Physics question pattern of final exam for classes 7,8 & 9

M.M: 80.       Time:2hr

 Section-A ( 40 marks)

Q1 MCQ  (1×15=15 marks)

Q2 short question answer  (2×6=12 marks)

Q3 short question answers     (13 marks)

Section -B (40 marks)

Attempt any four questions:

Q4     (1+2+3+4=10 marks)

Q5     (1+2+3+4=10 marks)

Q6.     (1+2+3+4=10 marks)

Q7.      (1+2+3+4=10 marks)

Q8.      (1+2+3+4=10 marks)

Practice questions based on chapter 8 ( electricity ) for class 8

 1. Define and explain electroscope with suitable diagram. Why are  there two metallic plates attached to the bellJar of an goldleaf electroscope?

2.An Atom is electrically neutral. When you comb your dry hair,it attracts pieces of paper.Explain it.

3. What is static electricity and  how is it produced? Explain the  process of induction with the help of suitable diagram.

4. Solar energy is available to all of us absolutely free of cost.Then why very few people have used solar panels to produce electricity?

5.An electrical appliance of power 2.5 KW is used for 8h each day. Find

A. The electrical energy consumed in kilowatt hour , each day.

B.The electrical energy consumed in 90 days.

6. In a premise 10 bulbs each of 100 watt, 4 fans each of 60 watt, 4 A.C's  each of 2 kilowatt are used for 8 hours Per day.Find

A. Total power consumed per day.

B. Total power consume in one year.

C. Total electrical energy consumed in 180 days.

D. The cost of electricity at the rate of₹ 8.25 per unit.

7. An electrical appliance is rated as 100W - 220V.

A. What do you understand by this statement?

B. How much current will flow through the appliance when in use?

8. Is it possible to charge a conductor? If yes then explain the different methods of charging a conductor.

9. How will you use a good leaf electroscope to find out whether the charge on a charged body is positive or negative? Draw a well labelled diagram of a gold leaf electroscope and describe its construction as well as working.

10. Why lightning occurs? Is it possible to protect our buildings from lightning? If yes then explain it in detail.

Practice questions based on chapter 7 (sound) for class 8

 1. Harsh is a musician,who bring about a change in the pitch of the sound produced by his instruments? Give the likely reason for bringing about such a change.

2. Anshuman is a 'tabla player' , he is playing on the Tabla and produce a sound of a given pitch and loudness. Suggest how he can increase the

A. Pitch of the sound without any significant change in its loudness

B. Loudness of the sound without any significant change in its pitch.

3. The value of a sound is 40 decibel. Find the intensity of the sound, as compared to the intensity of the just audible sound ?

4. We know that the frequency of the female voice in humans is more than the frequency of the male voice. We also know that for sound waves,

Velocity = frequency × wavelength

Can we conclude that the velocity of propagation of the female voice would be more than that of the male voice? If not, how can we understand the above quoted results?

5. Why is the sound of a siren in an ambulance or fire tender different from the horns used by other vehicles? Give reason For your answer.

6. During rainy season ,We See The Lightning before we hear the thunder. Explain it.

7. Floor and ceilings of the Auditorium, and theatres are covered with sound proof tiles. Why we do so?

8. What do you mean by characteristic  of sound? Explain it in detail. 

Is amplitude  is related to loudness of sound in any way? Explain this with suitable example.

9.ram and shyam produces sound of amplitude 3:4 .find loudness of shyam's sound in terms of ram's sound?

10. Is sound produced by a musical instrument is monotone?explain your by giving reason in support of your answer.

Practice questions based on Chapter 6 (heat )for class 8

 1. A Green colour spherical metallic ball has a cavity inside. What will happen to the diameter of the ball if the ball is heated?explain it.

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

3. We all know that evaporation of a liquid takes place primarily from its surface. Use this fact to explain how the common practice of " spreading out spilled water " helps to speed up its rate of evaporation.

4. Find out the reason  of ' pressure cookers' being more effective in cooking  pulses at mountains and after places at greater Heights.

5. Are we correct in saying that liquids not only expand on heating but they do so more than solids?

6. The dimensions of an object are P, P/500 and P/1000. When this object is heated, its thermal expansion can be referred to as an example of what and Explain factors affecting it.

7. A Yellow coloured glass stopper has got stuck in the neck of a bottle. It can be e removed by pouring hot water on the neck of the bottle. Explain why?

8.An empty glass bottle is kept with its mouth in a tub of water. When it is heated at the top, bubbles of air escaping into the water. Explain it by giving reason.

9.A blue colour iron plate of dimensions 20 cm× 50 cm , at 10°C is is heated in a furnace, when the final area of plate is 1006 cm². If if the coefficient of linear expansion of iron is 0.000012 / °C, find the temperature of the Furnace?

10. The constant difference in length of iron rod ( A) and brass rod ( B) is 1.5 cm, when both the rods are heated through same range of temperature. Find the length of each rod? Coefficient of linear expansion for A and B are 12 × 10-⁶ / °C and 18 × 10-⁶ / °C.

Practice questions based on chapter 5(light energy) for class 8

 1. Dr Ajay  sometimes uses a mirror on his forehead to examine the internal parts the ear ,nose, throatetc. What type of mirror do you think it is? Explain it by giving reason for your answer.

2. Class 8 boys are given three Mirrors one of which is is a plane mirror, the second a convex mirror and the third a concave one. How can you decide "which is which" by looking at the image of your face in them from a closed position?

3. Give reasons for the following:

A. A pond of water appears shallower than it actually is.

B. A Pencil appear bent when dipped partially in a beaker containing water.

4.what will you observe when a pencil is completely submerged in a glass full of water. Will it slightly bent or it will appear straight? Give Reason in support of your answer.

5.Md. sharib moves towards a plane mirror with a speed of 5m/s. How fast is  his image moving towards him?

6. An object P is kept at a distance of 25 cm from a concave mirror of focal length 15cm. Where its image will be formed?

7. Generally During morning and evening time, why does sky appears to be Orange- red coloured? 

8. What do you mean by Dispersion of light? Explain it with the help of a suitable diagram. What will happen if we pass a monochromatic Ray of light through a prism, will it under go dispersion and split into its constituent colours?

9. Draw a ray diagram in which an upright and magnified image will be formed in a concave mirror. Where Such Mirrors are used in our day to day life?

10. What do you mean by mirage? Explain its occurence with the help of a a suitable diagram. Is it anything to do with the total internal reflection?

Practice questions based on chapter 3(force and pressure) for class 8

 1. What do you mean by a moment of force? Why  do we prefer a longer spanner for tightening a large nut?

2. What is a liquid pressure? Describe an experiment to show that the pressure of a liquid on the bottom of its container increases with an increase in the depth of the liquid.

