68 free MCQs on Thermal Properties of Matter with worked answers and explanations. Temperature scales, thermal expansion, calorimetry, and the three modes of heat transfer including radiation laws.
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During a phase change (melting or boiling), temperature stays constant while heat is absorbed entirely as latent heat (Q=mL); temperature only rises again once the substance is fully in its new phase.
Q1.
What is the value of absolute zero on the Celsius scale?
Answer: B. -273.15°C
Why: Absolute zero, 0 K, corresponds to -273.15°C, the theoretical lowest possible temperature.
Q2.
Convert 25°C to Kelvin.
Answer: A. 298 K
Why: K = C + 273.15, so 25°C = 298.15 K, approximately 298 K.
Q3.
The coefficient of area expansion (β) is related to the coefficient of linear expansion (α) by:
Answer: B. β = 2α
Why: For small expansions, area expansion coefficient is approximately twice the linear expansion coefficient: β ≈ 2α.
Q4.
The SI unit of specific heat capacity is:
Answer: B. J/(kg·K)
Why: Specific heat capacity is heat per unit mass per unit temperature rise, so its SI unit is J/(kg·K).
Q5.
Which mode of heat transfer can occur even through a vacuum?
Answer: C. Radiation
Why: Radiation is heat transfer through electromagnetic waves and does not require any medium, hence it can occur through vacuum (e.g., sunlight reaching Earth).
Q6.
The latent heat of fusion of ice is the heat required to:
Answer: B. Convert ice to water at the same temperature (0°C)
Why: Latent heat of fusion is the heat needed to change a unit mass of a substance from solid to liquid without any change in temperature.
Q7.
According to the Stefan-Boltzmann law, the power radiated by a perfect black body is proportional to:
Answer: D. T⁴
Why: Stefan-Boltzmanns law states E = σT⁴, so radiated power per unit area is proportional to the fourth power of absolute temperature.
Q8.
Mayers relation for one mole of an ideal gas is:
Answer: A. Cp - Cv = R
Why: Mayers relation states that the difference between molar specific heats at constant pressure and constant volume equals the gas constant R.
Q9.
Newtons law of cooling is valid only when:
Answer: B. The temperature difference between body and surroundings is small
Why: Newtons law of cooling is an approximation valid only for small temperature differences between the body and its surroundings.
Q10.
Water has its maximum density at:
Answer: B. 4°C
Why: Water shows anomalous expansion, contracting from 0°C to 4°C and expanding above 4°C, so its density is maximum at 4°C.
Q11.
According to Wiens displacement law, as the temperature of a black body increases, the wavelength of maximum emitted intensity:
Answer: B. Decreases
Why: Wiens law states λ_m·T = constant, so as T increases, the peak wavelength λ_m decreases (shifts toward blue/shorter wavelengths).
Q12.
Heat transfer by the actual movement of heated fluid particles is called:
Answer: B. Convection
Why: Convection involves the bulk movement of a fluid (liquid or gas) carrying thermal energy from one place to another.
Q13.
Which of the following has the highest specific heat capacity among common substances?
Answer: C. Water
Why: Water has an unusually high specific heat capacity (about 4186 J/kg·K), much higher than most metals, which is why it is used as a coolant.
Q14.
Heat flows spontaneously from a body at higher temperature to a body at:
Answer: A. lower temperature
Why: Heat always flows from hot to cold until thermal equilibrium is reached.
Q15.
The SI unit of heat (a form of energy) is the:
Answer: A. joule
Why: Heat is energy, measured in joules (J).
Q16.
The SI unit of temperature is the:
Answer: A. kelvin
Why: The kelvin (K) is the SI base unit of temperature.
Q17.
Most substances ___ when heated:
Answer: A. expand
Why: Heating increases molecular motion, so most substances expand.
Q18.
Heat transfer through a solid without any bulk movement of the material is:
Answer: A. conduction
Why: In conduction, energy passes between neighbouring particles without their bulk movement.
Q19.
Heat transfer by the actual movement of a heated fluid is called:
Answer: A. convection
Why: Convection carries heat by the circulation of the fluid itself.
Q20.
The mode of heat transfer that requires no material medium is:
Answer: A. radiation
Why: Radiation travels as electromagnetic waves and needs no medium - as sunlight through space shows.
Q21.
A metal rod of length 1 m has a coefficient of linear expansion 2 × 10⁻⁵ /°C. If its temperature increases by 50°C, the increase in length is:
Answer: B. 1 mm
Why: ΔL = αLΔT = 2×10⁻⁵ × 1 × 50 = 1×10⁻³ m = 1 mm.
Q22.
