Heat and Thermodynamics PYQs – Physics, SSC & Railway

Answer: Option 3 — Absorbs all incident electromagnetic radiation and emits radiation characteristic of its temperature.
A black body is defined as a perfect absorber (absorptivity = 1) and, by Kirchhoff's law, it is therefore also a perfect emitter.
What it emits depends only on its temperature — hotter bodies radiate more, and at shorter wavelengths.
Revision note — Black-body radiation
• The term was coined by Gustav Kirchhoff in 1860. In practice a cavity with a small hole behaves as a black body; lamp black and platinum black come close.
• Kirchhoff's law: a good absorber is a good emitter — at a given temperature, emissivity equals absorptivity.
• Stefan–Boltzmann law: energy radiated per unit area per second E = σT⁴, with σ = 5.67 × 10⁻⁸ W m⁻² K⁻⁴. Doubling the absolute temperature raises the radiation 16 times.
• Wien's displacement law: the wavelength of peak emission falls as temperature rises, λ_max × T = 2.9 × 10⁻³ m K.
• This is why a heated iron glows red, then yellow, then white, and why blue stars are hotter than red stars.
• Planck's law (1900) explained the whole spectrum by assuming energy is emitted in packets (quanta) — the birth of quantum theory; classical physics had predicted the ultraviolet catastrophe.
• The Sun radiates almost as a black body at about 5,800 K, peaking near 500 nm in the visible range.
Do not mix these up
• Black body vs black object — a black body absorbs and emits perfectly; an ordinary black surface merely absorbs most visible light.
• Stefan's law vs Wien's law — Stefan gives how much is radiated (∝ T⁴); Wien gives at which wavelength the emission peaks (∝ 1/T).
Also asked from this topic: the law that explains the colours of stars from their temperature (Wien's displacement law); the value of the Stefan–Boltzmann constant (5.67 × 10⁻⁸ W m⁻² K⁻⁴).

Answer: Option 3 — Increase water's boiling point
A cooker traps the steam, so the pressure inside rises and water can no longer boil at 100 degree C.
It boils at about 120 degree C instead, and food cooks faster because it is being held at a higher temperature.
Revision note — Boiling point and pressure
• The boiling point of a liquid is the temperature at which its vapour pressure equals the surrounding pressure, so raising the pressure raises the boiling point.
• At sea level (1 atmosphere) water boils at 100 degree C = 212 degree F = 373.15 K; the melting point of ice is 0 degree C = 273.15 K.
• A domestic cooker holds about 1 atmosphere of extra pressure (about 15 psi), lifting the boiling point to roughly 120 degree C.
• On a hill station the air pressure is lower, so water boils below 100 degree C and food cooks slowly — the same principle working in reverse.
• The safety valve and the whistle (pressure regulator) release excess steam; without them the rising pressure would burst the vessel.
• Steam at 100 degree C burns worse than water at 100 degree C, because on condensing it releases an extra 2,260 J per gram of latent heat of vaporisation.
Do not mix these up
• Higher temperature vs uniform temperature — the cooker works by raising the boiling point, not merely by spreading heat evenly.
• Boiling vs evaporation — boiling happens throughout the liquid at a fixed temperature; evaporation happens only at the surface, at any temperature.
Also asked from this topic: why a pressure cooker cooks faster (pressure raises the boiling point above 100 degree C); the boiling point of water at sea level (212 degree F).

Answer: Option 2 — When temperature, pressure, and chemical potential remain uniform and constant throughout the system over time
Thermodynamic equilibrium means all three sub-equilibria hold at once — thermal (same temperature), mechanical (same pressure) and chemical (same chemical potential, no net reaction or diffusion).
Nothing then changes with time, and there is no net flow of heat, matter or work.
Revision note — Thermodynamic equilibrium and systems
• Thermal equilibrium = uniform temperature; mechanical equilibrium = uniform pressure with no unbalanced force; chemical equilibrium = uniform chemical potential with no net reaction.
• Zeroth law of thermodynamics: two bodies each in thermal equilibrium with a third are in thermal equilibrium with each other. Named by Ralph H. Fowler in 1931, it lets temperature be defined.
• Types of system: an open system exchanges both matter and energy; a closed system exchanges only energy; an isolated system exchanges neither — the ideal thermos flask.
• A diathermic wall lets heat pass between systems; an adiabatic wall blocks all heat exchange.
• At equilibrium the entropy of an isolated system is at its maximum, so it is constant, not rising.
• Pressure, temperature, volume, internal energy and enthalpy are state functions; heat and work are path functions, depending on how the change was brought about.
Do not mix these up
• Thermodynamic equilibrium vs absolute zero — at equilibrium molecules still move; motion is minimum only at 0 K (-273.15 degree C), and even there the energy is not zero.
• Equilibrium vs steady state — at equilibrium there is no net flow at all; a steady state can have a constant but non-zero flow through it.
• Zeroth law vs first law — the zeroth law defines temperature; the first law is the conservation of energy.
Also asked from this topic: who proposed the zeroth law in 1931 (Ralph H. Fowler); the system that exchanges neither matter nor energy (isolated); what a diathermic wall does (lets heat flow).

