Final temperature after mixing water
100 g of water at 80 °C is mixed with 300 g of water at 20 °C in an insulated container. What is the final temperature?
Answer B: explanation for every choice
A
30 °C is half the temperature difference, not the final temperature.
B
Correct. Heat lost = heat gained: 100(80 − T) = 300(T − 20), so 400T = 14,000 and T = 35 °C.
C
This is a simple average, ignoring the different masses.
D
This swaps the masses, weighting the hot water more heavily.
TakeawayFor the same substance, the final temperature is the mass-weighted average.
Equation usedHeat transfer
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