Selina Concise Class 8 Physics Solution Chapter No. 2- ‘Physical Quantities and Measurement’ For ICSE Board Students.
Selina Concise Physics Solutions for Class 8th Chapter 2 Physical Quantities and Measurement with easy language, expert answers and doubt clearing sections. The book is especially designed keeping in mind the syllabus of ICSE board and the difficulty level faced by the students of ICSE boards Class 8.
Selina Class 8 Physics Chapter 2 Solution:
A) Objective Questions :
1.) Write true or false for each statement :
a) Equal volumes of two different substances have equal masses.
Answer: False.
The ratio of mass and volume depends on density of the substance.
b) The density of a piece of brass will change by changing its size or shape.
Answer: False.
Density of substance does not changes by changing it’s dimensions.
c) The density of a liquid decreases with increase in its temperature.
Answer: True.
If the temperature of liquid increases then it’s volume also increases. Thus the density of that liquid decreases.
d) Relative density of water is 1.0
Answer: True.
Relative density of water is fixed and that is 1.0.
e) Relative density of a substance is expressed in g/cm³.
Answer: False.
Relative density is the ratio of same physical quantities. Therefore Relative density is dimensionless quality.
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.
Answer: False.
Buoyancy force is depend on the weight of the object which is partially or completely immersed in the water or liquid.
g) A body experiences the same buoyant force while floating in water or alcohol.
Answer: True.
Buoyant force does not depend on density of liquid.
h) A body experiences the same buoyant force when it floats or sinks in water.
Answer: False.
Buoyant force is depend level of liquid which displaced by immersing the object in it.
i) A body floats in a liquid when its weight becomes equal to the weight of the liquid displaced by its submerged part.
Answer:
(j) A body while floating, sinks deeper in a liquid of low density than in a liquid of high density.
2.) Fill in the blanks :
Answer:
a) 1 kg is the mass of 1000 mL of water at 4⁰ C.
b) Mass = density xvolume.
c) The S.I. unit of density is kg/m³ .
d) Density of water is1000 kg/m³
e) 1 g/cm³ =1000 kg/m³
f) The density of a body which sinks in water is more than 1000 kg/m³.
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 than 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:
Column A Column B
(a) kg/m³ (i) relative density
(b) No unit (ii) sinks in alcohol
(c) Relative density (iii) floats on water
(d) Iron (iv) density
(e) Wood (v) density bottle
Answer:
Column A Column B
(a) kg/m³ (iv) density
(b) No unit (i) relative density
(c) Relative density (v) density bottle
(d) Iron (ii) sinks in alcohol
(e) Wood (iii) floats on water
4.) Select the correct alternative :
a) The correct relation is
(i) Density = Mass x Volume
(ii) Mass = Density x Volume
(iii) Volume = Density x Mass
(iv) Density = Mass + Volume
Answer: (ii) Mass = Density x Volume
We know that,
Density = mass/volume.
Therefore, Mass = density × volume.
b) The relative density of alcohol is Its density is
(i) 0-8
(ii) 800 kg/m³.
(iii) 800 g/cm³
(iv) 0.8 kg/m³.
Answer: (ii) 800 kg/m³.
The relative density of alcohol is (ii) 800 kg/m³.
c) A block of wood of density 0.8 g/cm³ has a volume of 60 cm³. The mass of block is
(i) 60.8 g
(ii) 75 g
(iii) 48 g
(iv) 0.013 g
Answer: (iii) 48 g
Given, Density = 0.8 g/cm³,
Volume = 60 cm³.
We know that,
Density = mass/volume.
Therefore, Mass = density ×volume
Mass = 0.8 × 60
Mass = 48 g
d) The density of aluminium is 2.7 g /cm³ and that of brass 8.4 g/cm³. The correct statement is :
(i) Equal masses of aluminium and brass have equal volumes
(ii) The mass of a certain volume of brass is more than the mass of equal volume of aluminium
(iii) The volume of a certain mass of brass is more than the volume of equal mass of aluminium
(iv) Equal volumes of aluminium and brass have equal masses.
