to high kinetic energy and negligible
interparticle forces of attraction, the particles in a gas move with high
speeds in all directions and occupy all the space in the container.
(v) Shape – The external form or appearance of a
substance is called its shape. A solid has a fixed shape because the particles
in a solid are closely packed and their positions are fixed due to strong
forces of attraction between them. The liquids and gases do not have fixed shapes
because the positions of particles in them are not fixed due to comparatively
weaker forces of attraction between them.
(vi) Kinetic energy – The energy possessed by a
material due to the motion of its particles is called kinetic energy. At a
given temperature, the particles in a gas have the maximum kinetic energy
because they move with high speeds due to weakest forces of attraction among
them. Liquids have lesser kinetic eergy hereas solids have the least kinetic
energy at a given temperature.
(vii) Density – The mass per unit
volume of a material is called its density. Solids have high densities because
their particles are very close together. Liquids have usually lower densities
than solids because their particles are somewhat more loosely packed than that
in solids. Gases have the lowest densities because their particles are very far
apart from one another.
Question 7 . Give Reasons :
(a)
A gas fills
completely the vessel in which it is kept.
(b)
A gas exerts
pressure on the walls of the container.
(c)
A wooden table
should be called a solid.
(d)
We can easily
move our hand in air but to do the same through a solid block of wood, we need
a karate expert.
Answer . (a) The particles of a gas
have high kinetic energy and negligible forces of attraction amongst them. Due
to this the particles of a gas are constantly moving with high speeds in all
the directions and the gas completely fills the vessel in which it is kept.
(b) Because of high kinetic energy
and negligible forces of attraction, the particles of a gas move with high speeds
in all directions. When the fast moving gas particles hit the walls of its
container from inside, they exert a pressure. Thus the pressure exerted by a
gas is due to the constant collisions of the fast moving gas particles against
the inner wall of the container.
(c) A wooden table is a rigid
object having a definite shape and a definite volume. Since a wooden table has
these basic characteristics of a solid state, it should be called a solid.
(d) Air is a gas whose particles
are very far apart and there are very weak forces of attraction between them.
The extremely weak forces between particles of air can be overcome easily due
to which we can easily move our hand in air. On the other hand, the particles
of a solid block of wood are very closely packed and there are very strong
forces of attraction between the particles of wood. It needs a huge outside
force to overcome the strong interparticle attraction of a block of wood and
break it apart by moving hand which only a karate expert can apply.
Question 8.
Liquids generally have lower density as compared to solids. But you must
have observed that ice floats on water. Find out why ?
Answer . Ice is formed by the freezing of water.
When water freezes to form ice, then a number of empty spaces are created in
solid ice giving it a cage-like structure. Due to the presence of some empty
spaces, the volume of ice becomes more than equal mass of water. Because of its
greater volume, the density of ice decreases. And due to its lower density than
water, ice floats on water, even though it is a solid).
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