NCERT Solutions for Class 9 Chemistry

In order to help you in your Class IX preparation, NTSE Guru has prepared the detailed solutions for all the questions of the important topic of Chemistry of your Class IX NCERT Science book. NTSEGuru experts provide you with descriptive solutions for the chapter ‘Matter in our Surrounding’ of Class IX Science NCERT book. Matter In Our Surroundings

Matter In Our Surroundings

Questions

1.  Which of the following are matter?

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

Ans: Chair, air, almonds, cold-drink, smell of perfume are matter.

 2.Give reasons for the following observations. The smell of hot sizzling food reaches you several meters away, but to get smell from the cold food you have to go close?

Ans: The particles of matter possess kinetic energy and thus are constantly moving. At low temperatures, the kinetic energy is low and hence the particles diffuse slowly. But as the temperature rises, the kinetic energy increases accordingly and hence the particles diffuse at a faster rate. Now since the particles of hot vapours coming out of hot sizzling food diffuse in air at a faster rate, therefore, they easily reach us even when we are several meters away.

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

Ans: This observation shows that particles of matter have spaces between them & there are less inter particle force of attraction between the particles of water.

 4.What are the characteristics of particles of matter?

Ans: Some important characteristics of the particles of matter are listed below:

  • Matter consists of tiny particles.
  • There exist spaces in between the particles of matter.
  • The particles of matter are not stationary but are continuously moving.
  • The particles of matter are held together by forces of attraction.

 Questions:

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: The order of increasing density is: air, exhaust from chimneys, cotton, water, honey, chalk and iron.

 2. (a) Tabulate the difference in characteristics of states of matter.
     (b) Comment upon the following: rigidity, compressibility, fluidity, kinetic energy and density.

Ans: (a)

 

Property

Solid

Liquid

Gas

(1)

Inter-particle spaces

Inter-particle space is minimum.

Inter-particle space is moderate.

Inter-particle space is maximum.

(2)

Inter-particle force of attraction

Inter-particle force of attraction are the strongest.

Inter-particle force of attraction are weaker than those in solids but stronger than those in gases.

Inter-particle forces of attraction are the weakest.

(3)

Particle motion

Particles do not move but only vibrate at their mean position.

Particles can move freely to a limited extent.

Particles move freely.

(4)

Shape and volume

They have a definite shape & volume.

They have a definite volume but no definite shape, they take the shape of the container.

They have neither a definite shape nor a definite volume.

(5)

Compressibility

Solids are completely incompressible.

Liquids are almost incompressible.

Gases are highly compressible.

(6)

Rigidity / Fluidity

Solids possess rigidity.

Liquids can flow; therefore, they possess fluidity which is lower than that of gases.

Gases flow more easily than liquids and thus have the highest fluidity.

(7)

Density

They have highest density.

Their density is lower than those of solids but much higher than those of gases.

They have lowest density.

(8)

Diffusion

Generally they do not show the property of diffusion.

They show the property of diffusion greater than solids.

They have the highest rate of diffusion.

(9)

No. of free surfaces

Can have infinite number of free surfaces

Only one free surface

No free surface

 

 (b) Rigidity : Rigidity is the tendency to maintain shape when some external force is applied. Due to minimum inter particle spaces and strong inter particle force of attraction, generally solids possess rigidity.

            Compressibility: Compressibility is the tendency to decrease in volume or change shape, when some pressure is applied. Due to large inter-particle spaces in gases, their volume decreases when some pressure is applied on them. Therefore, gases possess high compressibility.

            Fluidity: Fluidity is the tendency to flow. Due to weak inter-particle forces of attraction, liquids & gases have fluidity.

            Kinetic energy : Kinetic energy is the energy possessed by particles due to motion. Due to large inter-particle distances and weaker forces of attraction, the particles of a gas have maximum freedom of movement. As a result, they move in different directions with high velocities.  Therefore, particles of a gas have the highest kinetic energy.

            Density: Density is the mass per unit volume of a substance. Since the particles of solid are closely packed, those of liquids are loosely held while those of gases are free to move, therefore, solids have the highest density, liquids have lower density than solids while gases have the least density.

3. Give reasons:

  • A gas fills completely the vessel in which it is kept.
  • A gas exerts pressure on the walls of the container.
  • A wooden table should be called a solid.
  • We can easily move our hand in air but to do the same through a solid block of wood we need a karate expert.

Ans: (a) A gas fills its container completely because due to high kinetic energy and negligible forces of attraction, the molecules of a gas move freely with high speed in all directions.

(b) The particles of a gas have negligible force of attraction and high kinetic energy, due to which, particles in a gas move randomly in all directions. Due to random movement and high kinetic energy, the particles hit each other and also the walls of the container. The force exerted by gas per unit area on the walls of container is the pressure. Therefore, a gas exerts pressure on walls of the container.

(c) A wooden table should be called a solid because it is rigid, has a fixed shape and fixed volume & it is incompressible.

(d) We can easily move our hand in air but to do the same through a solid block of wood we need karate expert because air particles have very small forces of attraction & large inter-particle spaces, whereas solid block has much higher force of attraction between the particles & minimum inter particle space.

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, some empty spaces are created. As a result, the volume increases and density (0.92 g/cm3) becomes less than water (1.0 g/cm3). Hence, ice floats on water.

Questions

1. Convert the following temperatures to Celsius scale :

(a) 300 K                     (b) 573 K.

