ICSE Selina Solution for Class 9 Chemistry Chapter 7 given in this document serves as the best study material for ICSE class 9 Chemistry exam preparation. This page has answers to all the exercise questions provided in the Selina publishers class 9 Chemistry textbook. Answers provided here are written by highly experienced teachers at BYJU’S.
ICSE Selina solutions for class 9 will help you lay a strong foundation of the concepts involved in class 9 syllabus. Detailed answers provided here will help to clear all the concepts involved in the syllabus. Exercise questions have MCQ’S, Short and long answer questions, numerical problems and questions based on formula derivations which are very important from the point of examinations.
Chapter 7 Study of Gas Laws is an important chapter of ICSE class 9 Chemistry syllabus, as it carries good weightage of marks in the examination. This chapter also serves as the basic chapter required to study Physical Chemistry topics in future. This document has a variety of questions like MCQ, filling the blanks, questions based on derivations and numerical problems.
ICSE Selina solutions for class 9 chemistry provided here explains the behaviour of gases under different temperatures and pressure. This solution page has questions regarding Charles law, Boyle’s law, mathematical calculation involved in Charle’s and Boyle’s law, the relationship between Kelvin and Celsius scale of temperature and their conversions.
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Exercise :7
1. What do you understand by gas ?
Solution:
Gas is a physical state of matter in which interparticle attraction is weak and interparticle space is so large. Because of this gas particles become completely free to move randomly in the entire available space.
2. Give the assumptions of the kinetic molecular theory
Solution:
i) Gases are made of tiny particles which move in all possible directions at all possible speeds. The size of molecules is small as compared to the volume of the occupied gas.
ii) There is no force of attraction between gas particles or between the particles and the walls of the container. So, the particles are free to move in the entire space available to them.
iii) The moving particles of gas collide with each other and with the walls of the container. Because of these collisions, gas molecules exert pressure. Gases exert the same pressure in all directions.
iv) There is large interparticle space between gas molecules, and this accounts for high compressibility of gases.
v) Volume of a gas increases with a decrease in pressure and increase in temperature.
vi) Gases have low density as they have large intermolecular spaces between their molecules.
vii) Gases have a natural tendency to mix with one and other because of large intermolecular spaces. So, two gases when mixed form a homogeneous gaseous mixture.
viii) The intermolecular space of a gas is reduced because of cooling. Molecules come closer resulting in liquefaction of the gas.
3. During the practical in the lab when hydrogen sulphide gas having offensive odour is prepared for some test, we can smell the gas even 50 metres away. Explain phenomenon.
Solution:
We can smell the gas even 50 meters away because of the diffusion of gas molecules allow thorough mixing of the gas.
4. What is diffusion ? Give an example to illustrate
Solution:
Diffusion is a process of mixing of two substanecs kept in contact. Diffusion is due to molecular motion.
If a jar of chlorine is opened in a large room, the odorous gas mixes with air and spreads to every part of the room. Although chlorine is heavier than air, it does not remain at the floor level but spreads throughout the room.
5. How is molecular motion related with temperature.
Solution:
Molecular motion is directly proportional to temperature. Hence it affects kinetic energy of the molecule.
6. State (i) the three variables for gas laws ii) S.I units of these variables.
Solution:
i) Three variables for gas laws: Volume (V), Pressure (P), Temperature (T)
ii) SI units of these variables:
Volume: Cubic metre (m^{3})
Pressure: Pascal (Pa)
Temperature: Kelvin (K)
7. (a) State Boyle’s Law.
(b) Give its
(i) mathematical expression,
(ii) graphical representation and
(iii) significance.
Solution:
a) Boyle’s law states that the volume of a given mass of a dry gas is inversely proportional to its pressure at a constant temperature.
b)
i. 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
ii. Graphical representation of Boyle’s Law:
1 : Variation in volume (V) plotted against (1/P) at a constant 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 the Xaxis is obtained.
iii) Significance of Boyle’s Law
On increasing pressure. volume decreases. The gas becomes denser. Thus at a constant temperature, the density of a gas is directly proportional to its pressure.
Atmospheric pressure is low at high altitudes, so air is less dense. Hence, a lesser quantity of oxygen is available for breathing. This is the reason why mountaineers have to carry oxygen cylinders with them.