3. The moment  of a force of 90 Newton, acting along a line at a perpendicular distance X from the the axis of rotation equals 18Nm. Find the value of X?

4. Give reasons for the following:

A. Why do  we feel a pressure on our finger when we used to cover a hole in a vessel filled with water?

B. It is easier to cut with a sharp knife than a blunt one.

C. Building and dams are made broader at the base than at the top.

D. Trucks and buses generally have Double wheels at the back.

5. a man is used to live in Mountain, if he comes down to plains,then What will happen to him ?

6. Harsh first stand and then lies down on a Sandy beach. In which case will he sink deeper and why?

7. A force of of hundred Newton acts along a line at a perpendicular distance of 20 cm from the axis of rotation. Find the moment of this force SI unit?

8. A force  of 200 Newton acts normally  over an area of 15 cm². Find the pressure exerted on the surface?

9. Find the ratio of the pressure due to 

A ) a force of 100 Newton acting over an area of 20 CM square and

B) a force of 50 Newton acting over an area of 50 cm square.

10. The  hand pump is a device that has been traditionally used for lifting out underground water. It's working is based on the existence and magnitude of atmospheric pressure. The atmospheric pressure at a given place can support a column of Mercury of height 75 cm. It is also known that Mercury has a density which is roughly 14 times that of water. The height of water column that the atmospheric pressure at that place can support. Given that the underground water at that place is present at a depth of 15 metre below the ground, will a hand pump be able to pump out this underground water?

Solved numerical of ch. 8 based on electricity

 1. An electrical appliance is rated as 60 watt - 150 volt.

a) what do you understand by this statement?

b) how much current will flow through the appliance when in use?

Ans. 1a. Electrical appliance is rated as 60 watt - 150 volt.

It means that if the appliance is used on a 150 volt supply the electric power consumed by it is 60 watt.i.e., 60 joule of electrical energy is consumed by the appliance in one second.

1b. Power rating as 60W -  150V.

P=60W

V= 150 V

I=?

P=VI

⇒P/V= I

⇒I=P/V

⇒I=60W / 150 V

⇒I= 0.4 A

2. An electric iron of power 1.5 KW is used for 30 minutes to press clothes. Calculate the electrical energy consumed in

a) kilowatt hour

b) joule

Ans. P=1.5 kW

t=30 minutes

  = 30 × 1/ 60  H

P= W/ t

⇒P × t = W

⇒ w = P × t

 ⇒ w= 1.5 kW × 1/2  

⇒ W = 0.75 kWh

2b) 

W= 0.75 kWh

     = 0.75 × 1 kWh

     = 0.75 × 3.6 × 10⁶ J    (  ∵ 1kWh = 3.6 × 10⁶ J )

     = 2.7 × 10⁶ J

3. Assuming the electric consumption per day to be 12 kilowatt hour and the rate of electricity to be rupees 6.25 per unit, find how much money is to be paid in a month of 30 days?

Ans. Energy consumption in one day= 12kWh

Energy consumption in 30 days = 12 kWh ×30

                                                           = 360 kWh

The rate of electricity is to be paid is 6.25 per unit.

Money to be paid in a month= energy consumed × cost of 1 unit

= 360 kWh × ₹ 6.25 / 1kWh

= ₹360× 6.25

= ₹ 2250.0

4. In a premise 5 bulbs each of 100 watt, 2fans each of 60 watt,2 AC s each of 1.5 kilowatt hour used for 5 hour per day.find:

A. Total power consumed per day

B. Total power consumed in 30 days

C. Total electrical energy consumed in 30 days,

D. The cost of electricity at the rate of rupees 6.25 per unit

Ans. Power of one bulb= 100W

∴ power of five bulbs = 5× 100W 

     =500W

Power of one fan= 60W

Power of 2 fan= 60W × 2 

  = 120W

Power of one AC = 1.5 kW

Power of 2 AC = 1.5kW × 2

  = 3.0 kW

  = 3.0 × 1kW

   = 3.0 × 1000W

   = 3000W

 ∴ total power of 5 bulbs 2 fans and 2 AC =500W + 120W + 3000 W

  = 3620 W

Total power consumed in 30 days = power consumed in one day × number of days

  = 3620W × 30 

   = 108600 W

    = 108.6 × 10³ W

    = 108.6 × 1 k W

    = 108.6 kW

Total electrical energy consumed in 30 days = P ×  t

   = 108.6 × 10³ W × 5 h

   = 543 kWh

The cost of electricity at the rate of 6.25 per unit = electrical energy consumed × cost of 1 unit

  = 543 kWh × ₹ 6.25 / 1kWh

   = 543 × ₹ 6.25

   = ₹ 3393.75

Solution of class test 4 of class 8

 1.The Gravitational unit of thrust in M.K.S system is

Ans.kgf

2.To reduce the pressure on a given area the total force applied is

Ans.  To reduce/ decrease.

3.1 Pascal is equal to

Ans. 1N/m²

4.An Elephant walk on any kind of ground

Ans.surface area occupied by its feet is more.

5.A man of mass 75 kg is sleeping on a bed .if the the area of a leg of the bed in contact with the ground is 7.5 cm square. find the pressure on the floor exerted by that man

Ans.mass = 75kg

     W=75 kg × g                                 (W= m×g)

        W= 75 Kgf

Area of  leg of bed = 7.5 cm²

In a bed number of leg =4

So , area of 4 legs of  bed= 4×7.5cm²

                                              =30.0 cm²

P=Thrust/ Area

P=75 Kgf / 30cm²

    = 7.5/3 kgfcm-²

    = 2.5 kgfcm-²

   = 2.5 × 10 N / 10-⁴ m². (1kgf= 9.8N ≈ 10N) & (1cm= 1/100 m ,1cm²= 10-⁴ m²)

   = 2.5 ×10⁵ Nm-². 

6.The thrust exerted by the blood in the veins our body is

Ans. Is slightly more than 1 atmospheric pressure.

7.Pressure exerted by a fluid is due to its

Ans weight.

8.Pressure inside a liquid increases with

Ans. Height of the liquid column.

9.To obtain a given moment of force for turning a body the force needed can be decreased by

Ans by applying force at the farthest point.

10.A person weighing 120 kgf stand on a platform  of dimension 2.5 cm × 0.5 cm. what pressure in Pascal does he exert

Ans.W= 120 Kgf

W= 120× 10N (1kgf=9.8N  ≈ 10N)

 = 1200 N

Dimensions means, length=2.5 cm

Breadth= 0.5cm

Area=length×breadth

        = 2.5/ 100 m × 0.5/100 m

       = 1.25/10000 m²

P= thrust/area

= 1200N/( 1.25/10000 m²)

 = 1200×10000/ 1.25  Nm-²

=960 ×10000 Nm-²

= 9.60×10²×10⁴ Nm-²

= 9.6 × 10⁶ pa (1Nm-² = 1Pa )

11.The moment of a force of of 50 Newton about a point is 2.5 Newton metre. find the perpendicular distance of force from that point

Ans. Moment of force= 2.5 Mm

force= 50N

Perpendicular distance=?