Answer: Option 2 — Entropy always increases in a closed system, providing a direction for time from past to future
⚠️ Note: option 2 says closed system; strictly, the second law says entropy never decreases in an isolated system — a closed system can lose entropy by giving out heat. Option 2 is still the only option that links entropy to the arrow of time, so the key stands.
Almost every law of physics works equally well forwards and backwards in time.
The second law of thermodynamics is the exception: the entropy of an isolated system never decreases, so the direction of rising entropy marks out past from future — the arrow of time.
Revision note — Entropy and the second law
• Entropy (S) is the measure of disorder or randomness of a system. Its SI unit is joule per kelvin (J/K), and the term was coined by Rudolf Clausius in 1865.
• Second law: for an isolated system change in entropy is greater than or equal to zero. Equality holds only for an ideal reversible process; every real process increases entropy.
• The phrase arrow of time was coined by the astronomer Arthur Eddington in 1928, exactly for this one-way behaviour.
• Boltzmann's relation, S = k log W, links entropy to the number of ways a state can be arranged; k is the Boltzmann constant, 1.38 × 10⁻²³ J/K.
• Consequences of the second law: heat flows hot to cold on its own, no engine can be 100 per cent efficient, and an ideal Carnot engine's efficiency depends only on the two reservoir temperatures.
• Third law: the entropy of a perfect crystal is zero at 0 K, and absolute zero can never actually be reached.
Do not mix these up
• Entropy vs rate of reaction — entropy is a state function measuring disorder; it says nothing about how fast a process runs, which is chemical kinetics.
• First law vs second law — the first law says energy is conserved; the second says it becomes steadily less available for work.
• Spontaneous vs fast — a spontaneous change is one the second law permits, not necessarily a quick one; rusting is spontaneous but slow.
Also asked from this topic: what an ideal heat engine's efficiency depends on (only the reservoir temperatures); which quantity is not a state function (work); Carnot's ideal engine (1824).

Answer: Option 1 — The hotter the object, the shorter the wavelength at which it emits the most energy
Wien's displacement law makes the peak wavelength inversely proportional to absolute temperature.
A hotter body therefore pushes its peak towards the shorter, bluer end of the spectrum.
Revision note — Wien's displacement law and thermal radiation
• λ(max) × T = b, where b = 2.898 × 10⁻³ m·K is Wien's displacement constant. Doubling T halves the peak wavelength.
• The colour sequence follows from it: heated iron glows dull red → orange → white-hot → bluish as its temperature rises.
• The Sun, at about 5,800 K, peaks near 500 nm, in the visible. The human body, near 310 K, peaks in the infrared — which is what a thermal camera sees.
• Stated by Wilhelm Wien, who won the Nobel Prize in Physics in 1911.
• The partner law is Stefan–Boltzmann: total power radiated per unit area E = σT⁴, with σ = 5.67 × 10⁻⁸ W m⁻² K⁻⁴.
• Both laws describe a black body, an ideal surface that absorbs and emits at all wavelengths. Together they explain the colours of stars from their surface temperature.
Do not mix these up
• Wien's law vs Stefan–Boltzmann law — Wien fixes where the peak lies; Stefan–Boltzmann fixes how much total energy is radiated.
• Inverse in T, not in T⁴ — the fourth power belongs to Stefan–Boltzmann. Wien's peak is inversely proportional to the first power of T.
Also asked from this topic: the law that explains the colours of the Sun and other stars from their temperature (Wien's displacement law); the value of the Stefan–Boltzmann constant (5.67 × 10⁻⁸ W m⁻² K⁻⁴).