8. Explain Boyle’s law on the basis of kinetic theory of matter.
Solution:
The kinetic theory states that the number of molecules present in a given mass and the average possessed by the particles is constant.
If the volume of a given mass of dry gas is reduced to half its original volume, the same number of particles will have half the space to move. As a result, the number of molecules striking at a unit area of the walls of the container at a given time will get doubled. Conversely, if the volume of a given mass of a gas is doubled at a constant temperature, the same number of molecules will have double the space to move about. Consequently, the number of molecules striking at a unit area of the walls of the container at a given time will become one half of the original value. Thus, the pressure of the gas will be reduced to half of its original pressure. Hence it is observed that if pressure increases, the volume of a given mass of gas decreases at a constant temperature.
9 The molecular theory states that the pressure exerted by a gas in closed vessel results from the gas molecules striking’ against the walls of the vessel. How will the pressure change if :
(a) the temperature is doubled keeping the volume constant
(b) the volume is made half of its original value keeping the temperature constant?
Solution:
a) Pressure will double.
b) Pressure remains the same.
10.
 State Charles law
 Give its
(i) Graphical representation,
(ii) mathematical expression and
(iii) Significance.
Solution:
a)
b)
i) Graphical representation of Charles’s 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.
ii)
V αT
Since V and T vary directly, we can equate them by making use of a constant k.
VT=constant=k
The value of k depends on the pressure of the gas, the amount of the gas and also on the unit of the volume.
VT = k ———– (I)
Let V1 and T1 be the initial volume and temperature of an ideal gas. We can write the equation I as:
V1T1=k ———– (II)
Let’s change the temperature of the gas to T2. Consequently, its volume changes to V2. So we can write,
V2T2=k ———– (III)
Equating equations (II) and (III),
V1T1=V2T2=k
Hence, we can generalize the formula and write it as:
(V1)(T1)=(V2)(T2)
Or
V1T2=V2T1
iii)
Significance of Charles’s Law: The volume of a given mass of a gas is directly proportional to its temperature; hence, the density decreases with temperature. This is the reason that
(a) Hot air is filled in balloons used for meteorological purposes. (b) Cable wires contract in winters and expand in summers.
11. Explain Charles’s law on the basis of the kinetic theory of matter.
Solution:
According to the kinetic theory of matter, the average kinetic energy of gas molecules is directly proportional to the absolute temperature. Thus, when the temperature of a gas is increased, the molecules would move faster and the molecules will strike the unit area of the walls of the container more frequently and vigorously. If the pressure is kept constant, the volume increases proportionately. Hence, at constant pressure, the volume of a given mass of a gas is directly proportional to the temperature (Charles’s law).
12. Define absolute zero and absolute scale of temperature. Write the relationship between °C and K.
Solution:
Absolute zero is – 273 °C. The value on the Celsius scale can be converted to the Kelvin scale by adding 273 to it.
Example: 20°C = 20 + 273 = 293 K .
13. (a) What is the need for the Kelvin scale of temperature?
(b) What is the boiling point of water on the Kelvin scale? Convert it into a centigrade scale.
Solution:
The behavior of gases cannot be express in temperature below 273.15°C. Hence there was need of Kelvin scale to express behaviour.
Boiling point of water on the Kelvin scale is 373 K.
In Centigare 372273= 100 °C
14. (a) Define S.T.P.
(b) Why is it necessary to compare gases at S.T.P.?
Solution:
The temperature of 0°C or 273 K, the pressure of 76mm Hg or 1 atm is considered as standard temperature and pressure.
It is necessary to compare gases at STP because the volume of a gas changes remarkably with a change in temperature.
15. Write the value of
a) Standard temperature in
i) °C ii) K
b) Standard pressure in
i) atm ii) mmHg iii) cm Hg iv) torr
Solution:
Standard temperature i) 0°C ii)273 K
b) Standard pressure i) 1 atmospheric pressure (atm) ii) 760 mmHg iii) 76 cm of Hg iv) 101325760 pascals
16. What is the relationship between the Celsius and the Kelvin scales of temperature?
Solution:
Temperature on the Kelvin scale (K)
= 273 + temperature on the Celsius scale
Or K = 273 + °C
(i) 273°C in Kelvin
t°C = t K – 273
273°C = t K – 273
t K = 273 + 273 = 546 K
∴ 273°C = 546 K
(ii) 293 K in °C
t°C = 293 – 273
t°C = 20°C
∴ 293 K = 20°C
17. State the laws which are represented by the following graphs.
Solution:
a) Charles’s law
b) Boyle’s law
18. Give reasons for the following
 All temperatures in the absolute (Kelvin) scale are in positive figures.