500 g of water at 20°C is heated to 80°C. Taking specific heat of water as 4200 J/(kg·K), the heat required is:
Answer: B. 126,000 J
Why: Q = msΔT = 0.5 × 4200 × 60 = 126,000 J.
Q23.
How much heat is needed to convert 10 g of ice at 0°C completely to water at 0°C? (Latent heat of fusion of ice = 334 J/g)
Answer: B. 3340 J
Why: Q = mL = 10 g × 334 J/g = 3340 J.
Q24.
Equal masses of two liquids A (specific heat 2000 J/kgK) and B (specific heat 4000 J/kgK) are mixed; A is at 80°C and B is at 20°C. What is the equilibrium temperature (assuming no heat loss)?
Answer: B. 33.3°C
Why: Heat lost by A = heat gained by B: m×2000×(80-T) = m×4000×(T-20). Solving: 2000(80-T)=4000(T-20) → 160000-2000T=4000T-80000 → 240000=6000T → T=40°C. Correct equilibrium temperature is 40°C.
Q25.
A metal sphere cools from 80°C to 60°C in 5 minutes when the surrounding temperature is 20°C. By Newtons law of cooling, the initial excess temperature of the sphere over its surroundings is:
Answer: C. 60°C
Why: Initial excess temperature = body temperature minus surrounding temperature = 80°C - 20°C = 60°C, the driving temperature difference that determines the initial cooling rate.
Q26.
A rod conducts heat at a steady rate of 20 W when the temperature difference across its ends is 10°C. If the temperature difference is increased to 25°C (all else unchanged), the new rate of heat conduction is:
Answer: B. 50 W
Why: By Fouriers law, H ∝ ΔT for fixed K, A, L. New rate = 20 × (25/10) = 50 W.
Q27.
For a diatomic ideal gas, if Cv = (5/2)R, what is the value of Cp?
Answer: B. (7/2)R
Why: Using Mayers relation Cp = Cv + R = (5/2)R + R = (7/2)R.
Q28.
Two rods of the same material and same length, but rod A has twice the cross-sectional area of rod B, conduct heat with the same temperature difference across their ends. The ratio of heat conducted per second by A to B is:
Answer: C. 2:1
Why: Rate of heat conduction H ∝ A (Fouriers law) for the same K, L, ΔT. Since A<sub>rod</sub> = 2×A<sub>rodB</sub>, ratio H<sub>A</sub>:H<sub>B</sub> = 2:1.
Q29.
A black body radiates energy at a rate E<sub>1</sub> at temperature T. If the temperature is doubled, the new rate of radiation E<sub>2</sub> in terms of E<sub>1</sub> is:
Answer: D. 16E<sub>1</sub>
Why: By Stefan-Boltzmanns law, E ∝ T⁴. Doubling T increases E by a factor of 2⁴ = 16, so E<sub>2</sub> = 16E<sub>1</sub>.
Q30.
A star has a surface temperature twice that of the Sun. According to Wiens law, the peak emission wavelength of the star compared to the Sun is:
Answer: B. Half as long
Why: Wiens law: λ_m ∝ 1/T. Doubling temperature halves the peak wavelength, so the star peak wavelength is half that of the Sun.
Q31.
Steam at 100°C is passed into water to raise its temperature. The large heat released by steam is mainly due to:
Answer: B. Latent heat of vaporization released during condensation
Why: When steam condenses to water at 100°C, it releases a large amount of latent heat of vaporization (2256 kJ/kg), which is much greater than what sensible heat alone would provide.
Q32.
Why does a thick glass tumbler crack when very hot water is poured into it suddenly, but not when poured slowly?
Answer: B. Sudden heating causes unequal/differential thermal expansion between inner and outer surfaces, creating stress that cracks the glass
Why: Glass is a poor thermal conductor; rapid heating causes the inner surface to expand quickly while the outer surface remains cooler, and the resulting differential expansion creates stress that can crack the glass.
Q33.
The heat needed to raise the temperature of 1 kg of a substance by 1 K is its:
Answer: A. specific heat capacity
Why: Specific heat capacity is the heat per unit mass per unit temperature rise.
Q34.
The heat needed to change the state of a substance at constant temperature is called the:
Answer: A. latent heat
Why: Latent heat drives a phase change without a temperature change.
Q35.
Water has its maximum density at a temperature of:
Answer: A. 4°C
Why: Owing to anomalous expansion, water is densest at about 4°C.
Q36.
The coefficient of linear expansion has units of:
Answer: A. per kelvin (K⁻¹)
Why: Linear expansion coefficient α has units of K⁻¹.
Q37.
The coefficient of volume expansion γ is related to the linear coefficient α by:
Answer: A. γ = 3α
Why: For an isotropic solid, γ = 3α.
Q38.
