Class 11 Physics Properties of Gases: CBSE Question Bank with Answers

-Dr.Sanjaykumar Pawar 

Properties of Gases Class 11 Physics: Important Questions & Formulas 

Complete CBSE Class 11 Physics study material, formula sheet, and practice questions for  Properties of Gases. 

 

Internal Link 

1. ​
   • ​Kinetic Theory of Gases (Class 11): "Before studying kinetic theory, review the fundamental [Properties of Gases and Pressure Formulas]."
   • ​Thermodynamics Notes:  [Zeroth Law of Thermodynamics and Temperature Scales]."
2. ​
   • Ideal Gas Equation or Boyle's Law to your detailed Kinetic Theory of Gases post.
   • ​ thermal conduction/radiation terms to your Thermal Properties of Matter article.

Class 11 Physics — Properties of Gases

CBSE Exam-Oriented Question Bank with Complete Solutions

A. Multiple Choice Questions (MCQs)

1. The SI unit of pressure is:

A. Newton

B. Pascal

C. Joule

D. Watt

Answer: B. Pascal

2. Pressure is defined as:

A. Force × Area

B. Area/Force

C. Force/Area

D. Mass/Area

Answer: C. Force/Area \[ \boxed{p=\frac FA} \]

3. The pressure exerted by a gas is mainly due to:

A. Weight of molecules

B. Collisions of molecules with container walls

C. Size of molecules

D. Colour of gas

Answer: B. Collisions of molecules with container walls

4. The relation between pressure and area for a constant force is:

A. \(p\propto A\)

B. \(p\propto A^2\)

C. \(p\propto 1/A\)

D. \(p\propto \sqrt A\)

Answer: C. \(p\propto 1/A\)

5. The pressure due to a liquid column of height \(h\) is:

A. \(\rho gh\)

B. \(\rho g/h\)

C. \(gh/\rho\)

D. \(\rho h/g\)

Answer: A. \(\rho gh\)

6. Which quantity is not a state variable of a gas?

A. Pressure

B. Volume

C. Temperature

D. Colour

Answer: D. Colour

7. The equation of an ideal gas is:

A. \(pV=nRT\)

B. \(pT=nRV\)

C. \(p=nRTV\)

D. \(V=nRpT\)

Answer: A. \(pV=nRT\)

8. The SI unit of thermodynamic temperature is:

A. °C

B. °F

C. K

D. J

Answer: C. K

9. \(0^\circ C\) is equal to:

A. 0 K

B. 100 K

C. 273.15 K

D. 373.15 K

Answer: C. 273.15 K

10. Absolute zero is:

A. \(0^\circ C\)

B. \(273.15^\circ C\)

C. \(-273.15^\circ C\)

D. \(100^\circ C\)

Answer: C. \(-273.15^\circ C\)

11. Two bodies are in thermal equilibrium when:

A. Their masses are equal

B. Their pressures are equal

C. Their temperatures are equal

D. Their volumes are equal

Answer: C. Their temperatures are equal

12. The Zeroth Law of Thermodynamics forms the basis of:

A. Barometer

B. Thermometer

C. Ammeter

D. Voltmeter

Answer: B. Thermometer

13. A wall that allows heat to pass through it is called:

A. Adiabatic

B. Diathermic

C. Rigid

D. Elastic

Answer: B. Diathermic

14. A wall that does not allow heat transfer is called:

A. Diathermic

B. Adiabatic

C. Conducting

D. Flexible

Answer: B. Adiabatic

15. Standard atmospheric pressure is approximately:

A. \(1.013\times10^3\text{ Pa}\)

B. \(1.013\times10^4\text{ Pa}\)

C. \(1.013\times10^5\text{ Pa}\)

D. \(1.013\times10^6\text{ Pa}\)

Answer: C. \(1.013\times10^5\text{ Pa}\)

B. Very Short Answer Questions

1. What is a gas?

Answer: A gas is a state of matter having neither a fixed shape nor a fixed volume.

2. What are the four variables used to describe the state of a gas?

Answer: Pressure \(p\), volume \(V\), temperature \(T\), and amount of substance \(n\).