Topic:Anomalous Expansion of Water
Water has maximum density at4°C.
When cooled from 4°C to 0°C:
Water expands (anomalous behavior)
Volume increases
Since Density = Mass/Volume
Mass remains constant, Volume increases
Therefore, Density decreases
Answer: Option 3 → Volume increases but density decreases

Topic:Heat Transfer - Conduction
Conduction:Heat transfer through direct contact
between particles in solids.
Metal spoon touches hot tea → heat flows from
tea to spoon through molecular vibrations.
Metals aregood conductorsof heat due to
free electrons that transfer energy rapidly.
Answer: Option 3 → Conduction

Heat Transfer Process
When a surface is heated, air molecules near it gain energy and become less dense.
Less dense warm air rises upward, while cooler denser air sinks down.
This circulation of air due to density differences is calledconvection.
Other processes:
Conduction → heat transfer through direct contact
Radiation → heat transfer through electromagnetic waves
Insulation → preventing heat transfer
Answer: Option 1 → Convection

Heat Transfer from Sun to Earth
Space between Sun and Earth is vacuum (no medium).
Conduction:Requires direct contact of particles.
Convection:Requires fluid medium (liquid/gas).
Radiation:Electromagnetic waves, no medium needed.
Sun's heat travels as electromagnetic radiation (light waves).
This is the only method that works through vacuum.
Answer: Option 2 → Radiation

Topic: Heat Absorption and Surface Color
Key Principle:Dark surfaces absorb more heat radiation than light surfaces.
Dark colors absorb most wavelengths of light and convert them to heat.
Light/white surfaces reflect most radiation, absorbing less heat.
During winters, we need maximum heat absorption to stay warm.
Therefore, dark-colored surfaces are preferred.
Answer: Option 2 → Dark-coloured surface

Correct Answer: 273.15 K — Option (c)
The melting point of ice (pure water at standard atmospheric pressure) is 0°C (Celsius) or 32°F (Fahrenheit). Converting to Kelvin: K = °C + 273.15, so 0°C = 0 + 273.15 = 273.15 K. The Kelvin scale is an absolute temperature scale where 0 K (absolute zero) represents the theoretical absence of all thermal energy. Key temperature conversions: Melting point of ice: 0°C = 273.15 K; Boiling point of water: 100°C = 373.15 K; Absolute zero: −273.15°C = 0 K. Note that 373.15 K is the boiling point of water, not the melting point of ice.
Option (a) 373.15 — Incorrect: 373.15 K is the boiling point of water (100°C = 373.15 K), not the melting point of ice; the melting point of ice is 273.15 K.
Option (b) 0 — Incorrect: 0 K is absolute zero (−273.15°C), the lowest possible temperature; the melting point of ice is 0°C, which equals 273.15 K, not 0 K.
Option (d) 100 — Incorrect: 100°C is the boiling point of water, not the melting point of ice; the melting point of ice is 0°C = 273.15 K.

Correct Answer: 4°C — Option (b)
Water exhibits a unique property called anomalous expansion: unlike most liquids, water expands (becomes less dense) when cooled below 4°C and also when heated above 4°C. At 4°C, water reaches its maximum density of approximately 1000 kg/m³ (1 g/cm³). Above 4°C, water expands normally as temperature increases (density decreases). Below 4°C, as temperature drops toward 0°C, water molecules begin to form hydrogen bonds that push them apart into the open hexagonal crystal lattice structure of ice — causing water to expand and become less dense. This is why ice floats on water (ice is less dense than liquid water at 4°C). This anomalous expansion has significant ecological importance as it ensures water bodies freeze from the top down, preserving aquatic life.
Option (a) 0°C — Incorrect: At 0°C, water either freezes into ice or is just at the freezing point; it has lower density than at 4°C because the hydrogen-bonded lattice structure begins forming below 4°C.
Option (c) 25°C — Incorrect: At room temperature (25°C), water is well above 4°C and has a density of approximately 997 kg/m³, which is less than the maximum density at 4°C.
Option (d) 100°C — Incorrect: At 100°C, water is at its boiling point and has a density of about 958 kg/m³, which is significantly lower than the maximum density at 4°C.

Topic:Thermodynamic Systems
Types of Systems:
1.Open System→ Exchange of both mass and energy
2.Closed System→ Exchange of energy only, no mass
3.Isolated System→ No exchange of mass or energy
Question asks: No exchange of energy OR mass
Isolated System→ Completely sealed from surroundings

Thermodynamic Systems Classification:
Open System:Both matter and energy can cross the boundary
Example: Boiling water in open vessel
Closed System:Only energy exchange, no matter exchange
Example: Sealed pressure cooker
Isolated System:No exchange of matter or energy
Example: Thermos flask (ideal)
Answer: Option 3 → Exchange of energy and matter between system and surroundings.

