NCERT Solutions For Class 9 Science Chapter 1 Matter in Our Surroundings

Question 2. Give reasons for the following observation:

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

Answer:

The smell of hot sizzling food reaches us from several metres away, as the particles of hot food have more kinetic energy than the cold food.

So, the rate of diffusion is higher in hot food compared to 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:

A diver is able to cut through water in a swimming pool. It shows that the particles of matter (Water) have space between them and have less intermolecular forces of attraction.

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

Answer:

There are three characteristics of the particles of matter -

1. The particle of matter has space between them.
2. Particles of matter are continuously moving.
3. The particle of matter attracts each other.

Question 2.(b) Comment upon the following: rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density.

Answer:

Rigidity- The tendency of a substance to maintain its shape when subjected to an external force.

Compressibility - It means the contraction in the volume of a substance under the applied pressure. Liquid and gas are compressible because they have empty space, but solid does not.

Fluidity- The tendency of a particle to flow is known as fluidity. Liquids and gases can flow.

Filling of a gas container- The gases can fill the container with a large amount when we apply external pressure.

Shape- Solids have fixed shapes and boundaries. Liquid and gases have no fixed shape and definite boundaries.

Kinetic energy- The energy possessed by particles due to their motion is known as kinetic energy. Gas has maximum kinetic energy because they have more random motion.

Question 3.(a) Give reasons

• A gas fills the vessel completely in which it is kept.

Answer:

A gas fills the vessel completely in which it is kept.

The molecules of gas can have the tendency to move in any random direction due to their high kinetic energy.

Question 3. (b) Give reasons

• A gas exerts pressure on the walls of the container.

Answer:

A gas exerts pressure on the walls of the container because the molecules of the gas are continuously in random motion because of their high kinetic energy.

So, the molecules of gas are vibrating and hitting the walls of the container, and as a result exert pressure on the walls.

Question 3.(c) Give reasons

• A wooden table should be called a solid.

Answer:

A wooden table should be called a solid because it has a definite shape, fixed volume and definite boundaries. Also, it cannot flow and is incompressible.

Question 3.(d) Give reasons

• 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.

Answer:

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 because, in air, there is less force of attraction between the particles. So, a very small amount of external force can break it.

But in the case of solids, the force of attraction is very strong and the molecular space is so high. Henc,e a large amount of force is required to break it.

Topic 1.4 Can matter change its state? (Page-9)

Question 1. Convert the following temperature to Celsius scale:

a. 300 K

b. 573 K.

Answer:

It is known that,
T_k = 273 + T_c
T_k = Temperature in Kelvin and

T_c = temperature in Celsius.

Therefore,

(i) Temperature(C) = 300 - 273 = 27 ºC

(ii) Temperature (C) = 573 - 273 = 300 ºC

Question 2. What is the physical state of water at:

a. 250 ºC
b. 100 ºC

Answer:

The primary state of water is liquid at room temperature. It changes to the gaseous state above 100 ºC. At 100 ºC, water (liquid) can be in equilibrium with the gaseous state.

Hence, (i) At 250 ºCphysical state of water is gas. and,

(ii) At 100 ºC physical state of water can be gas as well as liquid.

Question 3 For any substance, why does the temperature remain constant during the change of state?

Answer:

The temperature of the substance does not change because the heat is used to overcome the forces of attraction.

This heat energy is known as latent heat.

That is why for any substance, the temperature remains constant during the change of state.

Topic 1.5 - Evaporation (Page 10)

Question 1. Why does a desert cooler cool better on a hot dry day?

Answer:

A desert cooler cools better on a hot and dry day. It is because the inner walls of the cooler get sprinkled by the water continuously and due to warm, dry weather, this water gets evaporated.

Evaporation causes cooling of the present air inside of the cooler. This cold air is sent into the room by a fan.

Question 2. How does the water kept in an earthen pot (matka) become cool during summer?

Answer:

The water inside the earthen pot becomes cold during the summer because the earthen pot is porous with a lot of pores. So, water comes out on the surface of the earthen pot, and this water gets evaporated.

And thus the temperature of water present inside the pot is much lower than outside, and hence the water becomes cold.