3. Define pressure.

Answer: Pressure is the normal force acting per unit area. \[ p=\frac FA \]

4. What is the SI unit of pressure?

Answer: Pascal (Pa).

5. What is one pascal?

Answer: \(1\text{ Pa} = 1\text{ N m}^{-2}\)

6. What causes gas pressure?

Answer: Continuous collisions of gas molecules with the walls of the container.

7. What is atmospheric pressure?

Answer: The pressure exerted by Earth's atmosphere on surfaces.

8. What is a barometer?

Answer: An instrument used to measure atmospheric pressure.

9. What is standard atmospheric pressure?

Answer: \(1\text{ atm} = 1.01325\times10^5\text{ Pa}\)

10. What is mechanical equilibrium?

Answer: The condition in which there is no net mechanical motion of a movable boundary due to pressure difference.

11. What is thermal equilibrium?

Answer: A state in which two bodies in thermal contact have the same temperature and no net heat flows between them.

12. What is the SI unit of temperature?

Answer: Kelvin (K).

13. What is absolute zero?

Answer: The lowest limiting temperature, equal to \(0\text{ K}\) or approximately \(-273.15^\circ\text{C}\).

14. What is a diathermic wall?

Answer: A wall that allows heat transfer through it.

15. What is an adiabatic wall?

Answer: A wall that does not allow heat transfer through it.

C. Short Answer Questions

1. Explain why gases exert pressure.

Answer:
Gas molecules are continuously moving randomly. They repeatedly collide with the walls of the container. During these collisions, momentum is transferred to the walls, producing force. Force per unit area is pressure. \[ \boxed{p=\frac FA} \]

2. Why does a sharp needle produce greater pressure?

Answer:
Pressure is given by: \[ p=\frac FA \] For the same force, a smaller area produces greater pressure. A needle has a very small tip area, so it produces large pressure.

3. Explain mechanical equilibrium.

Answer:
Consider two gases separated by a movable piston. If their pressures are different, the piston moves toward the lower-pressure side. When the pressures become equal, the piston stops. This condition is called mechanical equilibrium. \[ \boxed{p_1=p_2} \]

4. Explain the working principle of a mercury barometer.

Answer:
A tube filled with mercury is inverted into a mercury reservoir. Mercury falls until the pressure due to the mercury column balances atmospheric pressure. At standard atmospheric pressure, the mercury column is approximately 760 mm high.
Thus: \[ \boxed{1\text{ atm} = 760\text{ mmHg}} \]

5. Derive \(p=\rho gh\).

Answer:
Consider a liquid column of area \(A\) and height \(h\).

Volume: \(V=Ah\)

Mass: \(m=\rho V=\rho Ah\)

Weight (Force): \(F=mg=\rho Ahg\)

Pressure: \(p=\frac FA\)

Therefore: \(p=\frac{\rho Ahg}{A}\)

Hence: \[ \boxed{p=\rho gh} \]

6. Differentiate between diathermic and adiabatic walls.

Answer:

Diathermic Wall	Adiabatic Wall
Allows heat transfer	Does not allow heat transfer
Thermally conducting	Thermally insulating
Example: Metal wall	Example: Ideal insulating wall

7. State the Zeroth Law of Thermodynamics.

Answer:
If body A is in thermal equilibrium with body B, and body B is in thermal equilibrium with body C, then A is also in thermal equilibrium with C.

8. Why is the Kelvin scale called an absolute scale?

Answer:
The Kelvin scale begins at absolute zero (\(0\text{ K}\)), the lowest limiting temperature. Therefore it is called an absolute temperature scale.

