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Q & A: phase equilibrium at constant pressure

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Most recent answer: 06/12/2016
A 1 kg of ice at 0 degrees Celsius is placed into a PERFECTLY INSULATED, SEALED CONTAINER that has 1 kg of water also at 0 degrees Celsius. The water and ice completely fill the container, but the container is flexible. After some time one can expect that: a) the water will freeze so that the mass of the ice will increase b) the ice will melt so that the mass of the ice will decrease c) both the amount of water and the amount of ice will remain constant d) both the amount of water and the amount of ice will decrease Answer is (B). But I don't know why.
- Shreya Singh (age 17)
Varanasi, Uttar Pradesh, India

I'm baffled that you were told the answer is (B). The flexible container means, I presume, that the pressure remains at 1 atmosphere, so that T=0°C is the melting/freezing temperature. If some of the ice melts, that would soak up latent heat, causing T to drop, causing freezing. If some more ice would form, that would release latent heat, raising T and causing ice to melt. In other words, the equilibrium between water and ice is stable. For a given amount of enthalpy (thermal isolation), the starting ratio remains the lowest free-energy condition.

Mike W.

p.s. On thinking more about it, here's how someone may have gotten confused. The external pressure would do a little positive work on the water as it melts because water has less volume than ice. You might think that would heat the material up and drive more melting. However, this small effect is already included in the latent heat.

(published on 06/12/2016)

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