Latent Heat & Phase Transitions — NEET Physics MCQs with Solutions
Free NEET Physics Latent Heat & Phase Transitions MCQs with step-by-step solutions (20 questions). Part of Calorimetry & Thermal Properties. Practise online on Prepizo — no login needed.
▶ Practise Latent Heat & Phase Transitions online (free)
Questions with solutions
Q1 — Latent Heat & Phase Transitions · easy · theory
The latent heat of a substance is the heat required to:
A. Change the phase of unit mass WITHOUT any change in temperature ✓ Correct
B. Raise the temperature of unit mass by 1°C
C. Break the substance into atoms
D. Raise the temperature of one mole by 1°C
Solution: Q = mL: latent ("hidden") heat changes the phase at constant temperature — it goes into breaking intermolecular bonds, not raising temperature.
Q2 — Latent Heat & Phase Transitions · easy · theory
During melting or boiling, the temperature of a pure substance:
A. Falls
B. Fluctuates randomly
C. Rises steadily
D. Remains constant even though heat is being supplied ✓ Correct
Solution: All supplied heat goes into the phase change (latent heat), so the temperature stays constant until the change is complete — the flat portions of a heating curve.
Q3 — Latent Heat & Phase Transitions · easy · theory
The latent heat of vaporization of water compared to its latent heat of fusion is:
A. About half
B. Much smaller
C. Equal
D. Much larger (540 vs 80 cal/g) ✓ Correct
Solution: L_v = 540 cal/g vs L_f = 80 cal/g: separating molecules completely into vapour needs far more energy than loosening the solid lattice.
Q4 — Latent Heat & Phase Transitions · medium · theory
The melting point of ice, with an INCREASE in pressure:
A. Decreases ✓ Correct
B. Stays exactly 0°C
C. First increases then decreases
D. Increases
Solution: Ice contracts on melting (water is denser), so increased pressure favours the liquid — the melting point drops. This is the basis of regelation.
Q5 — Latent Heat & Phase Transitions · easy · theory
The boiling point of water, with an increase in pressure:
A. Decreases
B. Increases (principle of the pressure cooker) ✓ Correct
C. Becomes 0°C
D. Is unchanged
Solution: Higher pressure makes it harder for vapour to form, raising the boiling point — a pressure cooker cooks faster at ~120°C. At altitude, lower pressure lowers the boiling point.
Q6 — Latent Heat & Phase Transitions · medium · theory
Regelation is the phenomenon in which ice:
A. Never melts
B. Evaporates directly to vapour
C. Melts only above 10°C
D. Melts under increased pressure and refreezes when the pressure is removed ✓ Correct
Solution: A weighted wire passes through an ice block: pressure melts ice under the wire; the water refreezes above it — melting by pressure + refreezing = regelation.
Q7 — Latent Heat & Phase Transitions · easy · theory
Sublimation is the direct change from:
A. Liquid to solid
B. Solid to vapour without becoming liquid ✓ Correct
C. Vapour to liquid
D. Solid to liquid
Solution: Substances like camphor, dry ice (solid CO₂) and iodine pass directly from solid to vapour — sublimation.
Q8 — Latent Heat & Phase Transitions · medium · theory
Steam at 100°C causes more severe burns than boiling water at 100°C because:
A. Water cannot cause burns
B. Steam is hotter than 100°C
C. Steam has a higher specific heat
D. Steam additionally releases its large latent heat of vaporization (540 cal/g) on condensing ✓ Correct
Solution: Same temperature, but condensing steam first delivers 540 cal/g of latent heat before cooling as water — far more total heat to the skin.
Q9 — Latent Heat & Phase Transitions · easy · numerical
The heat required to melt 50 g of ice at 0°C (L_f = 80 cal/g) is:
A. 4000 cal ✓ Correct
B. 80 cal
C. 400 cal
D. 8000 cal
Solution: Q = mL_f = 50 × 80 = 4000 cal.
Q10 — Latent Heat & Phase Transitions · easy · numerical
The heat required to vaporize 20 g of water at 100°C (L_v = 540 cal/g) is:
A. 10800 cal ✓ Correct
B. 5400 cal
C. 1080 cal
D. 540 cal
Solution: Q = mL_v = 20 × 540 = 10800 cal.