D. Numerical Short Questions

1. A force of 100 N acts on an area of \(2\text{ m}^2\). Find pressure.

\[ p=\frac FA \] \[ p=\frac{100}{2} \] \[ \boxed{p=50\text{ Pa}} \]

2. Convert \(27^\circ\text{C}\) into kelvin.

\[ T=\theta+273.15 \] \[ T=27+273.15 \] \[ \boxed{T=300.15\text{ K}} \quad (\text{or } \boxed{300\text{ K}}) \]

3. Convert 300 K into Celsius.

\[ \theta=T-273.15 \] \[ \theta=300-273.15 \] \[ \boxed{\theta=26.85^\circ\text{C}} \]

4. Calculate the pressure at a depth of 5 m in water. Take \(\rho=1000\text{ kg/m}^3\) and \(g=9.8\text{ m/s}^2\).

\[ p=\rho gh \] \[ p=1000\times9.8\times5 \] \[ \boxed{p=4.9\times10^4\text{ Pa}} \]

E. Long Answer Questions

1. Explain the concept of pressure in gases and derive the pressure formula.

Answer:
A gas consists of a very large number of molecules moving continuously and randomly. These molecules collide with the walls of the container. Each collision produces a small force on the wall. The combined effect of a huge number of collisions produces a measurable force.

Pressure is defined as force per unit area:

\[ \boxed{p=\frac FA} \]

Thus, pressure depends on:

• Magnitude of force
• Area over which the force acts

For a fixed force: \(p\propto\frac1A\)

Therefore, decreasing the area increases pressure.

2. Explain thermal equilibrium and the Zeroth Law of Thermodynamics.

Answer:
When two bodies at different temperatures are brought into thermal contact, heat flows from the hotter body to the colder body. This continues until their temperatures become equal.

At this stage: \(T_1=T_2\) and there is no net heat transfer. The bodies are said to be in thermal equilibrium.

The Zeroth Law states:
If A is in thermal equilibrium with B, and B is in thermal equilibrium with C, then A is in thermal equilibrium with C.

The law provides the basis for measuring temperature using thermometers.

3. Explain the Kelvin temperature scale and its relation with Celsius scale.

Answer:
The Kelvin scale is the SI thermodynamic temperature scale. Its unit is kelvin (K).

The lowest limiting temperature is absolute zero: \(0\text{ K} = -273.15^\circ\text{C}\)

The relationship is:

\[ \boxed{T(\text{K})=\theta(^\circ\text{C})+273.15} \]

For example: \(25^\circ\text{C} = 298.15\text{ K}\)

The size of one kelvin is equal to the size of one Celsius degree for temperature differences.

F. Assertion and Reason

Options:

A. Both Assertion and Reason are true, and Reason correctly explains Assertion.
B. Both are true, but Reason does not correctly explain Assertion.
C. Assertion is true, but Reason is false.
D. Assertion is false, but Reason is true.

---

1. Assertion: A sharp needle produces greater pressure than a blunt needle for the same applied force.
Reason: Pressure is inversely proportional to area.

Answer: A

2. Assertion: Gas pressure is produced by molecular collisions with the container walls.
Reason: Gas molecules are continuously moving randomly.

Answer: A

3. Assertion: \(0^\circ\text{C}\) is equal to \(0\text{ K}\).
Reason: Kelvin and Celsius scales have different zero points.

Answer: D

4. Assertion: Thermal equilibrium requires equal temperatures.
Reason: There is no net heat transfer between bodies in thermal equilibrium.

Answer: A

5. Assertion: Kelvin temperature must be used in the ideal gas equation.
Reason: The ideal gas equation uses absolute temperature.

Answer: A

6. Assertion: Pressure at the same depth in a liquid does not depend directly on the shape of the container.
Reason: Hydrostatic pressure is given by \(p=\rho gh\).

Answer: A

7. Assertion: An adiabatic wall allows heat to flow freely.
Reason: An adiabatic wall is thermally insulating.

Answer: D

8. Assertion: A barometer is used to measure atmospheric pressure.
Reason: Atmospheric pressure supports a column of mercury in a mercury barometer.

