Black latent heat

Joseph Black was a pioneer in careful measurements of heat transfer, making important contributions in the areas of calorimetry, specific heat, and latent heat. These exercises involve latent heat.

Common opinion had held that to melt a solid required only a tiny addition of heat once the solid had been raised to its melting temperature. Black noted that the quantity of heat required was actually substantial. He gave the term latent heat to the "hidden" heat required to bring about a phase transition in addition to heating a material to the proper temperature.

I have ... put a lump of ice into an equal quantity of water, heated to the temperature 176[°F], and the result was, that the fluid was no hotter than water just ready to freeze. Nay, if a little sea salt be added to the water, and it be heated only to 166 or 170, we shall produce a fluid sensibly colder than the ice was in the beginning, which has appeared a curious and puzzling thing to those unacquainted with the general fact.
1) Using modern data on the heat capacity and the latent heat of fusion of water, predict the final temperature that would result from the experiment Black describes. Assume that he placed 100. g of ice at 0.0°C in a well-insulated container with 100. g of water at 80.0°C (176°F). The latent heat of fusion of water is 333.5 J/g and the specific heat of liquid water is 4.18 J/g°C .

2) Qualitatively rationalize how Black's salt-water variation is possible: how can one mix equal masses of ice at 0°C and salt water at, say, 75°C, and end up with salt water at a temperature below zero?


Joseph Black, Lectures on the Elements of Chemistry delivered in the University of Edinburgh by the Late Joseph Black, M.D. ... published from his manuscripts by John Robison (1803)

Copyright 2003 by Carmen Giunta. Permission is granted to reproduce for non-commercial educational purposes.

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