Heat and Thermodynamics

31. The absolute zero temperature is 0 Kelvin. In °C unit, which one of th

The absolute zero temperature is 0 Kelvin. In °C unit, which one of the following is the absolute zero temperature?

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0 °C
-100 °C
-273·15 °C
-173·15 °C
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UPSC NDA-2 – 2018
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The correct answer is -273.15 °C.
Absolute zero is defined as 0 Kelvin (0 K). The relationship between the Celsius scale (°C) and the Kelvin scale (K) is given by the formula T(K) = T(°C) + 273.15.
To convert 0 Kelvin to degrees Celsius, we rearrange the formula: T(°C) = T(K) – 273.15. Substituting T(K) = 0, we get T(°C) = 0 – 273.15 = -273.15 °C. Absolute zero is the theoretical point where particles have minimum vibration and zero point energy, and it is the lowest possible temperature.

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32. A stainless steel chamber contains Ar gas at a temperature T and press

A stainless steel chamber contains Ar gas at a temperature T and pressure P. The total number of Ar atoms in the chamber is n. Now Ar gas in the chamber is replaced by CO₂ gas and the total number of CO₂ molecules in the chamber is n/2 at the same temperature T. The pressure in the chamber now is P’. Which one of the following relations holds true? (Both the gases behave as ideal gases)

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P' = P
P' = 2P
P' = P/2
P' = P/4
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UPSC NDA-2 – 2018
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The ideal gas law states that PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the ideal gas constant, and T is temperature. For the first scenario with Ar gas, we have PV = n_Ar RT. Since the number of atoms is n, and assuming R is taken such that n represents the total number of particles, the equation can be written as PV = nRT. For the second scenario with CO₂ gas, the number of molecules is n/2, and the temperature T and volume V (of the chamber) are the same. So, P’V = (n/2)RT. Dividing the second equation by the first gives (P’V)/(PV) = ((n/2)RT)/(nRT), which simplifies to P’/P = (n/2)/n = 1/2. Therefore, P’ = P/2.
The key principle is the ideal gas law (PV = nRT or PV = NkT, where N is the number of particles and k is Boltzmann’s constant). The pressure of an ideal gas depends only on the number of particles, volume, and temperature, not on the type of gas (as long as it behaves ideally). Since the number of particles is halved while temperature and volume are kept constant, the pressure must also be halved.
Avogadro’s hypothesis states that equal volumes of all ideal gases, at the same temperature and pressure, contain the same number of molecules. This question applies the ideal gas law inversely, showing that for a fixed volume and temperature, pressure is directly proportional to the number of moles or molecules.

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33. The statement that ‘heat cannot flow by itself from a body at a lower

The statement that ‘heat cannot flow by itself from a body at a lower temperature to a body at a higher temperature’, is known as

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Zeroth law of thermodynamics
First law of thermodynamics
Second law of thermodynamics
Third law of thermodynamics
This question was previously asked in
UPSC NDA-2 – 2017
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The correct answer is Second law of thermodynamics.
The statement that ‘heat cannot flow by itself from a body at a lower temperature to a body at a higher temperature’ is one of the standard formulations (specifically, the Clausius statement) of the Second Law of Thermodynamics. It describes the natural direction of heat flow.
The Zeroth law of thermodynamics defines thermal equilibrium and establishes temperature as a property. The First law of thermodynamics is a statement of the conservation of energy, relating heat, work, and internal energy. The Third law of thermodynamics deals with the entropy of a system at absolute zero temperature.

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34. Water boils at a lower temperature at high altitudes, because

Water boils at a lower temperature at high altitudes, because

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the air pressure is less
outside temperature is less
latent heat is less
None of the above
This question was previously asked in
UPSC NDA-2 – 2017
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Water boils at a lower temperature at high altitudes because the atmospheric pressure is lower. Boiling occurs when the vapor pressure of the liquid equals the surrounding atmospheric pressure. With lower external pressure, a lower temperature is needed for water’s vapor pressure to reach that level.
Boiling point is the temperature at which the vapor pressure of a liquid equals the ambient atmospheric pressure. Pressure decreases with increasing altitude, leading to a lower boiling point.
At sea level, the boiling point of water is 100°C (212°F) at standard atmospheric pressure. At significantly higher altitudes, such as in mountainous regions, the boiling point can be several degrees lower. For example, at an altitude of 5,000 feet (approx 1500m), water boils at about 95°C (203°F). This is why cooking times often need to be adjusted at high altitudes.

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35. If we plot a graph between volume V and inverse of pressure P (i.e., 1

If we plot a graph between volume V and inverse of pressure P (i.e., 1/P) for an ideal gas at constant temperature T, the curve so obtained is:

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straight line
circle
parabola
hyperbola
This question was previously asked in
UPSC NDA-2 – 2016
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A graph between volume V and inverse of pressure (1/P) for an ideal gas at constant temperature T is a straight line.
According to the Ideal Gas Law, PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the ideal gas constant, and T is temperature. For a fixed amount of gas (n is constant) at a constant temperature (T is constant), the product nRT is a constant. Let’s call this constant C. So, PV = C (This is Boyle’s Law). We want to plot V against 1/P. Rearranging the equation, we get V = C * (1/P). If we let Y = V and X = 1/P, the equation becomes Y = CX. This is the equation of a straight line passing through the origin with slope C (which is equal to nRT).
Plotting V versus P directly would yield a hyperbola (PV = C). A parabola is typically associated with quadratic relationships, and a circle with relationships involving the sum of squares.

