Heat and Thermodynamics

11. Which of the following are the most favourable conditions for liquefyi

Which of the following are the most favourable conditions for liquefying a gas ?

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Low pressure and high temperature
Low pressure and low temperature
High pressure and high temperature
High pressure and low temperature
This question was previously asked in
UPSC CAPF – 2019
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Liquefying a gas requires bringing its molecules closer together and reducing their kinetic energy so that intermolecular attractive forces become dominant. This is best achieved by applying high pressure (to reduce the space between molecules) and lowering the temperature (to reduce molecular motion).
– High pressure forces gas molecules closer, increasing the likelihood of intermolecular attractions.
– Low temperature reduces the kinetic energy of molecules, allowing attractive forces to overcome disruptive thermal motion.
– Every gas has a critical temperature above which it cannot be liquefied by pressure alone, regardless of how high the pressure is. Liquefaction is only possible at or below the critical temperature.
Below the critical temperature, the substance is called a vapor, and it can be liquefied by applying sufficient pressure. Above the critical temperature, it remains a gas even under high pressure; increasing pressure only increases its density, forming a supercritical fluid above the critical pressure. Therefore, high pressure and low temperature relative to the critical point are the most favorable conditions.

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12. Which one of the following processes is not a part of long-wave

Which one of the following processes is not a part of long-wave radiation?

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Conduction
Scattering
Convection
Radiation
This question was previously asked in
UPSC CAPF – 2018
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Scattering is not typically considered a part of the processes fundamentally defining or transferring energy as long-wave radiation in the Earth’s energy budget, unlike Conduction, Convection, and Radiation (emission/absorption).
Long-wave radiation refers to thermal radiation emitted by the Earth’s surface and atmosphere, primarily in the infrared spectrum. The transfer of this thermal energy within the Earth system involves processes like:
– **Radiation:** Emission and absorption of long-wave electromagnetic waves.
– **Conduction:** Heat transfer through direct contact, significant at the Earth-atmosphere interface.
– **Convection:** Heat transfer through the movement of air or water, driven by temperature differences resulting from radiative heating/cooling.
Scattering is a process where radiation is deflected in different directions by particles or molecules. While radiation of all wavelengths can be scattered, scattering is a dominant process affecting the path of incoming *short-wave* solar radiation (e.g., causing the blue sky). It is not a primary mechanism for the *transfer* or *distribution* of the heat energy associated with outgoing *long-wave* radiation in the same way as conduction, convection, and emission/absorption of radiation are.
The Earth’s energy balance involves the absorption of incoming short-wave solar radiation and the emission of outgoing long-wave terrestrial radiation. Conduction and convection are non-radiative heat transfer mechanisms that move heat absorbed from radiation within the atmosphere and surface.

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13. Lowering the atmospheric pressure on a liquid

Lowering the atmospheric pressure on a liquid

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increases the boiling point of the liquid
lowers the boiling point of the liquid
does not affect the boiling point of the liquid
increases the time required for it to boil
This question was previously asked in
UPSC CAPF – 2018
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The correct answer is B) lowers the boiling point of the liquid.
Boiling point is defined as the temperature at which the vapor pressure of a liquid equals the surrounding atmospheric pressure. When the atmospheric pressure is lowered, the liquid needs to reach a lower vapor pressure to start boiling. Since vapor pressure increases with temperature, a lower required vapor pressure is achieved at a lower temperature. Therefore, lowering the atmospheric pressure on a liquid lowers its boiling point.
This principle is why water boils at a lower temperature at high altitudes (where atmospheric pressure is lower) than at sea level. For example, water boils at 100°C at standard atmospheric pressure (1 atm), but at approximately 93°C in Denver, Colorado (altitude ~1600m), and even lower on top of Mount Everest. Conversely, increasing the pressure (e.g., in a pressure cooker) increases the boiling point of water, allowing food to cook faster.

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14. Which one of the following statements is not correct ?

Which one of the following statements is not correct ?