Answer: (ii) The mass of a certain volume of brass is more than the mass of equal volume of aluminium.
The density of brass is greater than density of aluminium. Therefore The mass of a certain volume of brass is more than the mass of equal volume of aluminium.
e) A density bottle has a marking 25 mL on it.
It means that :
(i) the mass of density bottle is 25 g
(ii) the density bottle will store 25 mL of an liquid in it
(iii) the density bottle will store 25 mL of water, but more volume of liquid denser than water
(iv) the density bottle will store 25 mL of water, but more volume of a liquid lighter than water.
Answer: (ii) the density bottle will store 25 mL of an liquid in it.
Density bottle store any liquid of same marking in it.
f) The correct statement is :
(i) The buoyant force on a body is equal to the volume of the liquid displaced by it.
(ii) The buoyant force on a body is equal to the volume of the body.
(iii) The buoyant force on a body is equal to the weight of the liquid displaced by it.
(iv) The buoyant force on a body is always equal to the weight of the body.
Answer: (iii) The buoyant force on a body is equal to the weight of the liquid displaced by it.
Force and weight has same dimensions.
g) A piece of wood floats on water. The buoyant force on wood will be:
(i) zero
(ii) more than the weight of the wood piece
(iii) equal to the weight of the wood piece
(iv) less than the weight of the wood piece.
Answer: (iii) equal to the weight of the wood piece.
h) The weight of a body is more than the buoyant force experienced by it, due to a liquid. The body will :
(i) sink
(ii) float with its some part outside the liquid
(iii) float just below the surface of liquid
(iv) float with whole of its volume above the surface of liquid.
Answer: (i) sink.
Downward force has greater magnitude than upward force.
B) Short/Long Answer Questions :
1) Define the term density of a substance.
Answer: the term of density is depend on mass and volume. It is defined as the ratio of mass divided by volume.
Density = mass/ volume.
2) Name the S.I. unit of density. How is it related to g /cm³ ?
Answer:
SI unit of density is kg/m³.
The relationship between kg/m³ and g /cm³.
As we know,
1 kg = 10³ g and
1 m = 10² cm.
Therefore,
1 Kg/ m³ = 10³/(10²)³ g/cm³
1 Kg/m³ = 10³/10⁶ g/cm³
1 kg/m³ = 10-3 g/cm³
This is the relation between kg/m³ and g /cm³.
3) The density of brass is 84 g/cm³. What do you mean by this statement ?
Answer: The density of brass is 84 g/cm³ means the mass of 1 cm cube of brass is 84 gm.
4) Arrange the following substances in order of their increasing density :
Iron. Cork. Brass, Water, Mercury.
Answer: The increasing order of density of above substances is as follows.
Cork<Water<iron < brass <mercury.
5) How does the density of a liquid (or gas) vary with temperature ?
Answer: We know that, Volume is directly proportional to temperature. If we increases the temperature then volume of liquid (gases) also increases and density decreases.
There is inverse relationship between temperature and density. If the temperature increases then density decreases.
6) A given quantity of a liquid is heated. Which of the following quantity will vary and how ?
(a) mass, (b) volume or (c) density.
Answer: If we supply heat then it’s volume and density changes but there is no change in mass. Volume is directly proportional to temperature. If we increases the temperature then volume of liquid (gases) also increases and density decreases.
There is inverse relationship between temperature and density. If the temperature increases then density decreases.
7) Describe an experiment to determine the density of the material of a coin.
Answer:
To determine the density of material of coin we have to find the mass of coin and volume of the coin also.
Since, density = mass/volume
Now, initially we can find the mass of the taken coin using beam balance as M g and then will find the volume of the coin using measuring cylinder.
Now take a measuring cylinder and fully fill it with water and note the volume of the water added as V1.
Now, insert the coin in the same measuring cylinder fully filled with water and note the volume of the water at that time as V2.
So, we can find the volume of the coin as,
Volume of coin = V2 – V1 = V cm³
Thus, the density of the material of the coin taken is given by,
Density of material of coin = mass of coin/ volume of coin = M/V g/cm³
In this way, we can easily find the density of the material of the coin using beam balance and the measuring cylinder as discussed above.