 Gases have lower density compared to that of solids or liquids.
 Gases exert pressure in all directions.
 It is necessary to specify the pressure and temperature of gas while stating its volume.
 Inflating balloon seems to violate Boyle’s law.
 Mountaineers carry oxygen cylinders with them.
 Gas fills completely the vessel in which it is kept.
Solution:
a) The behaviour of gases cannot be express in temperature below 273.15°C. Hence Kelvin scale is expressed in positive figures.
b) Gases have lower density compared to that of solids or liquids because the mass of a gas per unit volume is small due to large intermolecular spaces between the molecules.
c) At a given temperature, the number of molecules of gas striking against the walls of the container per unit time per unit area is the same. Thus, gases exert the same pressure in all directions.
d) It is necessary to specify the pressure and temperature of gas while stating its volume because the volume of a gas changes remarkably with a change in temperature and pressure, it becomes necessary to choose a standard value of temperature pressure.
e) According to Boyle’s law V ∝
When a balloon is inflated, the pressure inside the balloon decreases, and according to Boyle’s law, the volume of the gas should increase, but it will decrease violating Boyle’s law.
f) Atmospheric pressure is low at high altitudes. The volume of air increases and air becomes less dense because volume is inversely proportional to density. Hence, lesser volume of oxygen is available for breathing. Thus, mountaineers have to carry oxygen cylinders with them.
f) Mountaineers carry oxygen cylinders with them because in high altitudes atmospheric pressure is low which makes air less dense. This makes the volume of Oxygen for breathing very less.
g) Gas fills completely the vessel in which it is kept because interparticle attraction is weak and interparticle space is large in gases which makes particles completely free to move randomly in the entire available space.
19. How did Charles’s law lead to the concept of absolute scale of temperature ?
Solution:
Initially, Charles conducted some experiments to plot the variation of volume of a gas with the temperature, to study the relation between them when the other factors were kept constant ( pressure, no of moles etc). While doing so, he plot a graph between V and T, with volume in SI units and Temperature in Celsius. This turned out to be a straight line with a positive slope, but it cut the x axis at a point in the negative axis. This implied that in Celsius scale, zero volume could only be attained at that low temperature which turned out to be approximately – 273 Celsius. Hence, a new scale was adopted called Kelvin scale with – 273 C being the 0 of the Kelvin scale. This marked the absolute temperature which is the lowest temperature
20. What is meant by aqueous tension ? How is the pressure exerted by a gas corrected to account for aqueous tension ?
Solution:
When the gas is collected over water, it is moist and contains water vapour. The tptal pressure exerted by moist gas is equal to the sum of the partial pressure of the dry gas and the pressure exerted by water vapour. Partial pressure of water vapour is called as aqueous tension.
P_{total} = P_{gas }+ P_{water vapour}
P_{gas }= P_{total}– P_{water vapour}
Actual pressure of gas = Total pressure – Aqueous tension
21. State the following (a) Volume of a gas at 0 Kelvin.
(b) Absolute temperature of a gas at 7°C.
(c) Gas equation.
(d) Ice point in absolute temperature.
(e) S.T.P. conditions
Solution:
a) Volume of gas is zero.
b) Absolute temperature is 7 + 273 = 280 K.
c) The gas equation is
d) Ice point = 0 + 273 = 273 K
e) Standard temperature is 273 K or O°C.
Standard pressure is 1 atmosphere (atm) or 760 mmHg.
