Chapter 17

Why doesn't water freeze at 0 C when foreign ions are present?

Foreign ions decrease the number of water molecules at the interface between ice and water where freezing occurs.

Does the heat that is discharged at the condensation coils following the compression pump of a refrigerator involve vaporization or condensation of the refrigerating liquid?

Heat is discharged by condensation.

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.

Why do you feel uncomfortably warm on a hot and humid day?

Humid days have a lot of moisture in the air. So when its hot and humid, your sweat cannot evaporate as well as it does on hot, dry days - which makes you sweat even more.

Distinguish between humidity and relative humidity.

Humidity is a measure of the amount of water vapor. Relative humidity is the ratio of the amount of water vapor currently in the air at a given temperature to the greatest amount of water vapor the air can contain at that temperature.

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

Increase the boiling point of water.

Why does increasing the temperature of a solid make it melt?

Increased temperature means increased motion, which means more chance of molecule separation

Ice is put in a picnic cooler. To speed up the cooling of cans of beverage, it is important that the ice

Melts. For each gram of ice that melts, 540 calories is taken from the beverage.

Do the molecules in a liquid all have about the same speed, or do they have a wide variety of speeds?

No, they have a variety of speeds.

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?

One calorie; 80 calories; 540 calories.

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

Since the pressure at the bottom of a column of water is directly proportional to the depth, when water gushes out, the pressure at the bottom is reduced.

What are the four common phases of matter?

Solid, liquid, gas, and plasma.

Why is a steam burn more damaging than a burn from boiling water at the same temperature?

Steam contains more energy than boiling water of the same temperature.

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

The block doesn't separate into two pieces because the water above the tire refreezes when pressure on it is reduced.

What is sublimation?

The change of phase from solid to gaseous, bypassing the liquid phase.

What is evaporation?

The change oh phase from liquid to gas that occurs at the surface of a liquid.

Is it the boiling of water or the higher temperature of water that cooks food faster in a pressure cooker?

The higher temperature cooks food faster.

If bits of coals do not stick to your feet when firewalking, it's best if your feet are

Wet. The energy that vaporizes water is energy that doesn't burn your feet.

Distinguish between evaporation and boiling.

When evaporation occurs beneath the surface of a liquid, it is said to be boiling.

When will water boil at a temperature lower than 100 C?

When the air pressure above is reduced?

When snow forms in clouds, the surrounding air is

warmed. The change of phase is from gas to solid, which releases energy.

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

A liquid absorbs energy when it changes into a gas.

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

A liquid releases energy when it changes into a solid.

What is the basic difference between a cloud and fog?

Altitude.

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

As the air rises, it cools and condenses to form clouds.

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

Because H2O molecules form open structures in solid phase, the application of pressure lowers the melting point of ice. The crystals are crushed to the liquid phase.

Why doesn't energy added to boiling water increase the temperature of the water?

Because adding energy pulls apart the attraction among the molecules and therefore go into potential energy.

Why doesn't the water at the bottom of a geyser boil when it is at 100 C?

Because of the immense pressure at the bottom of the ocean is unable to turn to gas so does not actually boil. It remains in a super heated liquid state and quickly rises through the colder water until the water pressure is such that the water can expand into gas or cools to the temperature of the water around it.

Distinguish between condensation and evaporation.

Condensation is the changing of a gas to a liquid. Evaporation is the changing of a liquid to gas.

When you step out after a hot shower you feel cold, but you can feel warm again if you step back into the shower area. Which process is responsible for this?

Condensation.

Which involves the greatest number of calories?

Condensing 1 gram of 100ºC steam to 100º water

The process of boiling

Cools the water being boiled and depends on atmospheric pressure. