Selina Textbook Solutions: Study of Gas Laws | Chemistry Class 9 ICSE PDF Download

 Page 1


Chapter 2. Study of Gas Laws
Exercise 2
Solution 1.
The state of matter in which inter-particle attraction is weak and inter-particle space is so large that 
the particles become completely free to move randomly in the entire available space, is known as 
gas.
Solution 2.
The main assumption of kinetic molecular theory of gases are as follows:
1. All gases are made up of a large number of extremely small particles called molecules.
2. There are large vacant spaces between the molecules of a gas so that actual volume of the 
molecules of a gas is negligible as compared to the total volume occupied by the gas.
3. The molecules of a gas are always in a state of constant random motion in straight lines in all 
possible direction.
4. There are negligible attractive forces between the molecules of a gas.
5. There is no effect of gravity on the motion of the molecules of a gas.
6. The average kinetic energy of the molecules of a gas is directly proportional to that of the Kelvin 
temperature of the gas.
7. The molecules are perfectly elastic so that there is no net loss of energy during molecular 
collisions.
8. The pressure of a gas is due to the bombardment of the molecules of a gas against the walls of 
a container.
Solution 3.
In a laboratory, when hydrogen sulphide gas is prepared, it can be smelt even at 50 meters away. 
This is due to the phenomenon called Diffusion.
Diffusion is a process of intermixing of two substances kept in contact.
The inter-particle or inter molecular spaces in a gas are very large. When hydrogen sulphide gas is 
produced, its particle collides with air particles. Due to the collisions of particles, they start moving in 
all possible directions. As a result of which the two gases mix with each other forming a 
homogeneous mixture of a gas. Thus, the released gas can be smelt to a long distance.
Solution 4.
Pressure and volume relationship of gases- 
Experiment: Take a 10 ml syringe fitted with a piston. Raise the latter to the 10 ml mark and wrap 
an adhesive tape over its nozzle. Fit the wrapped nozzle tightly into a hole, bored half way through a 
rubber stopper.
Observation: On placing some weight on the piston (to put pressure), the piston moves downward 
and reduces the volume of air. Gradually, put more weight. The piston moves further downward and 
the volume of the air is further reduced.
Now remove the weights one by one. You will notice that, on decreasing the pressure, the piston 
moves upward as such the volume of the air increases.
Conclusion:
Page 2


Chapter 2. Study of Gas Laws
Exercise 2
Solution 1.
The state of matter in which inter-particle attraction is weak and inter-particle space is so large that 
the particles become completely free to move randomly in the entire available space, is known as 
gas.
Solution 2.
The main assumption of kinetic molecular theory of gases are as follows:
1. All gases are made up of a large number of extremely small particles called molecules.
2. There are large vacant spaces between the molecules of a gas so that actual volume of the 
molecules of a gas is negligible as compared to the total volume occupied by the gas.
3. The molecules of a gas are always in a state of constant random motion in straight lines in all 
possible direction.
4. There are negligible attractive forces between the molecules of a gas.
5. There is no effect of gravity on the motion of the molecules of a gas.
6. The average kinetic energy of the molecules of a gas is directly proportional to that of the Kelvin 
temperature of the gas.
7. The molecules are perfectly elastic so that there is no net loss of energy during molecular 
collisions.
8. The pressure of a gas is due to the bombardment of the molecules of a gas against the walls of 
a container.
Solution 3.
In a laboratory, when hydrogen sulphide gas is prepared, it can be smelt even at 50 meters away. 
This is due to the phenomenon called Diffusion.
Diffusion is a process of intermixing of two substances kept in contact.
The inter-particle or inter molecular spaces in a gas are very large. When hydrogen sulphide gas is 
produced, its particle collides with air particles. Due to the collisions of particles, they start moving in 
all possible directions. As a result of which the two gases mix with each other forming a 
homogeneous mixture of a gas. Thus, the released gas can be smelt to a long distance.
Solution 4.
Pressure and volume relationship of gases- 
Experiment: Take a 10 ml syringe fitted with a piston. Raise the latter to the 10 ml mark and wrap 
an adhesive tape over its nozzle. Fit the wrapped nozzle tightly into a hole, bored half way through a 
rubber stopper.
Observation: On placing some weight on the piston (to put pressure), the piston moves downward 
and reduces the volume of air. Gradually, put more weight. The piston moves further downward and 
the volume of the air is further reduced.
Now remove the weights one by one. You will notice that, on decreasing the pressure, the piston 
moves upward as such the volume of the air increases.
Conclusion:
1. An increase in pressure at constant temperature causes a decrease in the volume of a gas; 
conversely, if the volume of a fixed mass of a gas at constant temperature is decreased, the 
pressure of the gas increases.
2. A decrease in pressure at constant temperature causes a increase in the volume of a gas; 
conversely, if the volume of a fixed mass of a gas at constant temperature is increased, the 
pressure of the gas decreases.
Solution 5.
The molecular motion is directly proportional to the temperature. 
As temperature increases, molecular motion increases because the molecule possesses certain kinetic 
energy. And as the temperature decreases, molecular motion also decreases. Thus, when 
temperature is zero, molecular motion stops or ceases.
Solution 6.
The three variables for gas laws are:
1. Volume, V
2. Pressure, P
3. Temperature, T
These three are called as the Standard variables. S.I. unit of volume is cubic meter (m
3
). 
S.I. unit of pressure is Pascal (Pa). 
S.I. unit of temperature is Kelvin (K) or degree Celsius (
0
C).
Solution 7.
Boyle’s law: At constant temperature, the volume of a definite mass of any gas is inversely 
proportional to the pressure of the gas.                       Or 
Temperature remaining constant, the product of the volume and pressure of the given mass of a dry 
gas is constant.
Mathematical representation: 
Page 3