Question 3. Why does our palm feel cold when we put some acetone or petrol or perfume on it?

Answer:

When we put some acetone, petrol or perfume evaporate in our palm, they come into contact with the air and hence evaporation causes a cooling effect on our palm.

Question 4. Why are we able to sip hot tea or milk faster from a saucer rather than a cup?

Answer:

As we know, the evaporation rate is faster with the increased surface area. Tea in a saucer has a larger surface area than in a cup.

Therefore, the cooling of tea is more rapid in the saucer, and thus, we can sip hot tea or milk faster from a saucer rather than a cup.

Question 5. What type of clothes should we wear in summer?

Answer:

We should wear light coloured cotton clothes because the light colours reflect the solar radiation and cotton clothes have more porosity so that they allow sweat through it to evaporate faster, thereby causing a cooling effect.

NCERT Solutions for Class 9 Science Chapter 1 - Matter in Our Surroundings (Exercise Questions)

The Matter in Our Surroundings Class 9 exercise questions have been covered in these solutions. You can also take help from class 9 science chapter 1 notes.

Question 1 Convert the following temperatures to the Celsius scale.

(a) 293 K
(b) 470 K.

Answer:

It is known that,
T_k = 273 + Tc
T_k = Temperature in Kelvin and

Tc = temperature in Celsius.

Therefore,

(i) Temperature (ºC) = (293 - 273) = 20 ºC

(ii) Temperature (ºC) = 470 -273 = 197 ºC

Question 2. Convert the following temperatures to the kelvin scale.

(a) 25 ºC
(b) 373 ºC

Answer:

It is known that,
TK=273+Tc
T K = Temperature in Kelvin and

T C = temperature in Celsius.

Therefore,

(i) Temperature (K) = 273 + 25 = 298 K

(ii) Temperature (K) = 273 + 373 = 646 K

Question 3.(a) Give reason for the following observations.

• Naphthalene balls disappear with time without leaving any solid.

Answer:

Naphthalene balls disappear with time without leaving any solid because they can sublimate and directly convert into the gaseous state without leaving any solid.

Question 3.(b) Give reason for the following observations.

• We can get the smell of perfume sitting several metres away

Answer:

Perfumes contain a volatile solvent which diffuses very quickly and due to this, we can get the smell of perfume sitting several meters away.

Question 4. Arrange the following substances in increasing order of forces of attraction between the particles- water, sugar, oxygen.

Answer:

The general increasing order of forces of attraction in three states of matter is gas < liquid < solid.

Therefore, increasing order for the above-given substance is-

Oxygen < water < sugar

Question 6. (a) Give two reasons to justify-

• water at room temperature is a liquid.

Answer:

Water at room temperature is a liquid because-

1. Below 0 ºC it converts to ice (solid)
2. Above 100 ºC it converts into the gaseous state.

Question 6.(b) Give two reasons to justify-

• An iron almirah is a solid at room temperature.

Answer:

An iron almirah is a solid at room temperature because-

1. At room temperature, it has a definite shape and boundaries. Also, it has a fixed volume.
2. The melting point of iron is much higher than room temperature.

Question 7. Why is ice at 273 K more effective in cooling than water at the same temperature?

Answer:

At 273K, ice can easily absorb more heat energy from the surroundings in the form of latent heat and overcome the fusion to become water.

Water does not absorb this extra heat from the medium.

Thus, the cooling effect of ice is more than water at the same temperature.

Question 8. What produces more severe burns, boiling water or steam?

Answer:

Steam at 100 ºC produces more severe burns.

It is because extra heat is hidden in it, called latent heat, whereas the boiling water doesn't have any hidden heat.

Question 9. Name A,B,C,D,E and F in the following diagram showing change in its state

Answer:

In the above flow chart-

A = fusion/ melting/ liquefication

B = Vaporisation/ evaporation

C = Condensation (transformation of water from a gaseous state to a liquid state)

D = Solidification (the conversion of liquid into solid is known as solidification)

E = F = Sublimation

Direct conversion of solid into gas or gas into solid without attaining the liquid phase is called sublimation.