Answer: A

G. Fill in the Blanks

1. The SI unit of pressure is ________.
   Answer: Pascal
2. Pressure is force divided by ________.
   Answer: Area
3. The pressure due to a liquid column is ________.
   Answer: \(\rho gh\)
4. The SI unit of temperature is ________.
   Answer: Kelvin
5. \(0^\circ\text{C}\) is equal to ________ K.
   Answer: 273.15
6. Absolute zero is ________ K.
   Answer: 0
7. A wall that allows heat transfer is called a ________ wall.
   Answer: Diathermic
8. A wall that prevents heat transfer is called an ________ wall.
   Answer: Adiabatic
9. Atmospheric pressure is measured using a ________.
   Answer: Barometer
10. Standard atmospheric pressure is approximately ________ Pa.
    Answer: \(1.013\times10^5\)
11. The ideal gas equation is ________.
    Answer: \(pV=nRT\)
12. The universal gas constant is approximately ________.
    Answer: \(8.314\text{ J mol}^{-1}\text{K}^{-1}\)
13. Thermal equilibrium occurs when the temperatures are ________.
    Answer: Equal
14. Gas pressure is caused by molecular ________ with the container walls.
    Answer: Collisions
15. \(1\text{ bar}\) equals ________ Pa.
    Answer: \(10^5\)

H. Statement-Based Questions

Question 1

Consider the following statements:

1. Gas molecules are continuously moving.
2. Gas molecules exert pressure on container walls.
3. Gas has a fixed volume.
4. Gas molecules are widely separated.

Which statements are correct?

A. 1 and 2 only

B. 1, 2 and 4

C. 2 and 3 only

D. 1, 3 and 4

Answer: B

Question 2

Consider the following statements:

1. Pressure increases when area decreases for constant force.
2. Pressure is measured in pascal.
3. Pressure is a vector quantity.
4. \(p=F/A\).

Correct statements are:

A. 1, 2 and 4

B. 1 and 3

C. 2 and 3

D. All

Answer: A

Question 3

Consider the following statements:

1. Kelvin is an absolute scale.
2. \(0\text{ K} = -273.15^\circ\text{C}\).
3. \(0^\circ\text{C} = 273.15\text{ K}\).
4. \(25^\circ\text{C} = 25\text{ K}\).

Correct statements are:

A. 1 and 2

B. 1, 2 and 3

C. 2 and 4

D. All

Answer: B

I. Match the Columns

Question 1

Column A	Column B
A. Pressure	1. \(p=\rho gh\)
B. Hydrostatic pressure	2. Kelvin
C. Temperature SI unit	3. \(p=F/A\)
D. Ideal gas	4. \(pV=nRT\)

Answer: A → 3, B → 1, C → 2, D → 4

Question 2

Column A	Column B
A. Barometer	1. No heat transfer
B. Diathermic wall	2. Atmospheric pressure
C. Adiabatic wall	3. Heat can pass
D. Thermal equilibrium	4. Same temperature

Answer: A → 2, B → 3, C → 1, D → 4

J. Case Study-Based Questions

Case Study 1: Gas in a Cylinder

A gas is enclosed in a cylinder fitted with a movable piston. The molecules of the gas move randomly and continuously. When the gas is heated, the molecules acquire greater average kinetic energy. Depending on whether the piston is free or fixed, the volume or pressure may change.

1. What produces pressure in the gas?

A. Molecular collisions with the walls

B. Gravity only

C. Temperature only

D. Volume only

Answer: A

2. If the piston is movable and pressure remains constant, heating the gas generally causes:

A. Decrease in volume

B. Increase in volume

C. No change in volume

D. Zero pressure

Answer: B

3. Which equation describes an ideal gas?

A. \(pV=nRT\)

B. \(p=n/V\)

C. \(V=pT\)

D. \(pV=RT/n\)

Answer: A

4. What temperature scale should be used in the ideal gas equation?

Answer: Kelvin scale.

Case Study 2: Mercury Barometer

A mercury barometer is used to measure atmospheric pressure. A mercury column of approximately 760 mm is supported by atmospheric pressure under standard conditions.

1. The barometer measures:

A. Gas temperature

B. Atmospheric pressure

C. Volume

D. Density only

Answer: B

2. Standard atmospheric pressure corresponds approximately to:

A. 76 mmHg

B. 760 mmHg

C. 7600 mmHg

D. 7.6 mmHg

Answer: B

3. The pressure of a liquid column is given by:

\[ \boxed{p=\rho gh} \]

4. If the density of the liquid increases while height remains constant, pressure will:

A. Decrease

B. Increase

C. Become zero

D. Remain necessarily unchanged

Answer: B

K. Important 2-Mark Questions

1. Write two characteristics of gas molecules.

Answer:

1. They are in continuous random motion.
2. They are widely separated compared with molecules in liquids and solids.

2. Write two differences between heat and temperature.

Answer:

Heat	Temperature
Energy transferred because of temperature difference.	Indicates thermal state of a body.
Transfer occurs from higher to lower temperature.	Determines the direction of heat transfer.