Moment of force=perpendicular distance×force applied

Moment of force= d × F

d=moment of force/force

  = 2.5Nm/ 50N

  = 0.25/5 m

= 0.5 m

= 50cm.

= 500 mm

12.1 kgf is equal to

Ans. 9.8 N

13.What is the magnitude of thrust required in Newton to produce a pressure of 26500 Pascal on an area of 100 cm square?

Ans. Pressure=26500 pa

Area=100 cm²

         = 100 × 10-⁴ m²

         = 10-² m²

Pressure=thrust/area

thrust= pressure× area

           = 26500 Pa  × 10-² m²

          = 2.6500 ×10⁴ Nm-² × 10-² m² (1Pa = 1Nm-²)

         = 2.65 ×10² N

14.A normal force of 100 Newton can produce a pressure of 100000 Pascal .calculate the area in centimetre square on which the force shall act to exert the pressure

Ans.F= 100N

P= 10⁵ Pa

A=?

P= F/A

A= F/ P

    = 100 N/ 10⁵ Pa

    = 10² N / 10⁵ Nm-²

    = 10-³ m²

   = 10-³ × 10⁴ cm² (1m²= 10⁴ cm²)

 = 10¹ cm²

15.At sea level the thrust on an area of 6 square metre is equal to

Ans. 1atm=1.013× 10⁵ Nm-² (at sea level)

But,

 P = Thrust / area

Thrust = P × area

             = 1.013 × 10⁵ Pa × 6m²

             = 6.078 × 10⁵ Nm-² × m²

             = 6.078 × 10⁵ N

Solutions of physics online class test - 2 of class 8

 1.A body X floats on water, a body Y sinks in water.The density of the body X is .......... than the density of body Y.

Ans. Less

2.The buoyant force experienced by a body when floating in Indian ocean is .......... that when floating in Dead sea.

Ans.  equal to

3.A body while floating .......... in water than in Glycerine.

Ans.deeper

4.Relative density of A is 2.7 . 150 cubic cm of A would have a mass of......... .

Ans. Relative density  of A = 2.7

V = 150 cm³

Mass of 150 cm³ = ?

Relative density= density of substance/density of water at 4 degree Celsius.

RD × density of water=density of a substance

Density of a substance=RD × density of water

                                         = 2.7 × 1gcm-³

                                         = 2.7 gcm-³

ρ= M/ V

ρ× V=M

M=ρ× V

  = 2.7 gcm-³ × 150 cm³

  = 405.0 g

  = 0.405 kg

5.A body A sinks in Alcohol but floats in Mercury.Density of A is in between ............ and ............ .

Ans.0.8g per cubic cm ,13.60 g per cubic cm

6.......... is the volume of an object of mass 600 kg if its density is 0.8 g per cubic cm.

Ans.V= ?

M= 600 kg

ρ=0.8 gcm-³

   = 0.8 × 1gcm-³

   = 0.8 × 1000 kgm-³

   = 800 kgm-³

ρ= M/ V 

ρ× V = M

V = M / ρ

V = 600 kg / 800 kg m-³

   = 3/4 m³

  = 0.75  m³

= 7.5 × 10 -⁷ cm³

7.The mass of an empty density bottle is 100 g, it is 145 g when filled completely with water and 135 g when filled completely with a liquid.Then density of liquid is ......... .

Ans.M₁=100 g

M₂=145 g

M₃= 135 g

ρ=?

ρ=M₃ - M₁ / M₂ - M₁

   = 35g / 45 cm³

   = 7/ 9 gcm-³

   = 0.77777 gcm-³

   = 777.78 kg m-3

8.Name a substance whose density is 8.9 g per cubic cm as per your physics textbook.

Ans. Copper.

9.Volume of substances varies with change in temperature but .......... remains constant.

Ans.mass

10.The density of air is 1.30 g per cubic dm. Express it in 1) g per litre 2) g per cubic cm 3) kg per cubic m

Ans.ρ= 1.30 gdm-³

            = 1.30 g/ dm³

            = 1.30 g / 1 l ( 1L = 1dm³ )

            = 1.30 g / 1000 cm³ (1L = 1000 cm³)

            = 1.30/1000 gcm-³

            = 0.00130 gcm-³

            = 0.00130 × 1 gcm-³

            = 0.00130 × 1000 kgm-³ (1gcm-³ = 1000 kgm-³)

            = 1.30 kgm-3

So,the correct answer is option A.

1.30 ,0.00130 ,1.30.

Solution of physics online class test 3 of class 8

 1.A cylinder whose cross -sectional area is 200cm² and height is 20 dm, has a mass of 20 kg.The maximum capacity of cylinder is

Ans.A=200cm²

           = 200/10000 m²

           =2/100 m²

h= 20 dm

 =20×10-¹ m

= 20/10 m

 = 2 m

V= A× h

= 2/100 m² × 2 m

= 4/100 m³

= 0.04 m³

= 4×10⁴ cm³

=40× 10³ cm³

= 40 litre.

 = 40× 1 litre

 40 dm³ ( 1litre = 1 dm³) 

=40 × 10³ cm³

 = 4× 10⁴cm³

=4×10⁴× 1cm³ 

= 4× 10⁴× 10-⁶ m³  (1cm= 10-⁶ m³)

=4× 10-² m³

So, the correct option would be all of these.

2.A cylinder of radius 70 cm and a height of 100 cm has a maximum volume of

Ans.r= 70cm

h= 100cm

V=?

V= πr²h

V= 22/7 × 70cm × 70cm × 100cm.  (π=22/7)

= 22×10cm×7000cm²

=1540 × 10³ cm³

= 1540 L.        ( 1L=1000 cm³)

3.Density of an aluminium is 2.7 g cm-³.54 g of an aluminium block is put in a measuring cylinder with its reading as 27 mL.The level of liquid in cylinder raises to

Ans.M=54g

ρ=2.7 g cm-³

V₁ = 27 mL

V₂ = ?

ρ= M/V

ρ× V = M

 V= M/ ρ

= 54g / 2.7 g cm-³

= 20 cm³

V₂= V₁ + V

    = 27 mL + 20 mL

= 47 mL

4.Density of water is maximum at 4 °C and volume at that temperature would be

Ans. Minimum.

5.A specific gravity bottle is used to measure

Ans. Density.

6.The volume of water is maximum at

Ans. Hundred degree Celsius.

7.Density of a body can not be 

Ans.  negative.

8.On heating water beyond 10° C

Ans. Density decreases due to increase in volume because of thermal expansion.