8) Describe an experiment to determine the density of a liquid.
Answer: We can determine density of liquid by illustrating following experiment.
- Take a beaker of 100 mL and measure the mass of this beaker. Say mass of beaker is m1.
- Measure the given liquid with the help of 100 mL beaker.
- Now measure the mass of beaker with liquid and say it m2.
- Calculate difference between m1 and m2. We get mass of 100 mL of liquid.
- We can calculate density using formula,
Density = mass/volume.
9) What is a density bottle ? How is it used to find the density of a liquid ?
Answer:
Density bottle is the bottle specially used for measuring the density of the liquid.
It’s a glass bottle having glass stopper at its upper surface. Mostly the volume of density bottle is about 25ml or 50ml.
The diagram of density bottle is as,
The stopper which is at upper surface of a density bottle has a narrow hole through which we can pour liquid into it. When we insert the stopper then extra liquid will be removed from the hole and drained out. And hence, the volume of liquid filled in that density bottle is always same.
Following is the experiment or activity to measure the density of the liquid by using density bottle.
So to measure the volume of a liquid we have to measure the mass of the liquid and mass of the water using common balance and the value which gives the mass of water in density bottle is the volume of the liquid finally.
To measure the density of a liquid by density bottle we have to follow following steps:
Take a density bottle and dry it after washing. Now measure the mass of the empty density bottle as M1 g using common balance.
Now, by removing the stopper of the bottle pour the water into it. And make the outside part of the bottle dry and measure it’s mass as M2 g.
Now, we have to fill the bottle with the liquid whose volume is to be measured by washing and drying the density bottle again.
By replacing the stopper make outside part of the bottle dry and measure it’s mass as M3 g.
Now, determine the mass of water as (M2 – M1) g and mass of liquid as (M3 – M1) g.
We already know that, 1g/cm³ is the density of the water, then mass of the water which is poured into the density bottle gives the volume of the density bottle.
In this we can find the mass of the liquid as,
Mass of the liquid = (M3 – M1) g and
Volume of the liquid as,
Volume of the liquid= (M2 – M1) cm³
Thus,
Density of liquid is given by,
Density of liquid = mass of liquid/ volume of liquid
= (M3 – M1)/ (M2 – M1) g/cm³
In this way we can find the density of the liquid by using density bottle.
10) Define the term relative density of a substance.
Answer: Relative density is the ratio of density of the substance to density of water. It is the ratio of same quantity thus relative density is unit less quantity.
Relative density- The ratio of mass of any volume of a substance to the mass of water of same volume is Called relative density.
11) What is the unit of relative density ?
Answer: Relative density is the ratio of same quantity. Thus it does not have unit and dimensions.
12) Distinguish between density and relative density.
Answer:
Difference between density and relative density.
- Density- the ratio of mass and volume is called density.
Relative density- The ratio of mass of substance divided by mass of equal volume of water.
- SI unit of density is kg/cm³ But relative density does not have unit.
- Relative density is depend on Nature of substance but density depend on mass and volume only.
13) Explain the meaning of the statement ‘Relative density of aluminium is 2.7.
Answer: Relative density gives information about the mass of that substance is greater than mass of equal volume water. Relative density of aluminium is 2.7 means the mass of aluminum is 2.7 time greater than mass of equal volume of water.
14) How does the density of a body and that of a liquid determine whether the body will float or sink into that liquid?
Answer:
We can say that the object will float or sink into liquid on the basis of following conditions.
Condition for float an object on liquid-
If the density of object is less than the density of water then it will float on water.
Condition for sink an object into liquid-
If the density of object is greater than the density of liquid then it will sink intoliquid.
15) A cork piece floats on water surface while an iron nail sinks in it. Explain the reason.
Answer: We know the conditions for float and sink object into water.
The density of cork is less than density of water therefore it will float on water.
But the density of iron nail is greater than density of water therefore it will sink into water.
16) Which of the following will sink or float on water ? (Density of water = 1 g/cm³)
(a) Body A having density 500 kg/m³)
(b) Body B having density 2520 kg/m³
(c) Body C having density 1100 kg/m³
(d) Body D having density 0.85 g/cm³.