Chapter 2. Study of Gas Laws
Exercise 2
Solution 1.
The state of matter in which inter-particle attraction is weak and inter-particle space is so large that 
the particles become completely free to move randomly in the entire available space, is known as 
gas.
Solution 2.
The main assumption of kinetic molecular theory of gases are as follows:
1. All gases are made up of a large number of extremely small particles called molecules.
2. There are large vacant spaces between the molecules of a gas so that actual volume of the 
molecules of a gas is negligible as compared to the total volume occupied by the gas.
3. The molecules of a gas are always in a state of constant random motion in straight lines in all 
possible direction.
4. There are negligible attractive forces between the molecules of a gas.
5. There is no effect of gravity on the motion of the molecules of a gas.
6. The average kinetic energy of the molecules of a gas is directly proportional to that of the Kelvin 
temperature of the gas.
7. The molecules are perfectly elastic so that there is no net loss of energy during molecular 
collisions.
8. The pressure of a gas is due to the bombardment of the molecules of a gas against the walls of 
a container.
Solution 3.
In a laboratory, when hydrogen sulphide gas is prepared, it can be smelt even at 50 meters away. 
This is due to the phenomenon called Diffusion.
Diffusion is a process of intermixing of two substances kept in contact.
The inter-particle or inter molecular spaces in a gas are very large. When hydrogen sulphide gas is 
produced, its particle collides with air particles. Due to the collisions of particles, they start moving in 
all possible directions. As a result of which the two gases mix with each other forming a 
homogeneous mixture of a gas. Thus, the released gas can be smelt to a long distance.
Solution 4.
Pressure and volume relationship of gases- 
Experiment: Take a 10 ml syringe fitted with a piston. Raise the latter to the 10 ml mark and wrap 
an adhesive tape over its nozzle. Fit the wrapped nozzle tightly into a hole, bored half way through a 
rubber stopper.
Observation: On placing some weight on the piston (to put pressure), the piston moves downward 
and reduces the volume of air. Gradually, put more weight. The piston moves further downward and 
the volume of the air is further reduced.
Now remove the weights one by one. You will notice that, on decreasing the pressure, the piston 
moves upward as such the volume of the air increases.
Conclusion:
1. An increase in pressure at constant temperature causes a decrease in the volume of a gas; 
conversely, if the volume of a fixed mass of a gas at constant temperature is decreased, the 
pressure of the gas increases.
2. A decrease in pressure at constant temperature causes a increase in the volume of a gas; 
conversely, if the volume of a fixed mass of a gas at constant temperature is increased, the 
pressure of the gas decreases.
Solution 5.
The molecular motion is directly proportional to the temperature. 
As temperature increases, molecular motion increases because the molecule possesses certain kinetic 
energy. And as the temperature decreases, molecular motion also decreases. Thus, when 
temperature is zero, molecular motion stops or ceases.
Solution 6.
The three variables for gas laws are:
1. Volume, V
2. Pressure, P
3. Temperature, T
These three are called as the Standard variables. S.I. unit of volume is cubic meter (m
3
). 
S.I. unit of pressure is Pascal (Pa). 
S.I. unit of temperature is Kelvin (K) or degree Celsius (
0
C).
Solution 7.
Boyle’s law: At constant temperature, the volume of a definite mass of any gas is inversely 
proportional to the pressure of the gas.                       Or 
Temperature remaining constant, the product of the volume and pressure of the given mass of a dry 
gas is constant.
Mathematical representation: 
According to Boyle’s Law, 
 
Where K is the constant of proportionality if V’ and P’ are some other volume and pressure of the gas 
at the same temperature then, 
 
temperature, a straight line passing through the origin is obtained. 
 
2. V vs P : Variation in volume (V) plotted against pressure (P) at a constant temperature, a 
hyperbolic curve in the first quadrant is obtained. 
Page 4


Chapter 2. Study of Gas Laws
Exercise 2
Solution 1.
The state of matter in which inter-particle attraction is weak and inter-particle space is so large that 
the particles become completely free to move randomly in the entire available space, is known as 
gas.
Solution 2.
The main assumption of kinetic molecular theory of gases are as follows:
1. All gases are made up of a large number of extremely small particles called molecules.
2. There are large vacant spaces between the molecules of a gas so that actual volume of the 
molecules of a gas is negligible as compared to the total volume occupied by the gas.
3. The molecules of a gas are always in a state of constant random motion in straight lines in all 
possible direction.
4. There are negligible attractive forces between the molecules of a gas.
5. There is no effect of gravity on the motion of the molecules of a gas.
6. The average kinetic energy of the molecules of a gas is directly proportional to that of the Kelvin 
temperature of the gas.
7. The molecules are perfectly elastic so that there is no net loss of energy during molecular 
collisions.
8. The pressure of a gas is due to the bombardment of the molecules of a gas against the walls of 
a container.
Solution 3.
In a laboratory, when hydrogen sulphide gas is prepared, it can be smelt even at 50 meters away. 
This is due to the phenomenon called Diffusion.
Diffusion is a process of intermixing of two substances kept in contact.
The inter-particle or inter molecular spaces in a gas are very large. When hydrogen sulphide gas is 
produced, its particle collides with air particles. Due to the collisions of particles, they start moving in 
all possible directions. As a result of which the two gases mix with each other forming a 
homogeneous mixture of a gas. Thus, the released gas can be smelt to a long distance.
Solution 4.
Pressure and volume relationship of gases- 
Experiment: Take a 10 ml syringe fitted with a piston. Raise the latter to the 10 ml mark and wrap 
an adhesive tape over its nozzle. Fit the wrapped nozzle tightly into a hole, bored half way through a 
rubber stopper.
Observation: On placing some weight on the piston (to put pressure), the piston moves downward 
and reduces the volume of air. Gradually, put more weight. The piston moves further downward and 
the volume of the air is further reduced.
Now remove the weights one by one. You will notice that, on decreasing the pressure, the piston 
moves upward as such the volume of the air increases.
Conclusion:
1. An increase in pressure at constant temperature causes a decrease in the volume of a gas; 
conversely, if the volume of a fixed mass of a gas at constant temperature is decreased, the 
pressure of the gas increases.
2. A decrease in pressure at constant temperature causes a increase in the volume of a gas; 
conversely, if the volume of a fixed mass of a gas at constant temperature is increased, the 
pressure of the gas decreases.
Solution 5.
The molecular motion is directly proportional to the temperature. 
As temperature increases, molecular motion increases because the molecule possesses certain kinetic 
energy. And as the temperature decreases, molecular motion also decreases. Thus, when 
temperature is zero, molecular motion stops or ceases.
Solution 6.
The three variables for gas laws are:
1. Volume, V
2. Pressure, P
3. Temperature, T
These three are called as the Standard variables. S.I. unit of volume is cubic meter (m
3
). 
S.I. unit of pressure is Pascal (Pa). 
S.I. unit of temperature is Kelvin (K) or degree Celsius (
0
C).
Solution 7.
Boyle’s law: At constant temperature, the volume of a definite mass of any gas is inversely 
proportional to the pressure of the gas.                       Or 
Temperature remaining constant, the product of the volume and pressure of the given mass of a dry 
gas is constant.
Mathematical representation: 
According to Boyle’s Law, 
 
Where K is the constant of proportionality if V’ and P’ are some other volume and pressure of the gas 
at the same temperature then, 
 
temperature, a straight line passing through the origin is obtained. 
 
2. V vs P : Variation in volume (V) plotted against pressure (P) at a constant temperature, a 
hyperbolic curve in the first quadrant is obtained. 
 
3. PV vs P : Variation in PV plotted against pressure (P) at a constant temperature, a straight line 
parallel to X-axis is obtained. 
Significance of Boyles law:
According to Boyles law, on increasing pressure, volume decreases. The gas becomes denser. Thus, 
at constant temperature, the density of a gas is directly proportional to the pressure.
At higher altitude, atmospheric pressure is low so air is less dense. As a result, lesser oxygen is 
available for breathing. This is the reason that the mountaineers have to carry oxygen cylinders with 
them.
Solution 8.
Explanation of Boyle’s Law on the basis of kinetic theory of matter.
According to kinetic theory of matter, the number of particles present in a given mass and the 
average kinetic energy is constant.
If the volume of given mass of a gas is reduced to half of its original volume. The same number of 
particles will have half space to move.
As a result, the number of molecules striking the unit area of the walls of the container at given time 
will get doubled of the pressure will also get doubled.
Alternatively, if the volume of a given mass of a gas is doubled at constant temperature, same 
number of molecules will have double space to move. Thus, number of molecule striking the unit area 
of the walls of container at a given time will become one half of original value. Thus, pressure will 
also get reduced to half of original pressure. Hence, it is seen that if pressure increases, volume 
of a gas decreases at constant temperature and this is Boyle’s law.
Solution 9.
Page 5


Chapter 2. Study of Gas Laws
Exercise 2
Solution 1.
The state of matter in which inter-particle attraction is weak and inter-particle space is so large that 
the particles become completely free to move randomly in the entire available space, is known as 
gas.
Solution 2.
The main assumption of kinetic molecular theory of gases are as follows:
1. All gases are made up of a large number of extremely small particles called molecules.
2. There are large vacant spaces between the molecules of a gas so that actual volume of the 
molecules of a gas is negligible as compared to the total volume occupied by the gas.
3. The molecules of a gas are always in a state of constant random motion in straight lines in all 
possible direction.
4. There are negligible attractive forces between the molecules of a gas.
5. There is no effect of gravity on the motion of the molecules of a gas.
6. The average kinetic energy of the molecules of a gas is directly proportional to that of the Kelvin 
temperature of the gas.
7. The molecules are perfectly elastic so that there is no net loss of energy during molecular 
collisions.
8. The pressure of a gas is due to the bombardment of the molecules of a gas against the walls of 
a container.
Solution 3.
In a laboratory, when hydrogen sulphide gas is prepared, it can be smelt even at 50 meters away. 
This is due to the phenomenon called Diffusion.
Diffusion is a process of intermixing of two substances kept in contact.
The inter-particle or inter molecular spaces in a gas are very large. When hydrogen sulphide gas is 
produced, its particle collides with air particles. Due to the collisions of particles, they start moving in 
all possible directions. As a result of which the two gases mix with each other forming a 
homogeneous mixture of a gas. Thus, the released gas can be smelt to a long distance.
Solution 4.
Pressure and volume relationship of gases- 
Experiment: Take a 10 ml syringe fitted with a piston. Raise the latter to the 10 ml mark and wrap 
an adhesive tape over its nozzle. Fit the wrapped nozzle tightly into a hole, bored half way through a 
rubber stopper.
Observation: On placing some weight on the piston (to put pressure), the piston moves downward 
and reduces the volume of air. Gradually, put more weight. The piston moves further downward and 
the volume of the air is further reduced.
Now remove the weights one by one. You will notice that, on decreasing the pressure, the piston 
moves upward as such the volume of the air increases.
Conclusion:
1. An increase in pressure at constant temperature causes a decrease in the volume of a gas; 
conversely, if the volume of a fixed mass of a gas at constant temperature is decreased, the 
pressure of the gas increases.
2. A decrease in pressure at constant temperature causes a increase in the volume of a gas; 
conversely, if the volume of a fixed mass of a gas at constant temperature is increased, the 
pressure of the gas decreases.
Solution 5.
The molecular motion is directly proportional to the temperature. 
As temperature increases, molecular motion increases because the molecule possesses certain kinetic 
energy. And as the temperature decreases, molecular motion also decreases. Thus, when 
temperature is zero, molecular motion stops or ceases.
Solution 6.
The three variables for gas laws are:
1. Volume, V
2. Pressure, P
3. Temperature, T
These three are called as the Standard variables. S.I. unit of volume is cubic meter (m
3
). 
S.I. unit of pressure is Pascal (Pa). 
S.I. unit of temperature is Kelvin (K) or degree Celsius (
0
C).
Solution 7.
Boyle’s law: At constant temperature, the volume of a definite mass of any gas is inversely 
proportional to the pressure of the gas.                       Or 
Temperature remaining constant, the product of the volume and pressure of the given mass of a dry 
gas is constant.
Mathematical representation: 
According to Boyle’s Law, 
 