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The rate of evaporation depends on temperature
The rate of evaporation does not depend on surface area exposed to the atmosphere but on volume of the liquid
The rate of evaporation depends on humidity of the surroundings
The rate of evaporation depends on the wind speed
This question was previously asked in
UPSC CAPF – 2016
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The statement that the rate of evaporation does not depend on surface area exposed to the atmosphere but on volume of the liquid is not correct.
Evaporation is the process where liquid turns into gas below its boiling point. It occurs at the surface of the liquid. The rate of evaporation is influenced by several factors:
A) Temperature: Higher temperature increases the kinetic energy of liquid molecules, making it easier for them to escape the surface. (Correct statement)
B) Surface area exposed: A larger surface area allows more molecules to be at the surface and escape into the atmosphere per unit time, thus increasing the rate of evaporation. Volume of the liquid affects the total amount available for evaporation but not the instantaneous rate of evaporation at a given surface area. (Incorrect statement)
C) Humidity of the surroundings: Lower humidity means the air has a lower concentration of water vapor, creating a larger concentration gradient between the liquid surface and the air, leading to faster evaporation. (Correct statement)
D) Wind speed: Wind blows away the saturated air layer above the liquid surface, replacing it with drier air, which increases the rate of evaporation. (Correct statement)
Evaporation is a crucial part of the water cycle. Other factors influencing evaporation include air pressure and the presence of dissolved substances in the liquid.

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15. Water at 273 K is less effective in cooling than ice at the same tempe

Water at 273 K is less effective in cooling than ice at the same temperature. It is because :

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water is difficult to handle
water at 273 K has less energy than ice at the same temperature
water at 273 K has more energy than ice at the same temperature
water is not a cooling agent
This question was previously asked in
UPSC CAPF – 2016
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Water at 273 K (0°C) is less effective in cooling than ice at the same temperature because water at 273 K has more energy (internal energy) than ice at the same temperature.
When ice at 0°C (273 K) melts into water at 0°C, it absorbs heat from its surroundings. This absorbed heat, known as the latent heat of fusion (approximately 334 kJ/kg for water), is used to break the bonds holding the water molecules in a fixed solid structure and transition to the liquid phase, without causing a change in temperature.
Therefore, water at 0°C possesses this latent heat energy in addition to the energy contained in ice at 0°C.
When ice is used for cooling, it absorbs heat to melt, providing significant cooling due to the latent heat. The resulting water then absorbs further heat as its temperature rises. When water at 0°C is used for cooling, it only absorbs heat as its temperature rises, which provides less cooling capacity compared to the phase change process of ice.
Latent heat is the heat required to change the state of a substance at constant temperature and pressure. Specific heat capacity is the heat required to raise the temperature of a substance by one degree.

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16. According to the principle of energy conservation, when a piston in an

According to the principle of energy conservation, when a piston in an automobile engine compresses the gas in a cylinder, which of the following must occur ?

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Kinetic energy of gas must increase
The gas must undergo a change of state
The gas must undergo a chemical change
None of the above
This question was previously asked in
UPSC CAPF – 2014
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When a piston compresses the gas in a cylinder, work is done on the gas. According to the principle of energy conservation (specifically the First Law of Thermodynamics), this work done on the gas increases its internal energy. For an ideal gas, internal energy is directly related to its temperature, and temperature is a measure of the average kinetic energy of the gas molecules. Therefore, the kinetic energy of the gas must increase.
Work done *on* a system (like compressing gas) increases its internal energy. Increased internal energy in a gas manifests as increased temperature and thus increased average kinetic energy of the molecules.
Compression can lead to a change of state or chemical change under specific conditions (e.g., if the pressure and temperature exceed certain thresholds, or if the compression causes ignition in a fuel mixture). However, an increase in internal energy and kinetic energy is a direct and necessary consequence of work being done *on* the gas during compression, based purely on the principle of energy conservation.

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17. The process by which heat is transmitted from the Sun to the Earth is

The process by which heat is transmitted from the Sun to the Earth is called

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Conduction
Convection
Radiation
Cosmic disturbances
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UPSC CAPF – 2011
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The process by which heat is transmitted from the Sun to the Earth is called Radiation.
Radiation is the transfer of energy through electromagnetic waves, which can travel through a vacuum. The space between the Sun and the Earth is largely a vacuum.
Conduction is the transfer of heat through direct contact, and convection is the transfer of heat through the movement of fluids (liquids or gases). Neither conduction nor convection can efficiently transfer heat across the vacuum of space. The Sun emits energy primarily in the form of electromagnetic radiation (including visible light, infrared, and ultraviolet), which travels to Earth and is absorbed or reflected.

