Physics Ch. 17 Questions

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Four common phases of matter

solid, liquid, gas, plasma phases depend on temperature and pressure on the matter. Phase changes require a transfer of energy.

speed of molecules in a liquid

molecules in a liquid have a wide variety of speeds.


the fastest molecules at the surface of a liquid have escape velocity and leave to become gas gain KE by being pumped from below. "Billiard-ball physics"

Why is evaporation a cooling process?

when the faster surface molecules leave with gained KE, the slower, cooler ones remain.

What is sublimation? A heating or cooling process?

the fastest molecules at the surface of a solid have escape velocity and leave to become a gas. It leaves behind the slower, cooler molecules.


gas becomes a liquid. Warming process. Gas molecules attracted to liquid strike the surface with increased KE. opposite Evaporation

Why is a steam burn more painful that a boiling water burn at same temperature?

Steam releases considerable energy when it condenses to a liquid and wets the skin.

Why hot humid days make us feel uncomfortably warm?

cooling sweat does not evaporate well in humid air. "condensation counteracts the evaporation of perspiration"

distinguish "humidity" from "relative humidity"

humidity: mass of water vapor per unit volume of air. relative humidity: ratio of the amount of water vapor currently in the air (at a given Temperature) to the maximum amount of water the air could contain (at same Temperature)

why does water vapor in the air condense when the air is chilled?

cool air contains slower water molecules, which often stick together when they collide.

why does warm, moist air form clouds when it rises?

warm air rises, expands and cools. Water drops condense when moist air cools.

what is the difference between a cloud and fog?

the main difference is altitude. Fog is a cloud near the ground.

Distinguish evaporation from boiling.

evaporation happens at a liquid’s surface, while boiling occurs in the bulk of the liquid beneath the surface. Bubbles of vapor form and are buoyed to the surface to escape. Phase change throughout the liquid. Both are cooling processes.

Will increased atmospheric pressure increase or decrease the boiling point of water?

The boiling point is raised. Higher pressure collapses the bubbles that form. This increases the boiling point of water. Boiling cannot occur until the vapor Pressure within the bubbles overcomes the increased pressure on the water.

What cooks food faster in a pressure cooker?

The higher temperature of boiling water under pressure cooks the food faster. The tight lid holds pressurized water vapor above the water surface, inhibiting boiling. Water must boil at a higher temperature, increased boiling point.

why doesn’t water at the bottom of a geyser boil when it is at 100°C?

The water at the geyser’s bottom is under pressure from the column of water above it.

what happens to the water pressure at the bottom of a geyser when some of the water above gushes out? What is the result?

as water gushes out, the pressure on the remaining water is reduced; the water boils rapidly and erupts with great force. Afterwards, the geyser refills and repeats the cycle.

why doesn’t energy added to boiling water increase the temperature of the water?

Boiling is a cooling process; energy is removed as quickly as it is added.

when will water boil at a temperature lower that 100°C?

at pressures lower than atmospheric pressure. Higher altitudes have lower pressure. Ex.) Denver, CO, water boils at 95°C instead.

Evidence that water can boil at a temperature of 0°C?

Ice will form in boiling water in a vacuum chamber. Pressure is greatly reduced, water starts to boil. Continued boiling results in lowered temperature, and lower boiling point. Finally a boiling pt of 0°C is reached, cooling by boiling causes ice to form over the boiling water surface! Ex.) freeze-dried coffee.

why does increasing the temperature of solids make them melt?

increasing the temperature also increases molecular vibrations until attractive forces can no longer hold the molecules in one place.

why does decreasing the temperature of a liquid make it freeze?

cooling slows molecular motion until attractive forces between molecules can hold them in place. The molecules will vibrate in fixed positions and form a solid.

why doesn’t water freeze at 0°C when foreign molecules or ions are present?

foreign molecules and ions get between water molecules and ice crystals, this impeding crystal formation.

what happens to the hexagonal open structure of ice when sufficient pressure is applied to it?

the crystals are simply crushed to the liquid phase.

why doesn’t a wire simply cut a block of ice in two when it passes through the ice?

Pressure beneath the wire melts the ice without warming it, so the liquid flows above the wire, where it refreezes at lower pressure.

Does a liquid release or absorb energy when it changes into a gas? Into a solid?

liquid absorbs energy to evaporate; liquid releases energy when to freeze.

does a solid release or absorb energy when it changes into a liquid? when it sublimes into a gas?

solids absorb energy to melt; solids absorb energy to sublime.

Describe the condensation, evaporation, and heat flows that happen in a refrigerator.

Liquid evaporates inside the refrigerator, absorbing energy. Gas condenses outside the refrigerator, releasing energy.

How many calories are needed to change the temperature of 1 g of water by 1°C? To melt 1 g of ice at 0°C? To vaporize 1 g of boiling water at 100°C?

1 calorie is needed to change 1 g of water by 1°C. 80 calories are needed to melt 1 g of ice at 0°C. 540 calories are needed to vaporize 1 g of boiling water at 100°C.

Cite two reasons why firewalkers don’t burn their wetted feet when they walk barefoot on red-hot coals.

Hot coals have a low conductivity, and water on wet feet absorbs a lot of energy as it vaporizes. Energy that would normally go into burning skin instead does into changing the phase of water on the skin. The energy converts the water into vapor, an insulating layer between the feet and coals.

Air-conditioning units contain no water whatever, yet it is common to see water dripping from them when they’re operating on a hot day. Explain.

Does a common electric fan cool the air in a room? If not, then why is it used in an overly warm room?

Why will wrapping a bottle in a wet cloth at a picnic often produce a cooler bottle than placing the bottle in a bucket of cold water?

The human body can maintain its customary temperature of 37∘C on a day when the temperature is above 40∘C. How is this done?

Water will boil spontaneously in a vacuum – on the surface of the Moon, for example. Could you cook an egg in this boiling water? Explain.

What can go wrong if you grasp the handle of a hot skillet with a wet thin dishcloth?

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