By Newton’s law of cooling, the rate of heat loss of a body is proportional to its temperature difference with the:
Answer: A. surroundings
Why: The rate of cooling is proportional to the excess temperature over the surroundings.
Q39.
The peak wavelength of radiation from a hot body shifts to shorter wavelengths as its temperature rises. This is:
Answer: A. Wien's displacement law
Why: Wien’s displacement law: λ_max ∝ 1/T.
Q40.
A perfect black body is one that is a perfect:
Answer: A. absorber and emitter of radiation
Why: A black body absorbs all incident radiation and is also the best possible emitter.
Q41.
A solid sphere and a hollow sphere of the same material, radius, and surface conditions are heated to the same temperature and allowed to cool in identical surroundings. Which cools faster initially, and why?
Answer: B. Hollow sphere, because it has less mass for the same surface area, so smaller heat capacity
Why: Both have the same surface area and emissivity, so they radiate heat at the same rate, but the hollow sphere has less mass (lower heat capacity), so its temperature drops faster for the same heat loss.
Q42.
A calorimeter of negligible heat capacity contains 200 g of water at 25°C. 50 g of ice at 0°C is added. Given latent heat of fusion = 336 J/g and specific heat of water = 4.2 J/(g·K), the final temperature of the mixture is approximately:
Answer: B. 5°C
Why: Heat released by water cooling to 0°C = 200×4.2×25 = 21000 J. Heat needed to melt all the ice = 50×336 = 16800 J. Remaining heat = 4200 J raises the combined 250 g of water: 4200 = 250×4.2×ΔT, giving ΔT = 4°C, so final temperature is approximately 4 to 5°C.
Q43.
A composite slab is made of two materials of equal thickness with thermal conductivities K1 and K2 placed in series (heat flows perpendicular to the layers). The effective thermal conductivity of the slab is:
Answer: B. 2K1K2/(K1 + K2)
Why: For slabs of equal thickness in series, thermal resistances add (R = L/KA), giving an effective conductivity analogous to resistors in series: K<sub>eff</sub> = 2K1K2/(K1+K2).
Q44.
A liquid cools from 70°C to 60°C in 5 minutes and from 60°C to 50°C in 8 minutes, with the room temperature constant. Using Newtons law of cooling (approximate form), this tells us that:
Answer: B. The room temperature is below 50°C, consistent with the slower cooling rate at lower temperature difference
Why: Since the cooling slows down as the liquid temperature approaches the surroundings, the smaller temperature drop in a longer time (60 to 50 in 8 min vs 70 to 60 in 5 min) is expected behaviour consistent with Newtons law of cooling, with room temp below 50°C.
Q45.
A gas undergoes a process where Cp/Cv = γ. If the gas is diatomic with vibrational modes also active at high temperature (f=7), the value of γ is:
Answer: C. 9/7
Why: With f=7 (3 translational + 2 rotational + 2 vibrational), Cv=(7/2)R, Cp=(9/2)R, so γ=Cp/Cv=9/7 ≈ 1.29.
Q46.
Two spheres of the same material, one of radius r and another of radius 2r, are heated to the same temperature and left to cool by radiation. The ratio of their rates of fall of temperature (dT/dt) initially, smaller to larger, is:
Answer: B. 2:1
Why: Rate of cooling dT/dt ∝ (surface area)/(volume) ∝ 1/r for spheres of the same material and temperature. Ratio of smaller to larger = (1/r):(1/2r) = 2:1.
Q47.
A bimetallic strip used in a thermostat is made of two metals with different coefficients of linear expansion bonded together. When heated, the strip bends because:
Answer: B. One metal expands more than the other, causing differential expansion and bending toward the metal with lower α
Why: Since the two metals have different α, on heating, the metal with higher α expands more along its length, forcing the strip to curve, bending toward the side with the lower expansion coefficient.
Q48.
If the absolute temperature of a black body source is increased such that the total radiated power increases by a factor of 81, by what factor did the absolute temperature increase?
Answer: A. 3
Why: E ∝ T⁴, so if E increases by 81 = 3⁴, then T increased by a factor of 3.
Q49.
For an ideal gas, the molar specific heat at constant volume for a monatomic gas is (3/2)R. If instead the gas were polyatomic (nonlinear, no vibration, f=6), the ratio of Cv(polyatomic) to Cv(monatomic) would be:
Answer: B. 2:1
Why: Cv(monatomic) = (3/2)R, Cv(polyatomic, f=6) = (6/2)R = 3R. Ratio = 3R/(3R/2) = 2, so the ratio is 2:1.
Q50.
A metal sphere of radius r is heated and then allowed to cool by radiation in surroundings at a fixed temperature, following Newton's law of cooling. If an identical sphere of twice the radius starts cooling from the same initial temperature in the same surroundings, which sphere's temperature drops faster initially, and why?
