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Heat and Thermodynamics question

2002 · Shift 0 · Q148
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Heat and Thermodynamics question

2002 · Shift 0 · Q148

JEE MainPhysicsHeat and ThermodynamicsMCQ+4 / −1
If mass-energy equivalence is taken into account, when water is cooled to form ice, the mass of water should
  1. A
    increase
  2. B
    remain unchanged
  3. C
    decrease
  4. D
    first increase then decrease
View written solutionFree

Correct answer: C

  1. Use mass-energy equivalence

    Einstein’s relation is E=mc2E = mc^2E=mc2 so any decrease in internal energy corresponds to a decrease in mass: Δm=ΔEc2\Delta m = \frac{\Delta E}{c^2}Δm=c2ΔE​

  2. What happens when water cools and freezes?

    When water is cooled to form ice, it releases heat to the surroundings.

    • First, the water cools down to 0∘C0^\circ\text{C}0∘C.
    • Then, during freezing, it releases latent heat of fusion.

    Hence the internal energy of the system decreases: ΔE<0\Delta E < 0ΔE<0

  3. Effect on mass

    Since energy decreases, from Δm=ΔEc2\Delta m = \frac{\Delta E}{c^2}Δm=c2ΔE​ we get Δm<0\Delta m < 0Δm<0

    Therefore, the mass of water decreases slightly when it turns into ice.

  4. Check options

    • A: increase — Incorrect
    • B: remain unchanged — Incorrect
    • C: decrease — Correct
    • D: first increase then decrease — Incorrect

Therefore, the correct option is C.

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