9.A cuboidal piece of stone 25 cm long,12 cm wide and 8 cm thick has 4 kg mass

Ans.l=25 cm

b= 12cm

h= 8cm

V=?

V= l×b×h

    = 25/100 cm × 12/100 cm × 8/100 cm

    = 0.25 ×0.12×0.8 m³

   =0.024 m³

M= 4 kg

ρ= M/ V

  = 4 kg / 0.024 m³

  = 166.666 kgm-³

  = 166.67 kgm-³.

   Density in gcm-³ come out as 0.16667 gcm-³

So, the correct option is none of these.

10.A 2m heigh piece of ice is seen floating in a river .uniform cross-sectional area of ice is 4000 dm².The volume of ice would be

Ans.h=2 m

A= 4000 dm²

   =4000 × 1 / 100 m²

   =40 m²

V= l×b×h

    = A× h

 V= 40 m² × 2 m

    = 80 m³

This is the volume of ice which is above the water.

We know that 1/10th part of a ice is above the water.

so total volume of ice would be ten times of the volume of ice above the water surface.

V = 80 m³ ×10

     = 800 m³

11.The mass of density bottle is 51.50 g when empty,76.50 g when full of water and 71.85 g when full of oil,then

Ans.M₁=51.50 g

M₂=76.50 g

M₃= 71.86 g

V of water = M₂- M₁

                    = 76.50 g - 51.50g

                    = 25.0 g

since density of water is 1 gram per centimetre cube. 25 gram of water would occupy a volume of 25 cm cube.

V= 25 cm³

ρ= M/ V

   = 71.86 g - 51.50 g / 25 cm³

  = 20.36 g / 25 cm³

  = 0.814 gcm-³

V= 25 cm³ &  ρ=0.814 gcm-³ both are correct.

So right option is C) both a and b.

12.The mass of an empty density bottle is 29.5 g .when filled completely with water is 49.5 g and when completely filled with liquid it is 48.3 g

Ans.M₁= 29.5 g

M₂=49.5 g

 M₃= 48.3 g

ρ=?

ρ relative density= M₃ -M₁ /  M₂ - M₁

   = 48.3g - 29.5 g / 49.5 g - 29.5 g

  = 18.8g / 20g

  = 0.94

But relative density= density of a substance /density of water at 4 degree Celsius.

RD=ρₛ / ρ of water

RD × density of water= density of substance

0.94 × 1gcm-³ =density of

0.94× 1000 kgm-³ =density of a substance

940 kgm-3= density of substance.

13.Calculate volume of wood of mass 12000 kg if the density of wood is 0.8 g cm-³

Ans.m=1200kg

ρ= 0.8 gcm-³

V=?

ρ= M/ V

ρ× V = M

⇒V= M/ρ

      = 12000 kg / 800kgm-³

      = 120/8 m³

      = 15 m³.

Short/long answer questions of chapter 2 (physical quantities and measurement) of class 8

 1. Define the term density of substance.

Ans. Mass per unit of a substance is called as its density.

2. Name the SI unit of density. How is it related to gram per centimetre cube?

Ans. The SI unit of density is kg per metre cube ( kgm-³).

It is related to gcm-³ as

1kgm-³= 10-³ gcm-³

3. The density of brass is 8.4 gram per centimetre cube. What do you mean by this statement?

Ans. The density of brass is 8.4 gram per centimetre cube. It means that 8.4 gram of brass occupies a volume of 1 cm cube.

Or 1 centimetre cube of brass has a mass of 8.4 g.

4 . Arrange the following substances in order of their increasing density:

Iron, cork, brass, water, Mercury.

Ans. Iron=7.8gcm-³

Cork=0.25gcm-³

Brass=8.4 gcm-³

Water=1 gcm-³

Mercury=13.6 gcm-³

Increasing order means we have to start from the lowest density to higher density.

Cork < water < iron < brass < Mercury.

5. How does the density of liquid or gas vary with temperature?

Ans. The density of a liquid or gas decreases with rise in temperature.

The density of liquid or gas increases with decrease in temperature.

Because mass of a substance remains same but with the change in temperature volume varies. Volume increases with increase in temperature and decreases with decrease in temperature because of thermal expansion.

6. A given quantity of a liquid is heated. Which of the following quantity will vary and how?

Ans. Mass remains same.

Volume increases with increase in temperature and decreases with decrease in temperature cause of thermal expansion.

Density decreases with increase in temperature and vice - versa.

7. Describe an experiment to determine the density of the material of a coin.

Ans. Put the coin on a digital balance and measure its mass.

2. Find the volume of the coin by the liquid displacement method.

3. After finding mass and volume of the coin. Use the formula of density as

Density=mass/volume.

Explain  activity 1 or activity 2 of page number 22 of your physics textbook.

8. Describe an experiment to determine the density of a liquid.

Ans.  the density of a liquid can be determined by  using a density bottle.

1. Mass of empty bottle M₁

2. Mass of bottle filled with water M₂

3. Mass of bottle filled with liquid M₃

These masses are measured by using beam balance.

4. The mass of liquid M = M₃ - M₁

5. Volume of liquid. V=M₂ - M₁

6. By using formula of density as,

      Density=mass of liquid/volume of liquid.

 ρ=M/V

  ρ=(M₃ - M₁) / M₂ - M₁

9. What is a density bottle? How is it used to find the density of a liquid?

Ans. Density bottle is a specially designed bottle  which is used to determine the density of a liquid. The stopper has a narrow hole through which it which allows the excess liquid to drain out.The bottle always contains the same volume of liquid each time when it is filled.

It is is used to find the density of a liquid by measuring mass and volume of the liquid. 

10. Define the term relative density of a substance.

Ans. Relative density of a substance is the ratio of density of a substance to the density of water at 4 degree Celsius. Or,

Relative density of a substance is the ratio of the mass of any volume of the substance to the mass of an equal volume of water.

11. What is the unit of relative density?

Ans.relative density is a unitless quantity. Relative density is a pure, it has no unit.

12. Distinguish between density and relative density.

Ans.1. density of a substance is the mass per unit volume of  that substance. Whereas relative density of a substance is the ratio of density of that substance to the density of water at 4 degree Celsius.

2. Density is expressed in gcm-³ or kgm-³. Whereas relative density has no unit.

Note write these differences in a vertical column method. One side write density  and other side write relative density.

13. Explain the meaning of the statement'relative density of aluminium is 2.7'.

Ans. It means that the density of  aluminium is 2.7 times more than the density of water at 4 degree Celsius.

14. How does the density of a body and that of liquid determine whether the body will float or sink into that liquid?

Ans.1.if the density of body is more than the density of liquid then that body will sink in it.

2.and if the density of body is less than the density of the liquid than in that case the body floats in that liquid.

