Chapter 1 : Matter in Our Surroundings : Class - 9 : Question and Answer (Part 1)

 

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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