Where K is the constant of proportionality if V’ and P’ are some other volume and pressure of the gas 
at the same temperature then, 
 
temperature, a straight line passing through the origin is obtained. 
 
2. V vs P : Variation in volume (V) plotted against pressure (P) at a constant temperature, a 
hyperbolic curve in the first quadrant is obtained. 
 
3. PV vs P : Variation in PV plotted against pressure (P) at a constant temperature, a straight line 
parallel to X-axis is obtained. 
Significance of Boyles law:
According to Boyles law, on increasing pressure, volume decreases. The gas becomes denser. Thus, 
at constant temperature, the density of a gas is directly proportional to the pressure.
At higher altitude, atmospheric pressure is low so air is less dense. As a result, lesser oxygen is 
available for breathing. This is the reason that the mountaineers have to carry oxygen cylinders with 
them.
Solution 8.
Explanation of Boyle’s Law on the basis of kinetic theory of matter.
According to kinetic theory of matter, the number of particles present in a given mass and the 
average kinetic energy is constant.
If the volume of given mass of a gas is reduced to half of its original volume. The same number of 
particles will have half space to move.
As a result, the number of molecules striking the unit area of the walls of the container at given time 
will get doubled of the pressure will also get doubled.
Alternatively, if the volume of a given mass of a gas is doubled at constant temperature, same 
number of molecules will have double space to move. Thus, number of molecule striking the unit area 
of the walls of container at a given time will become one half of original value. Thus, pressure will 
also get reduced to half of original pressure. Hence, it is seen that if pressure increases, volume 
of a gas decreases at constant temperature and this is Boyle’s law.
Solution 9.
(a) Pressure will be doubled. 
(b) Pressure remains the same.
Solution 10.
Charless Law
At constant pressure, the volume of a given mass of a dry gas increases or decreases by 1/273 of its 
original volume at 00C for each degree centigrade rise or fall in temperature.
 
For Temperature = Conversion from Celsius to Kelvin 
1 K = 
0
C + 273 
For example, 
20
o
C = 20 + 273 = 293 K
Graphical representation of Charles law
T vs V: The relationship between the volume and the temperature of a gas can be plotted on a graph, 
A straight line is obtained. 
Significance of Charles’ Law: Since the volume of a given mass of gas is directly proportional to its 
temperature, hence the density decreases with temperature. This is the reason that:
(a) Hot air is filled in the balloons used for meteorological purposes. (b) Cable wires contract in 
winters and expand in summers.