15. A cork piece floats on water surface while and iron nail sinks in it. Explain the reason.

Ans. A cork piece  floats on water surface while an iron nail sinks in it because the average density of cork is less than the density of water where as the average density of iron is more than the density of water.

16. Which of the following with sink or float on water? Density of water=1gcm-3

A) body A having density of 500 kg per metre cube.

B) body B having density of 2520 kg per metre cube.

C) body C having density of 1100 kg per metre cube.

D) body D having density 0.85 g cm-³.

Ans. Density of water in SI unit is 1000kgm-³.

Since the density  of body B & C is more than the density of water, so both will sink in water.

On the other hand the density of body A & D is less than the density of water so these two bodies will float in water.

17. State the law of floatation?

Ans. According to the law of floatation,the weight of a floating body is equal to the weight of the liquid displaced by its submerged part.

18. The density of water is 1.0 gram per centimetre cube. Density of iron is 7.8 gram per centimetre cube. The density of mercury is 13.6 gram per centimetre cube. Answer the following:

A) will a piece of iron float or sink in water?

Ans. A piece of iron sinks in water because its density is more than the density of water.

B) will a piece of iron float or sink in Mercury?

Ans.a piece of iron will float in Mercury because its density is less than the density of Mercury.

19. The diagram given below shows a body floating in three different liquids A,B & C at different levels.

a) in which liquid does the body experience the greatest bite force?

b) which liquid has the least density?

c) which liquid has the highest density?

Ans. a) the body experience the same buoyant force in all three liquids.Because  for a floating body the buoyant force is equal to the weight of the body. Since three bodies are floating show buoyant force in all three cases would be same.

b) liquid A has  has the least density. Because less the density of liquid,  then larger portion of the floating body is inside the  liquid. By law of floatation the weight of a floating body should be equal to the weight of the liquid displaced by its submerge part.

Weight of the liquid displaced depends on the the volume submerged, density of liquid and acceleration due to gravity.

c) liquid C has the highest density because in that the submerged part of the body is very less .

20. For a floating body, how is its weight related to the buoyant force?

Ans. For a floating body the weight of the body should be equal to the the buoyant force or weight of the liquid displaced by it submerged part.

21. Why does a piece of ice float on water?

Ans.a piece of ice float on water because its density is less than the density of water.

22. Explain why an iron needle sinks in water but a ship made of iron floats on water?

Ans.an iron needle sinks in water but ship made of iron floats on water because the average density of iron needle  is more than the density of water where as the average density of ship made of iron is less than the average density of water so it floats on the surface of water.

23. Is it easier to swim in sea water than in river water? Explain the reason.

Ans.it is easier to swim in seawater than in river water because the density of seawater is more than the density of river water due to the presence of salt in the sea water.the weight of a man gets balanced by the less immersed part of his body in sea water as compared to that in river water.

24. Icebergs floating on sea water are dangerous for ships. Explain the reason.

Ans. Icebergs floating on sea water are dangerous for shipsbecause an iceberg floats in sea water with its large portions  submerged inside the water and only a little portion of it is above the surface of water. Ship can collide with invisible part of iceberg under the surface of water.

25. Explain why it is easier to lift a stone under water than in air.

Ans. It is easier to lift a stone under water than in air because when the stone is inside the water then a upthrust acts on it which helps us in lifting the load in upward direction. Although upthrust acts in air as well as in water but since the density of water is more compared to the density of air so buoyant force is more in water.

26. What is a submarine?how can it be made to dive in water and come to the surface of water?

Ans.a submarine is a fish shaped water tight- boat provided with several ballast tank in its front and rear parts. It can travel underwater like a ship.

1. To make the submarine dive, the tanks are filled with water so that the average density of the submarine becomes greater than the density of seawater and it sinks.

2. If a submarine is to rise,water from the ballast tanks is forced out into the sea by allowing the compressed air to enter the tank. Now the  average density  of submarine is less than that of sea water. So it it goes up and come to the surface of water.

27. A balloon filled with hydrogen rises in air explain the reason.

Ans. When light gas like hydrogen or helium is filled in a balloon, the weight of air displaced by the inflated balloon (upthrust) a more than the weight of the gas filled balloon and it rises up under the influence of the net upward force.

Objective questions of chapter 2(physical quantities and measurement) of class 8

 1. Write true or false for each statement:

A) equal volumes of two different substances have equal masses.

Ans false.

Equal volumes of two different substances have unequal masses.

B) the density of a piece of brass will change by changing its size or shape.

Ans. False

The density of a piece of brass will not change by changing its size or shape.

C) the density of a liquid decreases with increase in its temperature.

Ans. True.

D) relative density of water is 1.0

Ans. True.

E) relative density of a substance is expressed in gram per centimetre cube.

Ans.  false.

Relative density of a substance is a unit less quantity.

F) when a body is immersed in a liquid,the buoyant force experienced by the body is equal to the volume of the liquid displaced by it.

Ans. False.

When a body is immersed in a liquid,the buoyant force experienced by the body is equal to the weight of the liquid displaced by it.

G) a body experiences the same buyont force while floating in water or alcohol.

Ans. True.

H) a body experiences the same buoyant force when it floats or sinks in water.

Ans. False.

A body experiences the different buoyant force when it floats or sinks in water.

I) a body floats in a liquid when its weight becomes equal to the weight of the liquid displaced by it submerged part.

Ans. True.

J) a body while floating, sinks deeper in a liquid of low density than in a liquid of high density.

Ans. True

2. Fill in the blanks:

A) 1 kg is the mass of..1000..... mL of water at 4°C.

B) Mass= densit × .... volume...

C) the SI unit of density is....kgm-³....

D) density of water is...1000... kgm-³

E)1gcm-³ = ...1000... kgm-³

F) the density of a body which sinks in water is...more.. than 1000 kgm-³.

G) a body sinks in a liquid A, but floats in a liquid B. The density of liquid A is ..less..than the density of liquid B.

H) a body X sinks in water, but a body Y floats on water. The density of the body X is ..more... Then the density of body Y.

I) the buoyant force experienced by a body when floating in salt water is... equal to... That when floating in pure water.

J) the weight of a body floating in a liquid is.. zero....

3. Match the following:

A) kgm-³                  density

B) No unit               relative density

C) relative density   density bottle

D) iron                      sinks in alcohol

E) wood                   floats on water

4. Select the correct alternative:

A) the correct relation is

1. Density=mass×volume

2. Mass=density×volume

3. Volume=density×mass

4. Density=mass+volume

B) the relative density of alcohol is 0.8 . Its density is

1. 0.8

2. 800 kgm-³

3.800gcm-³

4.  0.8 khm-³

C) a block of wood of density 0.8gcm-³ has a volume of 60 cm³. The mass of block is

1. 60.8 g

2. 75 g

3. 48 g

4.  0.013g

D) the density of aluminium is 2.7 gcm-³ and that of brass is 8.4 gcm-³. The correct statement is:

1. Equal masses of aluminium and brass have equal.

2.the mass of a certain volume of brass is more than the mass of equal volume of aluminium.