3. State two applications of pressure.

Answer:

1. Sharp needles and knives use small surface areas to produce large pressure.
2. Hydraulic and pneumatic systems utilize uniform pressure propagation.

L. Important 3-Mark Questions

1. Explain three factors on which liquid pressure depends.

Answer:
From the hydrostatic formula \(p=\rho gh\), pressure depends on:

1. Density (\(\rho\)): Direct proportionality (\(p\propto\rho\))
2. Acceleration due to gravity (\(g\)): Direct proportionality (\(p\propto g\))
3. Vertical depth (\(h\)): Direct proportionality (\(p\propto h\))

2. Explain the significance of the Zeroth Law.

Answer:

1. Defines the fundamental concept of thermal equilibrium.
2. Establishes temperature as a measurable physical property.
3. Provides the operational basis for working principles of thermometers.

M. Important 5-Mark Questions

Question 1: Define pressure. Explain the origin of gas pressure and derive the expression for pressure due to a liquid column.

Answer Framework:

1. Definition: Pressure is defined as normal force acting per unit area: \[ p=\frac FA \]

2. Origin of Gas Pressure: Gas pressure arises from continuous, random elastic collisions of gas molecules against the container walls, transferring momentum per unit time.

3. Derivation for Liquid Column:

Consider a liquid column of surface area \(A\) and height \(h\) with density \(\rho\).

• Volume: \(V = Ah\)
• Mass: \(m = \rho V = \rho Ah\)
• Weight (Force on base): \(F = mg = \rho Ahg\)

Applying the pressure formula:

\[ p = \frac{F}{A} = \frac{\rho Ahg}{A} \implies \boxed{p=\rho gh} \]

Question 2: Explain thermal equilibrium, the Zeroth Law of Thermodynamics, and temperature scales.

Answer Framework:

• Thermal Equilibrium: State with no net heat flow between systems in contact, implying equal temperatures (\(T_1 = T_2\)).
• Zeroth Law: If A is in thermal equilibrium with B, and B is with C, then A is in thermal equilibrium with C.
• Importance: Serves as the fundamental basis for thermometer design.
• Temperature Scales & Conversions:

  Celsius Scale (\(^\circ\text{C}\)) vs Kelvin Scale (\(\text{K}\)).

  \[ \boxed{T = \theta + 273.15} \]

  Absolute zero is defined as \(\boxed{0\text{ K} = -273.15^\circ\text{C}}\).

N. Most Important Formulas for Examination

\[ \boxed{p=\frac FA} \]

\[ \boxed{F=mg} \]

\[ \boxed{p=\rho gh} \]

\[ \boxed{T=\theta+273.15} \]

\[ \boxed{\theta=T-273.15} \]

\[ \boxed{pV=nRT} \]

\[ \boxed{\frac{pV}{T}=\text{constant}} \]

\[ \boxed{pV=\text{const}\quad(T=\text{const})} \]

\[ \boxed{\frac VT=\text{const}\quad(p=\text{const})} \]

\[ \boxed{\frac pT=\text{const}\quad(V=\text{const})} \]

CBSE Exam Revision Priority

For this chapter, students should be particularly comfortable with:

1. \(p=F/A\)
2. Molecular origin of gas pressure
3. Barometer and atmospheric pressure
4. Derivation of \(p=\rho gh\)
5. Mechanical equilibrium
6. Thermal equilibrium
7. Zeroth Law of Thermodynamics
8. Celsius–Kelvin conversion
9. Absolute zero
10. Ideal gas equation \(pV=nRT\)
11. Boyle's, Charles' and pressure-temperature laws
12. Unit conversions and numerical problems