Chapter 4 Question Set.
6. Why do table of densities always include the temperature for which the listed values hold?
What would be true of the densities of most solids and liquids at a temperature higher than
the quoted one?
Answer:
a) Because the densities are temperature dependent.
b) The densities of most solids and liquids would be lower at higher temperatures than
the quoted one.
8. A wooden block is submerged in a tank of water and pressed down against the bottom of
the tank so that there is no water underneath it. The block is released. Will it rise to the
surface or stay where it is?
Answer:
It depends on the buoyancy of the wood. If the buoyancy is greater than that of the water, it
will rise to the surface. If it is less than that of the water, it will stay submerged.
9. When a person drinks a soda through a straw, where does the force come from that
causes the soda to move upward?
Answer:
The person reduces the pressure in the straw by sucking on it, and atmospheric pressure the
forces the liquid upward.
12. A U-shaped tube contains water and an unknown liquid separated by mercury as in
Figure 4-53 in your book. How does the density of the liquid compare with the density of
water? How do the pressures at A and B compare?
Answer:
a) The density of the unknown liquid is greater than the density of water.
b) They are equal (assuming the upper ends of the tube are open to equal pressures).
14. A bridge in Sweden carries the Göta Canal over a highway.
What, if anything, happens
to the load on the bridge when a boat passes across it in the canal?
Answer:
Nothing. The boat is floating on the water and its force down, due to gravity is the same as
that of the water it displaces.
16. An ice cube with an air bubble inside it is floating in a glass of water.
Compare what
happens to the water level in the glass when the ice melts with what would happen if the
cube had no bubble in it.
Answer:
The ultimate water level would be lower for the ice cube with the air bubble in it since the air
has displaced the water that would otherwise be added to the cup.
20. Gas molecules have speeds comparable with those of rifle bullets, yet it is observed that
a gas with a strong odor (ammonia, for instance) takes a few minutes to diffuse through a
room. Why?
Answer:
There is no order or uniformity to the speed or motion of gas molecules. Their velocity is an
average velocity in all directions so it takes time for enough gas molecules to move far
enough in any direction to be smelled.
22. The pressure on a sample of hydrogen is doubled, while its temperature is kept
unchanged. What happens to the average speed of the hydrogen molecules?
Answer:
Since the absolute temperature of a gas is proportional to the average kinetic energy of its
molecules (and, therefore their velocity2 [KE = ½ mv2]), their speed would remain unchanged.
24. Temperatures in both Celsius and Fahrenheit scales can be negative.
Why is a negative
temperature impossible on the absolute scale?
Answer:
Because absolute zero is the temperature at which molecules have zero kinetic energy.
Since kinetic energy cannot be less than zero, the temperature could not get any lower.
27. Why is a piece of ice at 0C more effective in cooling a drink than the same mass of cold
water at 0C?
Answer:
Because the ice can also contribute its Heat of Fusion to the cooling process. The cold water
can only contribute its specific heat.
28. Would it be more efficient to warm your bed on a cold night with a hot water bottle that
contains 1 kg of water at 50C or with a 1-kg gold bar at 50C? Why?
Answer:
It would be more efficient with the hot water bottle. The specific heat of water is a whopping
4.2 kJ/kg*C while that of gold is only 0.13 kJ/kg*C.
30. Why does evaporation cool a liquid?
Answer:
Evaporation is the loss of faster molecules at the liquid/air interface. Since the remaining
molecules are the slower ones, evaporation leaves cool liquids behind.
38. When salt is dissolved in water, do you think the entropy of the system of salt + water
increases or decreases? Why?
Answer:
Entropy increases. The salt molecules are now freed from their rigid crystalline structure and
able to move freely about in the solution. Less structure = greater entropy.