Ch 12 Answers

Chapter 12 continued
16. Heat Chefs often use cooking pans made of thick aluminum. Why is thick aluminum
better than thin aluminum for cooking?
Thick aluminum conducts heat better and does not have any “hot spots.”
17. Heat and Food It takes much longer to bake a whole potato than to cook french
fries. Why?
Potatoes do not conduct heat well. Increasing surface area by cutting a
potato into small parts increases heat flow into the potato. Heat flow from
hot oil to the potato is also more efficient than from hot air.
18. Critical Thinking As water heats in a pot on a stove, the water might produce
some mist above its surface right before the water begins to roll. What is happening,
and where is the coolest part of the water in the pot?
The heat flows from the burner (the hottest part) to the top surface of the
water (coldest). The water first transfers heat from bottom to top through
conduction, and then convection begins to move hot water in currents to
the top.
Practice Problems
12.2 Changes of State and the Laws of
Thermodynamics
pages 323–331
page 325
19. How much heat is absorbed by 1.0010 2 g of ice at 20.0°C to become water at
0.0°C?
Q mCT mH f
(0.100 kg)(2060 J/kg°C)(20.0°C) (0.100 kg)(3.3410 5 J/kg)
3.7510 4 J
20. A 2.0010 2 -g sample of water at 60.0°C is heated to steam at 140.0°C. How
much heat is absorbed?
Q mC water T mH v mC steam T
(0.200 kg)(4180 J/kg°C)(100.0°C 60.0°C) (0.200 kg)(2.2610 6 J/kg)
502 kJ
(0.200 kg)(2020 J/kg°C)(140.0°C 100.0°C)
21. How much heat is needed to change 3.0010 2 g of ice at 30.0°C to steam at
130.0°C?
Q mC ice T mH f mC water T mH v mC steam T
(0.300 kg)(2060 J/kg°C)(0.0°C (30.0°C)) (0.300 kg)
(3.3410 5 J/kg) (0.300 kg)(4180 J/kg°C)(100.0°C 0.0°C)
(0.300 kg)(2.2610 6 J/kg) (0.300 kg)(2020 J/kg°C)(130.0°C 100.0°C)
9.4010 2 kJ
274 Solutions Manual Physics: Principles and Problems
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