Answer: We can say that the object will float or sink into liquid on the basis of following conditions.
Condition for float an object on liquid-
If the density of object is less than the density of water then it will float on water.
Condition for sink an object into liquid-
If the density of object is greater than the density of liquid then it will sink into liquid.
Given,
Density of water = 1 g/cm³
But we know,
1 g = 10-3 kg and
1 cm = 10-2 m
1 g/cm³ = (10-3 )/ (10-2)³ kg/m³
1 g/cm³ = 10³ kg/m³
The density of water is 10³ kg/m³.
(a) Body A having density 500 kg/m³.
The density of body A is less than density of water, thus it will float on water.
(b) Density of body B = 2520 kg/m³.
The density of body B is greater than density of water thus it will sink into water.
(c) Density of body C = 1100 kg/m³
The density of body C is greater than density of water thus it will sink into water.
(d) Density of body D = 0.85 g/cm³.
The density of body D is less than density of water, thus it will float on water.
18)The density of water is 1g/cm³. The density of iron is 7.8 g/cm³. The density of mercury is 13. 6 g/cm³. Answer the following :
(a) Will a piece of iron float or sink in water ?
(b) Will a piece of iron float or sink in mercury ?
Answer:
(a) Given, density of water = 1 g/cm³ ,
Density of iron = 7.8 g/cm³.
The density of iron is greater than density of water. Thus iron sink into water.
(b) Given, density of mercury = 13.6 g/cm³ ,
Density of iron = 7.8 g/cm³.
The density of iron is less than density of mercury. Thus iron floaton mercury.
19) The diagram given below shows a body floating in three different liquids A, B and C at different levels.
(a) In which liquid does the body experience the greatest buoyant force ?
(b) Which liquid has the least density ?
(c) Which liquid has the highest density ?
Answer:
(a) Buoyancy force depends on weight of the object. It does not depend on the level of sink object in that liquid. Thus buoyancy force is same in each liquid.
(b) The level of immerse object in liquid A is greater therefore the density of that liquid is least.
(c) The level of immerse object in liquid C is minimum therefore the density of liquid C is greater.
20) For a floating body, how is its weight related to the buoyant force ?
Answer: Archimedes principle states that, when an object immersed in liquid then it displaced the level of liquid equal to weight of that object.
Condition for flailing object.
The weight of object = weight of liquid displaced by immersed part of that object.
Or we simply states that if the density of object and liquid is same then it float on that liquid.
21) Why does a piece of ice float on water ?
Answer: The density of piece of ice is less than density of water therefore it float on water.
22) Explain why an iron needle sinks in water, but a ship made of iron floats on water.
Answer: The density of iron is greater than density of water. Iron needle sinks in water because it has greater density.
But we know that density is inversely proportional to area of the substance. The density of ship reduces by increasing it’s area. Therefore it can float on water.
23) It is easier to swim in sea water than in river water. Explain the reason.
Answer: The density of sea water is greater than river water. Therefore we can easily float on sea water.
24) Icebergs floating on sea water are dangerous for ships. Explain the reason.
Answer: Iceberg floats on sea water because it has less density than sea water. But it’s only 1/9th part floats on sea water. We can not guess its size by seeing the its floating portions. Therefore icebergs floating on sea water are dangerous for ships.
25) Explain why it is easier to lift a stone under water than in air.
Answer: We know that, the density of water is greater than density of air. Water exerts buoyant force upwards direction so it is easy lift stone under water.
26) What is a submarine ? How can it be made to dive in water and come to the surface of water ?
Answer:
Submarine – A type of boat which can travels under water. It is based on the principle of flotation of object.
It is made up tanks which are filled with air therefore submarines can adjust their density.
27) A balloon filled with hydrogen rises in air. Explain the reason.
Answer: The density of hydrogen gas is less than air. Also we know that less dense object can floats on high dense object. Therefore Balloon filled with hydrogen gas rises in air.
C.) Numericals :
1) The density of air is 1.28 g/liter. Express it in : (a) g/cm³ (b) kg/m³.