3.the volume of a certain mass of brass is more than the volume of equal mass of aluminium.

4. Equal volumes of aluminium and brass have equal masses.

E) a density bottle has a marking 25ml on it. It means that:

1. The mass of density bottle is 25 g.

2. The density bottle will store 25 ml of any liquid in it.

3. The density bottle will store 25 ml of water, but more volume of liquid denser than water.

4. The density water will store 25 ml of water,but more volume of a liquid lighter than water.

F) the correct statement is:

1.the buoyant force on a body is equal to the volume of the liquid displaced by it.

2. The buoyant force on a body is equal to the volume of the body.

3.the buoyant force on a body is equal to the weight of the liquid displaced by it.

4. The buoyant force on a body is always equal to the weight of the body.

G) a piece of wood floats on water. Buoyant force on wood will be

1. Zero

2. More than the weight of the wood piece.

3. Equal to the weight of the wood piece.

4. Less than the weight of the wood piece.

H) weight of a body is more than the buoyant force experienced by it, due to a liquid. The body will:

1.sink

2. Float with its some part outside the liquid.

3. Float just below the surface of liquid.

4. Float with whole of its volume above the surface of liquid.

Short notes of chapter-2 (physical quantities and measurement) of class 8

 ☆ physical quantity-

• A quantity that can be measured is called a physical quantity. 

Or a measurable quantity is called a physical quantity.

• a physical quantity can be of two types:

     1. Scalar quantity- which can be expressed only by their magnitude.eg. mass, length, time ,distance, density ,volume, speed, temperature,  work, energy, power, pressure etc.

      2. Vector quantity- which requires the magnitude as well as the direction to express them.eg. force ,weight, velocity , displacement ,acceleration,magnetic field ,moment of force etc.

•  physical quantity= magnitude× unit

• basic quantities or fundamental quantities with their S.I. units:

1. Length - metre- m

2.Mass- kilogram- kg

3. Time- second-s

4. Temperature- Kelvin-K

5. Luminous intensity- candela-cd

6. Electric current- ampere-A

7. Amount of substance-mole-mol

☆ measurement-

- measurement is a comparison of an unknown quantity with a known fixed quantity of the same kind.

*Density:

- equal masses of different substances have different volumes.

•equal volumes of different substances have different masses.

*Mass per unit volume of a substance is called as its density.

-word formula:

Density=mass / volume.

ρ= M/ V

               ρ=density

               M=mass

                V=volume

-S.I. unit of density is kg/m³

-CGS unit of density is g/cm³

•relationship between SI & CGS unit of density

1kg/m³= 1kg / 1m×1m×1m

             =1000g / 100cm×100cm×100cm

             = 1g /1000cm³

             =1g /10³ cm³

            = 10-³ gcm-³

1kg/m³ = 10-³ gcm-³

1gcm-³ = 1000 kgm-³

-Density of iron=7.8 gcm-³

-density of copper=8.9 gcm-³

-density of water=1 gcm-³

•density of a substance does not change with any change in its shape or size.

•density decreases with increase in temperature and density increases with decrease in temperature for all substances except few.

*Maximum Density of water is 1gcm-³ at 4 degree Celsius.

 water contracts on heating from 0 degree Celsius to 4 degree Celsius and expands on heating above 4 degrees celsius.so the density of water increases from 0 degree Celsius to 4 degree Celsius and then decreases above 4 degree Celsius.

*Determination of density of a regular solid:

1. Measuring mass by putting the substance on a beam balance.

2. Finding volume by either formula method or liquid displacement method.

3. Putting the values of mass and volume in the formula 

      Density=mass/volume.

Formula for finding the volume of some regular solids:

1. Volume of a cube=side×side×side

2. Volume of a cuboid=length×breadth×height

3. Volume of a sphere=4/3 πr³

4. Volume of a cylinder=πr²h

5. Volume of a cone=1/3 πr²h

6. Volume of a lamina whose area is A and thickness is h, = A×h

Vessels for measuring volume:

1. Measuring cylinder -it is graduated in millilitre (mL).

-the graduation increase upwards.

-it comes in different capacities such as 100mL,200mL,500mL etc.,

•area of cross section of 10 centimetre square is is made up of either glass or plastic.

• it is is of capacity 100cm³ or 100mL.

• used in laboratory to measure the volume of liquids.

• used to find the volume of an irregular object by measuring the displacement of water or liquid by the object.

2. Measuring beaker-

-it is used to to take out fixed volume of liquids.

-the capacity of a measuring beaker is Marked on it.eg. 50mL,100mL,200mL,500mL,1L etc.,

3.Eureka can-

-the Beaker can contain a  volume of liquid up to the spout.

-any excess of liquid over flows through  the spout.

*Determination of density of an irregular solid:

1. Mass of a body is calculated by keeping it on on the beam balance.

2. Volume is calculated by liquid displacement method.

Liquid displacement method-

•volume of irregular shaped body is measured by using a measuring cylinder by the method of displacement of liquid.

• volume of a body of irregular shape= volume of liquid displaced by the body when it is immersed completely into the liquid.

• the difference in the final level from the initial level gives the volume of the liquid displacedi.e., V=V₂ -V₁.   V₂=final level ,V₁=initial level.

*Density bottle:

-a density bottle is a specially designed to determine the density of a liquid.

-sometimes it is also called as specific gravity bottle.

-it stores a fixed volume of a liquid.

-the stopper has a narrow hole through which the excess liquid drains out.

-it contains the same volume of liquid when it is filled.

-the volume of bottle is either 25 ml or 50 ml.

*Determination of density of a liquid using a density bottle:

1. Mass of empty bottle M₁

2. Mass of bottle filled with water M₂

3. Mass of bottle filled with liquid M₃

These masses are measured by using beam balance.

4. The mass of liquid M = M₃ - M₁

5. Volume of liquid. V=M₂ - M₁

6. By using formula of density as,

      Density=mass of liquid/volume of liquid.

 ρ=M/V

  ρ=(M₃ - M₁) / M₂ - M₁

Here M2 - M1= V we will get in centimetre cube.

Density  of water=mass/volume

Density×volume=mass

Volume=mass/density. But density  of water=1g/cm³

Units of mass and density's gram,gets cancelled and cm³ is left as a unit.

*Relative Density:

-the ratio of density of the substance to density of water is called relative density.or

-relative density is the ratio of the mass of any volume of the substance to the mass of an equal volume of water at 4°C.

-represented as R.D

-word formula:

R.D=density of the substance/density of water at 4 degree Celsius.

-R.D is a dimensionless quantity because it is the ratio of two same quantities.