Answer: B. The smaller sphere, because it has a larger surface-area-to-volume ratio, so it radiates heat faster relative to its heat capacity
Why: The rate of fall of temperature is proportional to surface area divided by mass (dT/dt ∝ A/(mc) ∝ 1/r for a sphere), so the smaller sphere, having a higher surface-to-volume ratio, cools faster initially.
Q51.
By the Stefan–Boltzmann law, the energy radiated per unit area by a black body is proportional to T raised to the power:
Answer: A. 4
Why: E = σT⁴, so the emitted power varies as the fourth power of absolute temperature.
Q52.
If the absolute temperature of a black body is doubled, the radiated energy increases by a factor of:
Answer: A. 16 times
Why: Since E ∝ T⁴, doubling T multiplies the emission by 2⁴ = 16.
Q53.
In the conduction relation Q/t = kA(T₁ − T₂)/L, the constant k is the:
Answer: A. thermal conductivity
Why: k is the thermal conductivity of the material of the rod.
Q54.
During a phase change such as ice melting, the temperature of the substance:
Answer: A. stays constant
Why: The absorbed latent heat changes the state, so the temperature stays constant during the change.
Q55.
The latent heat of fusion of ice is approximately:
Answer: A. 334 J/g
Why: About 334 J are needed to melt 1 g of ice at 0°C.
Q56.
Two rods of the same size but different conductivities are joined end to end. The pair conducts less heat than the better conductor alone because the poorer conductor:
Answer: A. limits the overall flow
Why: In series, the poorer conductor acts as a bottleneck, restricting the heat current.
Q57.
Wien’s displacement law can be written λ_max × T equals a:
Answer: A. constant
Why: λ_max·T = 2.9 × 10⁻³ m·K, a constant.
Q58.
The net radiation loss of a body at temperature T is proportional to (T⁴ − T₀⁴), where T₀ is the temperature of the:
Answer: A. surroundings
Why: T₀ is the surrounding temperature that the body also absorbs radiation from.
Q59.
Anomalous expansion means that between 0°C and 4°C, water actually:
Answer: A. contracts on heating
Why: Unusually, water contracts as it warms from 0°C to 4°C, then expands above 4°C.
Q60.
A good absorber of thermal radiation is also a good:
Answer: A. emitter
Why: By Kirchhoff’s law, a good absorber is an equally good emitter at the same temperature.
Q61.
Two rods of equal length and cross-section, of conductivities K₁ and K₂, are joined end to end. The effective thermal conductivity of the combination is:
Answer: B. 2K₁K₂/(K₁ + K₂)
Why: Rods in series add thermal resistances, giving the harmonic mean form: 2K₁K₂/(K₁ + K₂).
Q62.
A body cools from 60°C to 50°C in 10 minutes in surroundings at 30°C. Using Newton law of cooling, its temperature after the next 10 minutes is approximately:
Answer: B. 43.3°C
Why: First step: 1 = k(55 − 30) → k = 0.04. Next: (50 − T)/10 = 0.04((50 + T)/2 − 30) → T ≈ 43.3°C.
Q63.
A metal rod of length 1 m and expansion coefficient 2×10⁻⁵ per °C is heated by 100°C. Its increase in length is:
Answer: B. 2 mm
Why: ΔL = LαΔT = 1·2×10⁻⁵·100 = 2×10⁻³ m = 2 mm.
Q64.
The power radiated by a black body is proportional to the fourth power of its absolute temperature. If its temperature (in kelvin) is doubled, the radiated power becomes:
Answer: D. 16×
Why: By Stefan law P ∝ T⁴, so doubling T gives 2⁴ = 16 times the power.
Q65.
Two stars have surface temperatures 6000 K and 12000 K. The ratio of the wavelengths at which they radiate maximum energy (6000 K : 12000 K) is:
Answer: B. 2:1
Why: By Wien law λ_max ∝ 1/T, so ratio = T₂:T₁ = 12000:6000 = 2:1.
Q66.
100 g of water at 20°C is mixed with 50 g of water at 80°C. The equilibrium temperature is:
Answer: B. 40°C
Why: T = (100·20 + 50·80)/150 = (2000 + 4000)/150 = 40°C.
Q67.
The heat required to melt 10 g of ice at 0°C (latent heat 80 cal/g) is:
Answer: C. 800 cal
Why: Q = mL = 10·80 = 800 cal.
Q68.
A bimetallic strip bends when heated primarily because the two metals have different:
Answer: B. coefficients of linear expansion
Why: Different linear expansion coefficients cause unequal expansion, bending the strip toward the metal that expands less.