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18. Steam at 100°C is more effective in heating than water at the same tem

Steam at 100°C is more effective in heating than water at the same temperature because

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steam is in the gaseous state and water is in the liquid state
steam has an additional heat known as 'latent heat of vaporization'
water has hydrogen bonds but steam does not
transfer of heat from steam is easier than water
This question was previously asked in
UPSC CAPF – 2011
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The correct option is B. Steam at 100°C is more effective in heating than water at 100°C because steam possesses additional energy in the form of latent heat of vaporization. This is the energy absorbed by water to change its state from liquid to gas at its boiling point (100°C) without a change in temperature. When steam comes into contact with a cooler surface, it condenses back into water at 100°C, releasing this large amount of latent heat, which is then transferred to the surface being heated. Water at 100°C does not have this stored latent heat to release upon changing state.
The question asks why steam at the same temperature as water is a more efficient heating medium. The key concept is latent heat.
The latent heat of vaporization of water at 100°C is approximately 2260 kJ/kg (or 540 cal/g). This means that condensing 1 kg of steam at 100°C to water at 100°C releases 2260 kJ of heat, which is significantly more than the heat released by simply cooling 1 kg of water from 100°C. This large amount of heat released upon condensation makes steam very effective for heating applications like steam engines, industrial processes, and steam burns.

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19. Which among the following is/are the reasons behind using Mercury in t

Which among the following is/are the reasons behind using Mercury in thermometers ?

  • 1. Mercury does not wet the inner sides of the thermometer.
  • 2. It can be seen easily in a thin capillary tube of the thermometer.
  • 3. It is a good conductor of heat.
  • 4. It is non-toxic.

Select the correct answer using the code given below :

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1 only
1 and 2 only
1, 2 and 3
3 and 4
This question was previously asked in
UPSC CAPF – 2010
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The reasons behind using Mercury in thermometers among the given options are that Mercury does not wet the inner sides of the thermometer, it can be seen easily in a thin capillary tube, and it is a good conductor of heat.
Mercury possesses several properties suitable for use in thermometers: 1) It has high surface tension and does not wet (stick to) the glass tube, allowing for accurate readings of the meniscus. 2) It is opaque and reflective, making the column easily visible against the glass. 3) It is a good conductor of heat, which facilitates faster transfer of heat from the object being measured to the mercury, allowing it to quickly reach thermal equilibrium. 4) Mercury is highly toxic, which is a disadvantage, not a reason for its use.
Other properties of mercury that make it suitable for thermometers include a relatively low freezing point (-38.83 °C) and a high boiling point (356.73 °C), giving it a wide range of use, and a uniform coefficient of thermal expansion within its liquid range. However, due to its toxicity, mercury thermometers are being increasingly replaced by alcohol-based or digital thermometers.

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20. Given below are the four cases in which certain heat transfer is takin

Given below are the four cases in which certain heat transfer is taking place :

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  • 1. Ice is melting in a glass full of water
  • 2. Water is boiling in an open container
  • 3. A metal rod is heated in a furnace
  • 4. A cup of coffee is allowed to cool on a table

In which of the above cases, the Newton’s Law of Cooling is applicable?

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1 only
4 only
1 and 4 only
1, 2 and 3
This question was previously asked in
UPSC NDA-2 – 2024
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Newton’s Law of Cooling states that the rate of heat loss from an object is proportional to the temperature difference between the object and its surroundings. This law is generally applicable when heat transfer is primarily by convection and radiation, and the temperature difference is relatively small.
1. Ice melting: Involves a phase change at a constant temperature (0°C). The rate of heat absorption is constant if the surroundings are at a constant temperature, but the process is melting, not cooling according to the temperature-difference proportionality of Newton’s law.
2. Water boiling: Involves a phase change at a constant temperature (100°C at atmospheric pressure). This is a process of heating (adding heat to cause vaporization), not cooling.
3. Metal rod heated in a furnace: This is a process of heating the rod by transferring heat *from* the furnace *to* the rod. Newton’s Law of Cooling describes heat *loss* (cooling).
4. Cup of coffee cooling on a table: The hot coffee loses heat to the cooler surroundings (air and table) via convection and radiation, causing its temperature to decrease. This scenario directly fits the conditions and description of Newton’s Law of Cooling, especially as the temperature difference between the coffee and the surroundings decreases over time.
Newton’s Law of Cooling applies to the cooling of an object where the rate of heat loss is proportional to the temperature difference between the object and its surroundings. It is typically applicable for heat transfer by convection and radiation and for relatively small temperature differences.
The formula for Newton’s Law of Cooling is often given as dT/dt = -k(T – T_s), where T is the temperature of the object, T_s is the temperature of the surroundings, t is time, and k is a positive constant. This shows that the rate of temperature change (cooling if T > T_s) is proportional to the temperature difference (T – T_s).

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