Heat to boil off water
How much heat is needed to vaporize 2.0 kg of water already at 100 °C? (Lv = 2,260 kJ/kg)
Answer C: explanation for every choice
A
This halves the mass.
B
This is the heat for 1 kg.
C
Correct. q = mL = (2.0)(2,260) = 4,520 kJ.
D
This doubles the correct value.
TakeawayPhase change at constant temperature: q = mL.
Equation usedHeat transfer
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