Q11 — Latent Heat & Phase Transitions · easy · numerical
The heat needed to convert 10 g of ice at 0°C into water at 20°C is:
A. 1000 cal ✓ Correct
B. 200 cal
C. 1200 cal
D. 800 cal
Solution: Melt: 10 × 80 = 800 cal; warm: 10 × 1 × 20 = 200 cal. Total = 1000 cal.
Q12 — Latent Heat & Phase Transitions · easy · numerical
The heat required to convert 5 g of water at 100°C completely into steam at 100°C is:
A. 2900 cal
B. 540 cal
C. 500 cal
D. 2700 cal ✓ Correct
Solution: Q = mL_v = 5 × 540 = 2700 cal (temperature unchanged — pure phase change).
Q13 — Latent Heat & Phase Transitions · easy · numerical
The heat released when 15 g of steam at 100°C condenses into water at 100°C is:
A. 1200 cal
B. 8100 cal ✓ Correct
C. 4050 cal
D. 540 cal
Solution: Condensation releases the same latent heat: Q = 15 × 540 = 8100 cal.
Q14 — Latent Heat & Phase Transitions · medium · numerical
The heat needed to take 10 g of water from 20°C to steam at 100°C is:
A. 800 cal
B. 5400 cal
C. 7000 cal
D. 6200 cal ✓ Correct
Solution: Warm: 10 × 1 × 80 = 800 cal; vaporize: 10 × 540 = 5400 cal. Total = 6200 cal.
Q15 — Latent Heat & Phase Transitions · medium · numerical
The energy required to melt 2 kg of ice at 0°C in SI units (L_f = 3.36 × 10⁵ J/kg) is:
A. 6.72 × 10⁴ J
B. 3.36 × 10⁵ J
C. 1.68 × 10⁵ J
D. 6.72 × 10⁵ J ✓ Correct
Solution: Q = mL_f = 2 × 3.36 × 10⁵ = 6.72 × 10⁵ J.
Q16 — Latent Heat & Phase Transitions · medium · numerical
An ice cube of 100 g at 0°C receives heat at 100 cal/s. The time to melt it completely (L_f = 80 cal/g) is:
A. 100 s
B. 8 s
C. 800 s
D. 80 s ✓ Correct
Solution: Q = 100 × 80 = 8000 cal; t = 8000/100 = 80 s.
Q17 — Latent Heat & Phase Transitions · medium · numerical
The heat released when 10 g of water at 0°C freezes into ice at 0°C is:
A. 0 cal
B. 800 cal ✓ Correct
C. 80 cal
D. 8000 cal
Solution: Freezing releases the latent heat of fusion: Q = 10 × 80 = 800 cal.
Q18 — Latent Heat & Phase Transitions · medium · numerical
A 500 W heater is used to boil away 0.5 kg of water already at 100°C (L_v = 2.26 × 10⁶ J/kg). The time required is about:
A. ≈ 18.8 min
B. ≈ 10 min
C. ≈ 37.7 min ✓ Correct
D. ≈ 75 min
Solution: Q = 0.5 × 2.26 × 10⁶ = 1.13 × 10⁶ J; t = Q/P = 1.13×10⁶/500 = 2260 s ≈ 37.7 min.
Q19 — Latent Heat & Phase Transitions · medium · numerical
The ratio of heat required to vaporize m grams of water at 100°C to that required to melt m grams of ice at 0°C is:
A. 4 : 27
B. 1 : 1
C. 27 : 4 ✓ Correct
D. 80 : 540
Solution: L_v/L_f = 540/80 = 27/4. Speed-trick: remember 540/80 = 6.75 = 27/4.
Q20 — Latent Heat & Phase Transitions · medium · numerical
The total heat needed to convert 1 g of ice at 0°C to steam at 100°C is:
A. 800 cal
B. 720 cal ✓ Correct
C. 540 cal
D. 620 cal
Solution: Melt 80 + warm 100 + vaporize 540 = 720 cal. Speed-trick: memorise "80 + 100 + 540 = 720" per gram.