Answer: Given,
Density of air = 1.28 g / liter.
a) We know that,
1 liter = 1000 cm³.
Therefore,
1.28 g /liter = 1.28 g / (1000 liter)
1.28 g/liter = 0.00128 g /cm³.
b) we know that,
1 liter = 0.001 m³, and
1 g = 0.001 kg
Therefore ,
1.28 g/liter = (1.28 × 0.001 kg)/(0.001 m³
1.28 g/liter = 1.28 kg/m³.
2) The dimensions of a hall are 10 m x 7 m x 5 m. If the density of air is 1.11 kg/m³, find the mass of air in the hall.
Answer:
Given, dimensions of hall.
Length = 7 m, breadth = 5 m and height = 10 m.
Density of air = 1.11 kg/m³.
As we know that,
Density = mass/volume
Mass = density × volume
Mass = 1.11 × ( 7 × 5 × 10)
Mass = 388.5 kg
The mass of air in room is 388.5 kg.
3) The density of aluminium is 2.7 g/cm³. Express it in kg/m³.
Answer:
Given density of aluminium = 2.7 g/cm³.
As ,
1 g = 10-3 kg and
1 cm = 10-2 m
2.7 g/cm³ = ( 2.7× 10-3 )/ (10-2)³ kg/m³
2.7 g/cm³ = 2.7 × 10³ kg/m³
2.7 g/cm³ = 2700 kg/m³.
4) The density of alcohol is 600 kg/m³. Express it in g/xm³.
Answer:
Given, density of alcohol = 600 kg/m³.
But we know that,
1 kg = 10³ g and
1 m = 10² cm.
Therefore,
600 kg/m³ = (600 × 10³)/(10²)³ g/cm³
600 kg/m³ = 600 × 10-3
600 kg/m³ = 0.60 g/cm³
The density of alcohol is 0.60 g/cm³.
5) A piece of zinc of mass 438.6 g has a volume of 86 cm³. Calculate the density of zinc.
Answer: Given,
Mass = 438.6 g, Volume = 86 cm³.
We know that,
Density = mass/volume
Density= 438.6/86
Density = 5.1 g/cm³.
The density of piece of zinc is 5.1 g/cm³.
6) A piece of wood of mass 150 g has a volume of 200 cm³. Find the density of wood in (a) C.GS. unit, (b) S.I. unit.
Answer: Given,
Mass = 150 g, Volume = 200 cm³
We know that,
Density = mass/volume
Density= 150/200
Density = 0.75 g/cm³.
The density of piece of wood is 0.75 g/cm³. Now we convert this value in SI or MKS unit system.
1 g = 10-3 kg and
1 cm = 10-2 m
0.75 g/cm³ = ( 0.75× 10-3 )/ (10-2)³ kg/m³
0.75 g/cm³ = 0.75 × 10³ kg/m³
2.7 g/cm³ = 750 kg/m³.
The density of zinc-
In CGS unit system= 0.75 g/cm³.
In SI or MKS unit system = 750 kg/m³.
7.) Calculate the volume of wood of mass 6000 kg if the density of wood is 0.8g/cm³.
Answer:
Given, Mass = 6000 kg,
Density = 0.8 g/cm³.
We concert this value in SI unit system.
We know that,
1 g = 10-3 kg and
1 cm = 10-2 m
0.8 g/cm³ = ( 0.8× 10-3 )/ (10-2)³ kg/m³
0.8 g/cm³ = 0.8 × 10³ kg/m³
0.8 g/cm³ = 800 kg/m³.
The density of wood = 800 kg/m³.
As,
Density = mass/volume
Volume = mass/density.
Volume = 6000/800
Volume = 7.5 m³
The volume of that wood is 7.5 m³.
8) Calculate the density of a solid from the following data :
(a) Mass of solid = 72 g
(b) Initial volume of water in measuring cylinder
= 24mL
(c) Final volume of water when solid completely immersed in water = 42 mL
Answer: Given,
Mass of solid = 72 g,
Initial volume of liquid = 24 mL,
Final volume of liquid = 42 mL.