-*measurement of relative density of a liquid:

-the relative density of a liquid is measured by using a density bottle.

1. Mass of empty bottle= M₁ g

2. Mass of bottle filled with water =M₂ g

3. Mass of bottle filled with liquid= M₃ g

4. Mass of water=(M₂ - M₁)g

5. Mass of liquid=(M₃ -  M₁)g

These masses are measured by using a beam balance.

Relative density of a liquid=mass of liquid/mass of equal volume of water.

R.D= (M₃ -  M₁)g/(M₂ - M₁)g

       =(M₃ - M₁)/(M₂ - M₁)

*Specific gravity means relative density with respect to water.

*Density of substance in its different states:

--the substance is highly dense in solid state, less dense in liquid state and still less dense in gaseous state.

-but the density of ice is 0.917g cm-³,water is 1.00gcm-³ & steam is 0.00057 gcm-³.

*Floating and sinking:

-a body floats on a liquid if its density is less than the density of the liquid, while a body sinks in a liquid if its density is more than the density of the liquid.

-principle of floatation-

-when a body is placed in a liquid then two forces act on it:

1. The weight of the body which axe in vertically downward direction and has a tendency to sink the body.

2. Buoyant force/upthrust acts vertically upwards and it has tendency to move the body up.

Case 1-the weight of body is greater than the buoyant force-the body will sink in the liquid to the bottom under the the influence of the resultant force.

Case 2- the weight of the body is equal to the buoyant force.-the body will float just inside the surface of liquid.

case 3-the weight of the body is less than the buoyant force. Body floats partially submerged in liquid.only that much portion of the body will immerse inside the liquid by which the weight of the liquid displaced balance is the total weight of the body.

*Law of floatation:

- weight of the floating body= weight of the liquid displaced by its immersed part.

-the apparent weight of a floating body is zero.

*Body while floating,sinks deeper in a liquid of low density than in a liquid of high density.

*Some applications of floatation:

1. Floatation of an iron ship-average density of ship is less than the density of water.

2 flotation of man-the weight of a man gets balanced by the less immersed part of his body in sea water as compared to that in river water. Density of seawater is more than the river water because of presence of salt.

3. Floatation of ice on water- 9/10 th part of ice is inside the water & only 1/10 th part of it outside water. Because the density of ice is 0.9gcm-³ & water is 1.0gcm-³.

4. Submarine-  submarine is provided with water tanks to vary its average density.

5.icebergs-very huge and large pieces of ice floating on sea water.it floats in water with its large person submerged inside the water and only a little portion of it is above the surface of water.

6.whales-whales have a special organ called swim bladder to vary its average density.

7.Balloons-the buoyant force experienced by the balloon due to air, becomes greater than the weight of the balloon. So, the balloon rises up under the influence of the net upward force.

-

Objective questions of chapter 1 based on matter of class 8.

 1. Write true or false for each statement:

A) The temperature of a substance remains unaffected during its change of state.

Ans.True.

B) ice melts at 100 degree Celsius.

Ans. False.

Ice melts at 0 degree Celsius.

C) water at hundred degree Celsius has more heat than steam at hundred degree Celsius.

Ans. False.

Water at hundred degree Celsius has less heat than steam at hundred degree Celsius.

D) evaporation of a liquid causes cooling.

Ans. True.

E) water evaporates only at hundred degree Celsius.

Ans. False.

Water evaporates at all temperatures.

F) boiling takes place at all temperatures.

Ans. False.

Boiling takes place at 100 degree Celsius.

G) evaporation takes place over the entire mass of the liquid.

Ans. False.

Evaporation takes place at the surface of liquid.

H) the process of a gas converting directly into solid is called the vaporization.

Ans. False.

The process of a gas converting directly into solid is called deposition or solidification.

I) at higher altitudes, water boils above 100 degree Celsius.

Ans. False.

At higher altitudes, water boils below 100 degree Celsius.

J) the melting point of ice is zero degree celsius.

Ans. True.

2. Fil in the blanks:

A) evaporation takes place at...all.... Temperatures.

B).. freezing..... Process is just the reverse of melting.

C)... Sublimation... Is a process that involves direct conversion of a solid into its vapour on heating.

D) the temperature at which a solid converts into a liquid is called its... Melting point....

E) the smallest unit of matter that exist freely in nature is called.. molecule......

F) molecules of a substance are always in a state of .... Random motion.... And so they possess. Kinetic energy.......

G) inter molecular space is maximum in... Gases....., less in... Liquids.... And the least in . Solids.....

H) intermolecular force of attraction is maximum in. Solids......., Less in.. liquids.... And the least in.. gases.......

3. Match the following:

A) molecules-  matter.

B) 100°C - water boils.

C) 0°C -  water freezes.

D) at all temperatures- evaporation.

E) camphor- changes from solid to gas.

4. Select the correct alternative:

A) the intermolecular force is maximum in:

1. Solids

2. Gases

3. Liquids

4. None of the above.

B) the intermolecular space is maximum in

1. Liquids

2. Solids

3. Gases

4. None of these.

C) the molecules can move freely anywhere in:

1. Gases

2.Liquids

3. Solids

4. None of the above.

D) the molecules move only within the boundary in :

1. Liquids

2. Gases

3. Solids

4. None of the above.

E)the temperature at which a liquid gets converted into its vapour state is called its:

1. Melting point

2. Boiling point

3. Dew point

4. Freezing point.

F) rapid conversion of water into steam is an example of:

1. Evaporation

2. Freezing

3. Melting

4. Vaporization.

G) evaporation takes place from the:

1. Surface of liquid

2. Throughout the liquid

3. Mid portion of the liquid

4. Bottom of liquid.

H) boiling takes place from:

1. The surface of the liquid

2. Throughout the liquid

3. mid portion of the liquid

4. None of the above.

Short notes of chapter 1( based on matter )of class 8

*Matter

- matter is something which occupies space, has mass and can be perceived by our senses.

Composition-

•Indian philosophers- 5 tatvas- akash,vayu,Tejas(Agni),jal & prithvi.

•Maharishi kanada - tiny particles-anu

•John Dalton-tiny particles-molecules.

A molecule can exist free in nature and it has all the properties of that substance.

-a molecule can be of:

*one atom-monoatomic molecule.eg helium, neon,argon ,etc.

*two atoms-diatomic molecule.eg hydrogen molecule, oxygen molecule, nitrogen molecule, etc.

*more than two atoms-Polyatomic molecule.eg water, ammonia, carbon dioxide, etc.

*Characteristics of molecules:

1. Molecules are very small in size-

     -nearly 10-¹⁰ m

     -•crystals of potassium permanganate-dark purple colour (to demonstrate that matter is composed of tiny particles)

2. The molecules of matter have spaces between them- inter molecular space.

S<L<G.