Ans: The temperature of the Kelvin scale can be converted into Celsius scale by subtracting 273 from the temperature on the Kelvin scale. Thus,

(a) 300 K = 300 – 273 = 27°C             (b)  573 K = 573 – 273 = 300° C

2. What is the physical state(s) of H2O at

(a) 0oC                         (b) 100oC                     (c) 25oC           (d) 250oC

Ans:    (a) Physical state at 0oC is either solid or liquid.

             (b) At 100oC it is either liquid or gas (steam).

             (c) Liquid                                                        

             (d) Gas (Steam)

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

Ans: During change of state of a substance at its melting point or the boiling point, temperature remains constant because the heat energy supplied to the substance is used up in overcoming the inter-particle forces of attraction without increasing its kinetic energy. As a result, the thermometer does not show any rise in temperature till the entire substance undergoes change of state.

4. What are the two factors that determine the state of a substance? Suggest methods to liquefy a gas.

Ans: Two factors that determine the state of a substance are:

               i) Temperature        ii) Pressure

         Gases can be liquefied by:      

               i) Applying pressure ii) Lowering temperature

Questions

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

Ans: When a liquid evaporates, the particles of liquid absorb energy from the surroundings to compensate for the loss of energy during evaporation. This makes the surroundings cool. In a desert cooler, the water inside it is made to evaporate. This leads to absorption of energy from the surroundings, thereby cooling the surroundings. Again, we know that evaporation depends on the amount of water vapour present in air (humidity). If the amount of water vapour present in air is less, then evaporation is more. On a hot dry day, the amount of water vapour present in air is less. Thus, water present inside the desert cooler evaporates more, thereby cooling the surroundings more. That is why a desert cooler cools better on a hot dry day.

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

Ans: An earthen pot (Matka) has a large number of extremely small pores in it. Some of the water continuously seeps out through these pores to the outside of the pot and evaporates continuously. During evaporation, it takes the latent heat required for vaporization from its surroundings & the water present inside the pot. In this way, the water inside the pot loses heat and becomes cool.

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

Ans: Acetone or petrol or perfume evaporates by absorbing latent heat from our skin (palm) and therefore gives cooling sensation.

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

Ans: We are able to sip hot tea or milk faster from a saucer rather than a cup because in the saucer, the surface area of tea or milk increases, therefore the rate of evaporation increases and tea or milk cools down rapidly.

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

Ans: In summer, we perspire more, therefore, to keep our body cool, we must wear cotton clothes, since cotton clothes are good absorber of water, they absorb the sweat quickly and expose it to the atmosphere for easy evaporation, and sweat absorbs latent heat from our skin, evaporates & makes our body cool.

EXERCISE

1. Convert the following temperature to the Celsius scale.

(a) 293 K                                             (b) 470 K

Ans: Kelvin is an SI unit of temperature, where 0°C = 273.16 (approximately 273 K)

(a) 293 K = (293 – 273)°C = 20­°C          (b) 470 K = (470 – 273)°C = 197°C

2. Convert the following temperatures to the Kelvin scale.

(a) 25°C                                              (b) 373°C.

Ans: The temperature on the Celsius scale can be converted into the Kelvin scale by adding 273 to the temperature on the Celsius scale. Thus,

(a) 25°C  = 273 + 25 = 298 K, and     

(b) 373°C = 273 + 373 = 646 K.

3. Give reasons for the following observations.
  • Naphthalene balls disappear with time without leaving any solid.
  • We can get the smell of perfume sitting several meters away.

Ans: (a) Naphthalene undergoes sublimation slowly at room temperature. As a result, solid naphthalene gets converted into vapours which become a part of air around us. Therefore, naphthalene balls disappear without leaving any solid.

(b) We get the smell of perfume sitting several meters away due to diffusion. The particles of the perfume get diffused in air and the smell of perfume reaches us even if we are sitting several meters away.

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

Ans: The forces of attraction are the strongest in solids, followed by liquids and weakest in gases. Since sugar is a solid, water a liquid and oxygen a gas, therefore, the forces of attraction between particles increases in the order : oxygen < water < sugar.

5. What is the physical state of water at –

(a) 25°C                      (b) 0°C                                    (c) 100°C?

Ans: (a) Water at 25°C is present in the liquid state.

(b) At 0 °C, water can exist as both solid and liquid. At this temperature, after getting the heat equal to the latent heat of fusion, the solid form of water i.e., ice starts changing into its liquid form i.e., water.

(c) At 100 °C, water can exist as both liquid and gas. At this temperature, after getting the heat equal to the latent heat of vaporization, water starts changing from its liquid state to its gaseous state, i.e., water vapours.

6. Give two reasons to justify :
(a) Water at room temperature is a liquid.
(b) An iron almirah is a solid at room temperature.

Ans:   

(a)  (i)  Water has a fixed volume but it does not have a fixed shape.

(ii) Water has the tendency to flow i.e. it shows the property of fluidity.

(b)  (i) An iron almirah has a fixed shape as well as fixed volume.

(ii) An iron almirah is incompressible.

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

Ans: Ice at 273 K absorbs more heat energy in the form of latent heat of fusion as compared to water at 273K. Thus, ice is more effective in cooling than water at the same temperature.

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

Ans: Steam at 373 K has more heat energy, in the form of latent heat of vaporization than boiling water at 373 K, therefore, steam produces more severe burns than boiling water.

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

Liquid

Ans.     

A – Melting/Fusion                                 B – Vaporisation         

C – Condensation/liquefaction             D – Freezing/Solidification

E – Sublimation                                       F – Sublimation

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