Energy and the Phase Change of Water

Conceptual Physics

Text Correlation: Section 23.8

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Question 1

• On the graph, why is the slope of the curve for ice steeper than the slope for water and steam?

• Answer: The slope is (temperature change/heat change), which is the specific heat capacity, c. For ice, c = 0.50 cal/g °C; water c = 1.0 cal/g °C; steam, c = 0.50 cal/g °C. So we see the slopes for ice and steam are twice as steep as for water.

Question 2

• How does the 540 cal/gram heat of vaporization for water compare to the heat needed to bring a gram of 0 kelvin ice up to boiling water?

• Answer: The 540 cal/gram value is a large value, more than the energy needed to bring 0 kelvin ice to boiling. To bring a gram of ice at absolute zero to o°C ice would take less than 273 calories (the specific heat of ice is not constant all the way down to absolute zero) .

• Then to melt the ice would take another 80 calories, and to warm the water to boiling would take another 100 calories. All total, we have only about 300 calories, appreciably less than 540 calories!