Heat absorbed or released during phase change ( solid↔liquid↔gas ) at constant temperature is calledLatent Heat.
It is measured per unit mass (J/kg or cal /g).
Types: Latent heat of fusion (melting/freezing) and Latent heat of vaporization (boiling/condensation).
Example: Ice at 0°C → Water at 0°C requires latent heat.
Answer: Option 1 → Latent heat

During melting, temperature remains stable because all heat energy supplied is used to break the forces of attraction between particles in ice.
This energy required to break these forces is called latent heat of fusion.
Hence, temperature does not increase until the entire ice melts.
Therefore, heat energy is used to overcome particle attraction during melting.
Final answer: Option 4

A Thermometer is the instrument used to measure temperature. It works on the principle of thermal expansion of liquids (mercury or alcohol) or uses electronic sensors to detect temperature changes. It is expressed in Celsius (°C), Fahrenheit (°F), or Kelvin (K).
Why other options are incorrect:
- Barometer: Measures atmospheric pressure, not temperature.
- Hygrometer: Measures humidity (moisture content) in the air.
- Hydrometer: Measures the specific gravity (density) of liquids.

The correct answer is Option 3 — Cotton clothes absorb sweat and help in its evaporation, which cools the body. Cotton is a good absorber of water/sweat. When sweat evaporates from the surface of cotton cloth, it takes away heat from the body (latent heat of vaporization), thus cooling the body. Option 1 is wrong — cotton does allow evaporation. Option 2 is wrong — cotton does not trap heat. Option 4 is wrong — cotton is highly absorbent.

Correct Answer: 2 – Water vapour in the air condenses upon contact with the cold glass.
This is the phenomenon of condensation. The outer surface of the glass is cold (due to the ice-cold water inside). When warm, moist air comes in contact with the cold surface, the water vapour in the air loses heat and condenses into liquid water droplets on the glass surface. This is the same reason dew forms on cold surfaces.
Option 1 is incorrect – the ice-cold water inside does not evaporate through the glass.
Option 3 is incorrect – glass does not absorb moisture; the water forms due to condensation.
Option 4 is incorrect – the cold glass cools (not heats) the surrounding air.

Correct Answer: To utilize the large latent heat of vaporisation of water for cooling.
When water is sprinkled on a hot surface, it evaporates (vaporises). This evaporation process absorbs a large amount of heat energy from the surface, called the Latent Heat of Vaporisation (approximately 2260 J/g for water). This absorbed heat cools the surface and the surrounding air, providing relief from heat.
Why other options are wrong:
To increase surface temperature - Evaporation DECREASES temperature, not increases it.
To help water absorb heat - This is partially true but incomplete; the key is the phase change (evaporation) and the LATENT heat involved.
To make surface slippery - Not the scientific reason for this practice.

The correct answer is To record the highest and lowest temperatures over a period (Option 2).
A maximum-minimum thermometer (also called Six's thermometer) is designed to record both the highest and lowest temperatures that occur over a specific time period (typically 24 hours). It is widely used in meteorology and agriculture to monitor daily temperature variations without continuous observation.
Option 1 - Measure boiling point: A thermometer to measure boiling points is used in laboratory settings; a maximum-minimum thermometer is not specifically designed for this.
Option 3 - Temperature of gases in industrial settings: Industrial gas temperature measurement uses specialized pyrometers or industrial thermometers.
Option 4 - Temperature of chemical reactions: Reaction temperatures are measured using lab thermometers or probes, not maximum-minimum thermometers.

The correct answer is It prevents the mercury level from falling on its own (Option 2).
The kink (constriction) in a clinical thermometer is a narrow bend just below the bulb. When the thermometer is removed from the patient's body, the mercury column cannot flow back past the kink due to surface tension and gravity. This allows the temperature reading to remain fixed until the thermometer is manually reset (shaken).
Option 1 - Increases accuracy: The kink does not improve accuracy; it preserves the reading by preventing mercury from receding.
Option 3 - Helps in quick measurement: The kink plays no role in the speed of temperature measurement.
Option 4 - Prevents breaking: The kink's purpose is not structural protection against breakage; it serves to retain the mercury reading.