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

Which one of the following statements is not correct?

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Conduction can occur easily in solids, less easily in liquids but hardly at all in gases
Heat energy is carried by moving particles in a convection current
Heat energy is carried by electromagnetic waves in radiation
The temperature at which a solid changes into a liquid is called the boiling point
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UPSC NDA-2 – 2015
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The statement “The temperature at which a solid changes into a liquid is called the boiling point” is not correct. The temperature at which a solid changes into a liquid is called the melting point (or fusion point). The boiling point is the temperature at which a liquid changes into a gas.
This question tests knowledge of basic heat transfer mechanisms and phase change terminology.
Conduction is the transfer of heat through direct contact, most effective in solids. Convection is the transfer of heat through the movement of fluids (liquids or gases). Radiation is the transfer of heat through electromagnetic waves, which can occur even in a vacuum.

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37. The silvering in thermos flasks is done to avoid heat transfer by:

The silvering in thermos flasks is done to avoid heat transfer by:

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Convection
Conduction
Radiation
Both convection and conduction
This question was previously asked in
UPSC NDA-2 – 2015
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The correct answer is C. The silvering in thermos flasks is done to avoid heat transfer by radiation.
– A thermos flask is designed to minimize heat transfer through all three mechanisms: conduction, convection, and radiation.
– The vacuum between the double walls reduces heat transfer by conduction and convection.
– The stopper reduces convection (and some conduction).
– The silvered surfaces (typically vacuum-deposited aluminum) on the inner and outer walls are poor emitters and excellent reflectors of thermal radiation. This significantly reduces heat transfer by radiation across the vacuum.
Thermal radiation is heat transferred in the form of electromagnetic waves. Shiny, reflective surfaces are effective at minimizing radiation transfer because they absorb very little and reflect most of the incident radiation.

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38. The absolute zero, i.e., temperature below which is not achievable, is

The absolute zero, i.e., temperature below which is not achievable, is about:

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0 °C
–273 K
–273 °C
–300 °C
This question was previously asked in
UPSC NDA-2 – 2015
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Absolute zero, the theoretical lowest possible temperature, is approximately -273 °C.
Absolute zero is defined as 0 Kelvin (0 K). The Kelvin temperature scale is directly related to the Celsius scale by the formula K = °C + 273.15. Therefore, 0 K is equal to -273.15 °C. The option -273 °C is the closest approximation among the choices.
At absolute zero, particles theoretically cease all fundamental motion. According to the third law of thermodynamics, absolute zero is unattainable in any physical system, although temperatures very close to it have been achieved experimentally.

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39. Consider the two statements given below: Statement-1 : Infrared waves

Consider the two statements given below: Statement-1 : Infrared waves are also called heat waves. Statement-2 : Water molecules readily absorb infrared waves. Select the correct answer using the code given below:

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Both the statements are individually true and Statement-2 is the correct explanation of Statement-1.
Both the statements are individually true, but Statement-2 is not the correct explanation of Statement-1.
Statement-1 is true, but Statement-2 is false.
Statement-2 is true, but Statement-1 is false.
This question was previously asked in
UPSC NDA-1 – 2024
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Both Statement-1 and Statement-2 are individually true, and Statement-2 is the correct explanation of Statement-1.
– **Statement-1: Infrared waves are also called heat waves.** This is true. Infrared radiation is a form of electromagnetic radiation that is strongly associated with heat transfer. Objects at room temperature or higher emit infrared radiation. When infrared radiation is absorbed by a material, it increases the kinetic energy of its molecules, leading to an increase in temperature (i.e., heat).
– **Statement-2: Water molecules readily absorb infrared waves.** This is also true. Water molecules have vibrational modes that efficiently absorb infrared radiation, particularly in certain wavelengths within the infrared spectrum. This strong absorption is why water heats up when exposed to infrared sources (like sunlight or a heater) and why infrared spectroscopy is useful for studying water.
Statement-2 provides a key reason why infrared waves are considered “heat waves”. Because common substances like water (present in living organisms, the atmosphere, etc.) readily absorb infrared radiation and convert it into thermal energy, infrared radiation is a primary means of radiative heat transfer in many contexts, including between the sun and the Earth, and between objects at different temperatures. Thus, the property described in Statement-2 (ready absorption by water) contributes significantly to the heating effect associated with infrared waves, as stated in Statement-1.

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40. Evaporation from the surface of a given liquid takes place more rapidl

Evaporation from the surface of a given liquid takes place more rapidly when

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the temperature is high and the surface area of the liquid is large
the temperature is low and the surface area of the liquid is large
the temperature is low and the surface area of the liquid is small
the temperature is high and the surface area of the liquid is small
This question was previously asked in
UPSC NDA-1 – 2022
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Evaporation is a surface phenomenon where liquid turns into gas. The rate of evaporation is influenced by several factors, including temperature and surface area.
Higher temperature increases the kinetic energy of liquid molecules, making it easier for them to escape the liquid surface and become vapour. A larger surface area exposes more liquid molecules to the air, thus increasing the rate at which they can evaporate. Therefore, a combination of high temperature and large surface area leads to more rapid evaporation.
Other factors that increase the rate of evaporation include increased wind speed (removes saturated air above the surface) and decreased humidity (lower concentration of water vapour in the surrounding air allows more liquid to evaporate). Conversely, factors like low temperature, small surface area, still air, and high humidity decrease the rate of evaporation.

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