22. Choose the correct answer (a) The graph of PV vs P for a gas is
(i) parabolic
(ii) hyperbolic
(iii) a straight line parallel to Xaxis
(iv) a straight line passing through origin
(b) The absolute temperature value that corresponds t 27°C is
(i) 200K
(ii) 300K
(iii) 400K
(iv)246K
c) VolumeTemperature relationship is given by
(i) Boyle
(ii) Gay Lussac
(iii) Dalton
(iv) Charles
(d) If the pressure is doubled for a fixed mass of gas, its volume will become
(i) 4 times
(ii)1/2 times
(iii)2 times
(iv) No change
Solution:
a) (iii) Straight line parallel to the Xaxis.
b) (ii) 27°C = 27 + 273 = 300 K
c) (iv) Charles
d) (ii) 1/2 times
23. Match the following
Column A 
Column B 
a) Cm^{3} 
i) Pressure 
b) Kelvin 

c) Torr 
iii) Volume 
d) Boyle’s law 
iv) = 
e) Charles law 
v) = 
vi)temperature 
Solution:
Column A 
Column B 
a) Cm^{3} 
iii) Volume 
b) Kelvin 
vi)temperature 
c) Torr 
i) Pressure 
d) Boyle’s law 
ii) PV= P_{1}V_{1} 
e) Charles law 
iv) = = 
24. Correct the following statements.
(a) Volume of a gas is inversely proportional to its pressure at constant temperature.
(b) Volume of a fixed mass of a gas is directly proportional to its temperature, pressure remaining constant.
(c) 0°C is equal to zero Kelvin.
(d) Standard temperature is 25°C.
(e) Boiling point of water is 273 K.
Solution:
a) Volume of a given mass of a dry gas is directly proportional to its absolute temperature , if the pressure is kept constant.
b) Volume of a fixed mass of a gas is inversely proportional to its temperature, pressure remaining constant.
c) 0°C is equal to 273 Kelvin
d) Standard temperature is 0°C or 273 K.
e) Boiling point of water is 373 K.
25, Fill in the blanks
(a) The average kinetic energy of the molecules of is proportional to the ………….
(b) The temperature on the Kelvin scale at which molecular motion completely ceases is called ……….
(c) If temperature is reduced to half, …….would, also reduce to half.
(d) The melting point of ice is ……….. Kelvin.
Solution:
(a) The average kinetic energy of the molecules of is proportional to the Absolute temperature.
(b) The temperature on the Kelvin scale at which molecular motion completely ceases is called Absolute zero
(c) If temperature is reduced to half volumewould, also reduce to half.
(d) The melting point of ice is 273 Kelvin.
Important topics covered in ICSE Selina Solution for Class 9 Chemistry Chapter 7
 Behaviour and characteristic properties of Gases
 Molecular motion: Relationship of Temperature, Pressure and Volume
 Gas Laws
 Pressure and Volume Relationship in gases
 Temperature Volume relationship in gases
 Absolute Zero
 Absolute or Kelvin Scale of Temperature
 Conversion of temperature from Celsius scale to Kelvin scale and viceversa
 The Gas equation
 Standard temperature and pressure
 Effect of moisture or pressure
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Frequently Asked Questions on Study of Gas laws
What do you understand by gas ?
Gas is a physical state of matter in which interparticle attraction is weak and interparticle space is so large. Because of this gas particles become completely free to move randomly in the entire available space.
What is diffusion ?
Diffusion is a process of mixing of two substanecs kept in contact. Diffusion is due to molecular motion.
How is molecular motion related with temperature?
Molecular motion is directly proportional to temperature. Hence it affects kinetic energy of the molecule.
State the three variables for gas laws?
Three variables for gas laws: Volume (V), Pressure (P), Temperature (T)
State S.I units of these variables?
SI units of these variables:
Volume: Cubic metre (m3)
Pressure: Pascal (Pa)
Temperature: Kelvin (K)
State Boyle’s Law?
Boyle’s law states that the volume of a given mass of a dry gas is inversely proportional to its pressure at a constant temperature.
Explain Boyle’s law on the basis of kinetic theory of matter?
The kinetic theory states that the number of molecules present in a given mass and the average possessed by the particles is constant.
State Charles law?
Charles’ law (also known as the law of volumes) is an experimental gas law that describes how gases tend to expand when heated.
Explain Charles’s law on the basis of the kinetic theory of matter?
According to the kinetic theory of matter, the average kinetic energy of gas molecules is directly proportional to the absolute temperature. Thus, when the temperature of a gas is increased, the molecules would move faster and the molecules will strike the unit area of the walls of the container more frequently and vigorously. If the pressure is kept constant, the volume increases proportionately. Hence, at constant pressure, the volume of a given mass of a gas is directly proportional to the temperature (Charles’s law).
Define S.T.P?
The temperature of 0°C or 273 K, the pressure of 76mm Hg or 1 atm is considered as standard temperature and pressure.