The correct answer is Radiation (Option 3).
Radiation is the mode of heat transfer that does not require any medium (solid, liquid, or gas). Heat is transferred through electromagnetic waves (infrared radiation) and can travel through vacuum. This is how the Sun's heat reaches the Earth through the vacuum of space.
Option 1 - Conduction: Conduction requires a solid medium (or liquids/gases to a lesser extent) for heat to be transferred through particle-to-particle collision.
Option 2 - Convection: Convection requires a fluid medium (liquid or gas) where heat transfer occurs through the movement of bulk fluid masses.
Option 4 - Induction: Induction is a method of electromagnetic energy transfer (not a classical heat transfer mode); it also requires a conducting medium.

The correct answer is Radiation (Option 3).
Heat from the sun reaches the Earth through radiation, specifically electromagnetic radiation (infrared, visible light, and ultraviolet). Radiation can travel through the vacuum of space without requiring any medium, unlike conduction or convection.
Option 1 - Conduction: Conduction requires direct physical contact between molecules to transfer heat. Since space is a near-vacuum, conduction cannot transmit solar energy to Earth.
Option 2 - Convection: Convection requires a fluid medium (liquid or gas) for heat transfer through circulation. The vacuum of space prevents convection.
Option 4 - Evaporation: Evaporation is a phase change process (liquid to gas) that absorbs heat; it is not a mode of heat transfer from the sun to Earth.

The boiling point of water at sea level is 212°F (100°C) . At this temperature, the vapour pressure of water equals the atmospheric pressure at sea level (1 atmosphere or 760 mmHg). When this condition is met, water molecules can escape freely into the air, causing boiling. Factors like altitude affect this boiling point: at higher altitudes (lower pressure), water boils at a lower temperature, while at higher pressures (like in a pressure cooker), it boils at a higher temperature. The other options are incorrect: 217.4°F (103°C) and 208.4°F (98°C) do not represent the standard boiling point at sea level, and 203°F (95°C) is too low and only occurs at higher elevations.

The correct answer is option 1. In a pressure cooker, the pressure inside increases, not decreases . This increased pressure allows water to boil at a temperature higher than 100°C, which in turn cooks food faster (Statement II is correct). Additionally, the superheated steam generated inside transfers heat more efficiently to the food, further speeding up the cooking process (Statement III is correct). Statement I is wrong because it incorrectly says the pressure decreases. Thus, only II and III are correct .

Correct Answer: Option 1 – latent
The heat energy required to change 1 kg of a solid into liquid at its melting point (at constant atmospheric pressure) without any change in temperature is called Latent Heat of Fusion. 'Latent' means 'hidden' — the heat is absorbed without a temperature rise; it is used to break intermolecular bonds.
Why other options are wrong:
Option 2 (boiling): Boiling is the phase change from liquid to gas; latent heat of vaporisation applies there.
Option 3 (melting): While 'melting' heat is a colloquial term, the scientifically correct term is 'latent' heat of fusion. The candidate incorrectly selected this.
Option 4 (absolute): Absolute is related to temperature scale (Kelvin); not a type of heat of fusion.

Correct Answer: Both I and II
Evaporative cooling occurs when a liquid evaporates and absorbs latent heat from the surroundings, causing a cooling effect.
Statement I: Pouring acetone on the palm - CORRECT. Acetone has a very low boiling point and evaporates rapidly, absorbing heat from the palm and causing a cooling sensation.
Statement II: Sprinkling water on the roof after a sunny day - CORRECT. Water evaporates from the hot roof, absorbing heat from the surface and cooling it down. This is a common practice in summer.
Both are valid examples of cooling due to evaporation.
Why 'Only I', 'Only II', and 'Neither' are wrong: Both statements correctly describe cooling due to evaporation.

Statement 1 is incorrect — reversible (quasi-static) expansion produces the maximum work, not less.
Statement 2 is correct — Carnot efficiency depends only on hot and cold reservoir temperatures.
Statement 3 is correct — heat flows from cold to hot only with external work (second law / refrigerator).
Hence statements 2 and 3 only are correct.

Steam at 100 °C carries additional 2260 J/g of latent heat of vaporisation , which is released when it condenses on the skin to water at 100 °C.
This extra energy makes steam burns more severe than burns from boiling water.
Steam is not hotter, nor does it have higher specific heat per se.

Correct Answer: Option 2 – Thermometer
A thermometer is the standard instrument for measuring temperature. It works on the principle that physical properties of matter (volume of liquid mercury/alcohol, electrical resistance, infrared radiation) change predictably with temperature. Common types: mercury thermometer (clinical), digital thermometer (electronic), infrared thermometer (non-contact), thermocouple (industrial). Temperature scales: Celsius (°C), Fahrenheit (°F), and Kelvin (K).
Option 1 (Barometer): Measures atmospheric pressure (invented by Evangelista Torricelli, 1643). Used in weather forecasting. Not for temperature.
Option 3 (Speedometer): Measures the speed of a vehicle (typically in km/h or mph). Found in dashboards of automobiles. Not for temperature.
Option 4 (Altimeter): Measures altitude (height above sea level), typically using atmospheric pressure or GPS. Used in aircraft and mountaineering. Not for temperature.