Change in volume of liquid = 42 -24 = 18 mL.
18 mL = 18 cm³.
We know that,
Density = mass/volume
Density = 72 / 18
Density = 4 g/cm³.
The density of that solid is 4 g/cm³.
9) The mass of an empty density bottle is 21.8 g when filled completely with water it is 41.8 g and when filled completely with liquid it is 40.6 g. find:
(a) the volume of density bottle
(b) the relative density of liquid.
Answer: Given,
Mass of empty density bottle = 21.8 g,
Mass of density bottle with water = 41.8 g.
Mass of density bottle with liquid= 40.6 g.
a)
The mass ofwater in bottle = (mass of Mass of density bottle with water) – (Mass of empty density bottle)
The mass of water in bottle = 41.8 – 21.8
The mass of water in bottle = 20 g.
We know that, Density of water = 1 g/cm³.
As,
Density = mass /volume
Volume = mass × density
Volume = 20 × 1
Volume = 20 cm³.
The volume of density bottle is 20 cm³ or 20 mL.
b)
The mass of liquid = (mass of Mass of density bottle with alcohol) – (Mass of empty density bottle)
The mass of liquid= 40.6 – 21.8
The mass of liquid = 18.8 g.
We know that,
Relative density of liquid = mass of liquid/ mass of equal volume of water.
Relative density of liquid = 18.8/20
Relative density of liquid = 0.94 .
The relative density of liquid is 0.94 .
10) From the following observations, calculate the density and relative density of a brine solution.
Mass of empty density bottle = 22 g
Mass of bottle + water = 50 g
Mass of bottle + brine solution = 54 g
Answer: Given,
Mass of empty density bottle = 22 g,
Mass of density bottle with water = 50 g.
Mass of density bottle with brine solution= 54 g.
Firstly we calculate, density of brine solution. For this we have to calculate volume of bottle.
The mass of water in bottle = ( Mass of density bottle with water) – (Mass of empty density bottle)
The mass of water in bottle mass = 50 – 22
The mass of water in bottle = 28 g.
We know that, Density of water = 1 g/cm³.
As,
Density = mass /volume
Volume = mass × density
Volume = 28 × 1
Volume = 28 cm³.
The volume of density bottle is 28 cm³ or 28mL.
As,
Density of brine solution = mass of brine solution/volume
Density of brine solution = (mass of bottle with brine solution- mass of empty bottle/ volume.
Density of brine solution = (54-22)/28
Density of brine solution =32/28
Density of brine solution =1.14 g/cm³.
The density of brine solution is 1.14 g/cm³.
Now we calculate Relative density.
As,
Relative density of brine solution= mass of brine solution/ mass of equal volume of water.
Relative density of brine solution= 32/28
Relative density of brine solution= 1.14
The relative density of brine solution = 1.14.
11) The mass of an empty density bottle is 30 g. it is 75 g when filled completely with water and 65 g when filled completely with a liquid. Find
(a) volume of density bottle,
(b) density of liquid, and
(c) relative density of liquid.
Answer: Given,
Mass of empty density bottle = 30 g,
Mass of density bottle with water = 75 g.
Mass of density bottle with liquid= 65 g..
a)
The mass of water in bottle = (mass of Mass of density bottle with water) – (Mass of empty density bottle)
The mass of water in bottle = 75 – 30
The mass of water in bottle = 45 g.
We know that, Density of water = 1 g/cm³.
As,
Density = mass /volume
Volume = mass × density
Volume = 45 × 1
Volume = 45 cm³.
The volume of density bottle is 45 cm³ or 45mL.
b) Now we calculate density of liquid.
density of liquid = mass of liquid/volume
Density of liquid= (mass of bottle with liquid- mass of empty bottle/ volume.
Density of liquid= (65- 30)/45
Density of liquid =35/45
Density of liquid=0.77 g/cm³.
The density of liquid is 0.77 g/cm³.
c)
We know that,
Relative density of liquid = mass of liquid/ mass of equal volume of water.
Relative density of liquid = 35/45
Relative density of liquid = 0.77
The relative density of liquid is 0.77