•salt+water experiment = the molecules of salt occupy the spaces between the molecules of water.

3. Molecules are in constant motion

   Solid-vibrate about their mean positions without leaving their position.

Liquid-free to move within the boundary of the container.

Gas-can move in random manner everywhere

•Sprayed perfume experiment

•Lycopodium powder (yellow coloured dust powder/jash powder)-zig zag part of a fine particle of lycopodium powder.

4. Molecules attract each other:

   -inter-molecular force-is effective between the molecules only up to a separation of 10-⁹ m.

S>L>G

  •force of cohesion-intermolecular force of attraction between the molecules of the same substance.

•force of adhesion-the force of attraction between the molecules of two different substances.

•the particles of coal are bound by the strong forces of attraction amongst themselves.

*States of matter:

Three physical States of a substance can be changed by changing its physical condition such as temperature and pressure.

-•solids-rigid, definite size (volume) & definite shape.

•liquids-not rigid, definite volume but no definite shape.

•gases-not Rigid, neither a definite volume nor a definite shape.

 The properties which decide the state of a substance:

1. Intermolecular space

2. Intermolecular force

3. Kinetic energy of molecules due to their motion.

*Molecular model of solids:

1. Made up of very tiny particles-molecules.

   Like tiny rigid balls.

2. Inter molecular spacing is very small.

3. Can only vibrate to and fro about their mean positions. They do not leave their positions.

4. Closely packed due to the strong inter-molecular force between them.

*Molecular model of liquids:

1. made up of very tiny particles -molecules.

2. Intermolecular spacing in liquids are more than solids.

3.can move about freely within the boundary of The vessel in which the liquid is kept.

4.  less closely packed because the intermolecular forces in a liquid are weak in comparison to that in solids.

*Molecular model of gases:

1. Made up of very tiny particles-molecules.

   Like non-rigid balls.

2. Intermolecular spacing is very large.

3. can move about freely in the space available to them.

4. Are wide apart because the intermolecular forces in them are very weak.

    -the process of change from one state to another state either by absorption or rejection of heat at a constant temperature is called change of state.

Melting/fusion-the process of change from solid state to  liquid state by absorption of heat at a constant temperature.

Melting point-the constant temperature at which solid changes into liquid by absorbing heat energy.

•The wax melts at 55 degree Celsius during which heat is supplied, but temperature does not rise. After The melting of whole wax, the temperature begins to rise.energy used in increasing the potential energy of molecules.

*Melting point of ice decreases with increase of pressure on it,but the melting point of wax increases with increase of pressure on it.

*melting point of ice decreases when salt is added to it. This mixture of salt added to Ice is called freezing mixture. It is used for preparing kulfis.

•

Freezing-the process of change from liquid state to solid state by rejection of heat at a constant temperature.

*Water expands on freezing whereas wax, lead etc., Contract on freezing.

Freezing point-the constant temperature at which liquid changes into solid state by rejecting heat energy.

*Freezing point and melting point are the same.

*Explanation of melting by molecular model-

-on heating a solid,the kinetic energy of molecules increases due to which they begin to vibrate more violently.the molecules acquire sufficient kinetic energy to become far separated from each other by overcoming the forces of attraction between them & gets converted into liquid state.

AT MELTING POINT-the heat energy absorbed by the substance does not change the temperature of the substance,but is utilised in doing work against the forces of attraction in increasing the separation between the molecules that is their potential energy increases.

Boiling/vaporization-the process of change from liquid state to gaseous state by absorption of heat at a constant temperature.

Condensation-the process of change from gaseous state to liquid state by rejection of heat at a constant temperature.

* boiling point and condensation point are same.

*Explanation of vaporization by molecular model-on heating, the average kinetic energy of molecules of liquid increases.the molecules acquire sufficient kinetic energy to overcome the forces of attraction between themselves and they become free to leave the surface,thus converted into gases state.

AT BOILING POINT-The heat energy absorbed does not change the kinetic energy of the molecules,but is utilised in doing work against the force of attraction in increasing the separation between the molecules that is an increasing their potential energy.

Sublimation-the process of change from solid state to gaseous state directly by absorption of heat at a constant temperature .eg. ammonium chloride, iodine, camphor, naphthalene and solid carbon dioxide (dry ice).

Sometimes sublimation also takes place without heating. example naphthalene balls (moth balls) which is used to protect clothes, decreases in size with time & becomes small.

Deposition/solidification-process of change from gaseous state to solid state directly by rejection of heat at a constant temperature.

Hidden heat/latent heat-the quantity of heat given to or taken from the substance during the the change of state at a constant temperature.

• *Evaporation-the change from liquid state to gasses state on absorbing heat energy at all temperatures.
• *Explanation of evaporation by the molecular model:
• The molecules are in motion within its boundary and collide with each other. Some molecules below the surface of liquid acquire sufficient kinetic energy to overcome the force of attraction of other molecules and their intermolecular spacing increases such molecules have  to move to the surface of the liquid and they got it from surroundings so as to escape out into the atmosphere with atmospheric air molecules.
• *Factors affecting rate of evaporation:
• The temperature of liquid
• The area of the exposed surface cloth dries up faster if it is spread out than if it is folded up.
• The nature of liquid-volatile liquid such as alcohol, spirit, ether etc. evaporates much faster than water. The force of attraction among molecules is negligible in volatile liquids . So, they escape out more easily.
• The flow of air above  the liquid.to cool the milk, we blow air above it. On blowing air,the air molecules take away the liquid molecules from the surface with them and other molecules of the liquid occupy their place. This increases the rate of evaporation.
• The presence of moisture or humidity.wet clothes dry faster in dry summer days than in rainy season.
• *Evaporation produces cooling:
• When a liquid changes into vapour, it requires heat. Heat is supplied by the surroundings of the liquid .this results in fall in temperature in the surroundings.
• *Applications of evaporation:
• 1. Water gets cold  in an earthen pot.the heat required for evaporation is taken from water inside the pot.
• 2. The the strips of wet cloth on the fore head of a patient having high fever. During evaporation water takes heat it from the body of the patient.
• 3.tea in a saucer, the surface area of tea increases and evaporation becomes faster.
• 4.sweat from our body helps to maintain the body temperature at 37°C or 98.6°F. when sweat evaporates, it requires heat which. it takes away from our body.
• The average kinetic energy of molecules of a substance is a measure of its temperature.
• D the average kinetic energy of its molecules increases due to which its temperature rises. But is a substance gives out heat,the average kinetic energy of its molecules decreases due to which its temperature falls.
• 
  
  
  
  
  
• *Explanation of sublimation by molecular model:
• the intermolecular forces of attractions are weak in some solids, on heating the molecules acquire sufficient kinetic energy to overcome the force of of attraction and they become free to move and thus intermolecular separation becomes very large and they change to vapour directly.