Correct Answer: Option 4 – Heating water on a stove
Convection is the transfer of heat through the movement of a fluid (liquid or gas). When water is heated on a stove, the water at the bottom gets hot, becomes less dense, rises to the top, and the cooler denser water from the top sinks to the bottom — creating convection currents that eventually heat the entire water. This is a classic example of convective heat transfer.
Option 1 (Warming hands by a fire): This is primarily radiation — infrared radiation from the fire travels through air to warm the hands.
Option 2 (Heat through vacuum): Only radiation can travel through a vacuum (no medium needed); conduction and convection require a medium.
Option 3 (Heat transfer in metal): Heat transfer through a solid metal (from atom to atom without bulk movement) is conduction, not convection.

Correct Answer: Option 4 — large latent heat of vaporisation of water helps to cool the hot surface
Latent heat of vaporisation is the heat that 1 kg of a liquid must absorb to change completely into vapour at its boiling point, without any rise in temperature. For water this value is unusually large — 22.5 × 10⁵ J/kg (2260 kJ/kg) — the highest among common liquids.
When water is sprinkled on a hot roof, it begins to evaporate. To escape as vapour, every gram of that water must draw its large latent heat from the roof and from the air just above it. This heat is taken out of the surface, so the roof loses energy and its temperature falls. The surface therefore feels distinctly cooler after sprinkling.
The same principle explains why water in an earthen pot (matka) stays cool, why sweating cools the body, and why a desert cooler works well only in dry weather.
❌ Why the other options are incorrect
✗ less latent heat of vaporisation of water helps to cool the hot surface — water's latent heat of vaporisation is large, not small; and a small latent heat would remove less heat, giving poor cooling.
✗ large latent heat of vaporisation of water helps to heat the cold surface — evaporation absorbs heat from the surface, it never gives heat to it; the surface is cooled, not heated.
✗ large latent heat of vaporisation of water does not help to cool the hot surface — the exact opposite of the truth; the large latent heat is precisely the reason the cooling works.
📌 SSC & Railway Exam Facts
• Latent heat of vaporisation of water = 22.5 × 10⁵ J/kg; latent heat of fusion of ice = 3.34 × 10⁵ J/kg.
• Evaporation is a surface phenomenon and a cooling process; it occurs at all temperatures below the boiling point.
• Evaporation is faster with higher temperature, larger surface area, greater wind speed and lower humidity.
• During a change of state the temperature stays constant, because the heat supplied is used as latent heat.
• Steam at 100 °C causes a worse burn than water at 100 °C, as it carries this extra latent heat.

Correct Answer: Option 2 — 25 degree Celsius
📖 Given
• Temperature on the Kelvin scale = 298 K
• To find: the same temperature in degree Celsius (°C)
✍️ Formula
K = °C + 273, and therefore °C = K − 273
🔢 Solution
• °C = 298 − 273
• °C = 25 °C
So water at 298 K is at 25 °C, which is ordinary room temperature — exactly the temperature at which most laboratory values, such as standard pH and solubility data, are quoted.
Note that the kelvin (K) is the SI unit of temperature, and by convention the degree symbol is never written with it — we write 298 K, not 298 °K. The Kelvin scale is also called the absolute scale, because its zero is absolute zero, the lowest temperature possible.
❌ Why the other options are incorrect
✗ 28 degree Celsius — this would correspond to 301 K; it comes from subtracting 270 instead of 273.
✗ 30 degree Celsius — this corresponds to 303 K, five kelvin higher than the value given.
✗ 32 degree Celsius — this corresponds to 305 K; 32 is also the Fahrenheit value of 0 °C, which is a common source of confusion.
📌 SSC & Railway Exam Facts
• K = °C + 273; °C = K − 273; °F = (9/5 × °C) + 32.
• 0 °C = 273 K = 32 °F; 100 °C = 373 K = 212 °F; −40 °C = −40 °F.
• Absolute zero = 0 K = −273.15 °C, at which particle motion theoretically ceases.
• Normal human body temperature = 37 °C = 98.6 °F = 310 K.
• The kelvin is one of the seven SI base units, and the degree sign is never used with it.