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#### Class Nine Science Chapter 1 NCERT Solutions

Chapter 1: Matter in Our Surroundings NCERT Solution

PAGE 3

Q.1. Which of the following is matter?

Chair, air, love, smell, hate, almonds, thought, cold, cold-drink, the smell of perfume.

Ans. Chair, air, almonds, cold-drink is matter.

Q.2. Give reasons for the following observation :

The smell of hot sizzling food reaches you several metres away, but to get the smell from cold food, you have to go close.

Ans. This happens because the rate of diffusion of a gas increases with increase in temperature. In the case of hot food, diffusion of smell is faster than cold food.

Q.3. A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?

Ans. A diver is able to cut through water in a swimming pool because the matter is not continuous as it made up of particles which have intermolecular space between them.

Q.4. What are the characteristics of particles of matter?

Ans. The main characteristics of particles of matter are:

(a) Particles of matter have vacant spaces between them.

(b) Particles of matter are continuous in motion. They possess kinetic energy.

(c) The kinetic energy of particles increases with the increase of temperature.

(d) Particles of matter have attractive forces acting between them which keep the particles together and vary from one kind of matter to others.

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Q.1. 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.

Ans. Increasing order of density : Air < exhaust from chimneys, < cotton, < water, < honey, < chalk < iron.

Q.2. Tabulate the differences in the characteristics of states of matter (b) Comment upon the following:

Rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density.

Ans. (a) Differences in the characteristics of states of matter are tabulated as follows.

(b) (i) Rigidity: The property due to which an object retains its shape and size when subjected to outside force is called rigidity solids are rigid while liquids and gases are not.

(ii) Compressibility: The property due to which a substance reduced to its lower volume when force is applied is called compressibility. Gases are more compressible than liquid while solids are not.

(iii) Fluidity: The tendency of particles to flow is called fluidity. Liquids and gases can flow, hence they are called fluids.

(iv) Filling a gas container: The gaseous particles move randomly in all directions.

Hence, it takes the shape of the container.

(v) Shape: The intermolecular force in solid is maximum and has a definite shape whereas liquids and gases take the shape of the container.

(vi) Kinetic energy: It is defined as the energy possessed by the particles due to their motion. Gases have the highest kinetic energy followed by the and liquid the solids have the least.

(vii) Density: It is defined as the ratio of the mass of the particles to the volume occupied by it. Solids have the highest density followed by the liquid and gases have the least.

3. Give reasons :

(a) A gas completely fills 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 the air, but to do the same through a solid block of wood we need a karate expert.

Ans. (a) The molecules of a gas have large intermolecular spaces and kinetic energy, but extremely small intermolecular forces. Thus, the molecules of the gas spread in the entire space of the containing vessel on account of high kinetic energy and practically no intermolecular forces and hence, fill the entire space of the vessel.

(b) The molecules of a gas have very large kinetic energy. When these molecules strike against the walls of the containing vessel, they exert certain average force per unit area. The force per unit area is known as pressure; therefore, gases exert pressure on the sides of the containing vessel.

(c) Solids are rigid, incompressible and have definite shape and volume. Since the table has all the above-mentioned properties, therefore, it is solid.

(d) The intermolecular forces between the molecules of a gas are almost negligible and intermolecular spaces are very large. Thus, we can easily move our hand in the air, without any appreciable force.

The intermolecular forces between the molecules of a solid are very large and intermolecular spaces are very small. Thus, a lot of force is required to separate the molecules of a solid. It is for the same reason that we need a karate expert to break a block of wood.

4. Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why?

Ans. When water freezes to form ice, its volume increases as the molecules of water are not closely packed in ice but have large spaces between them and hence, its density decreases. Now as ice has a lower density than water, therefore, it floats on water.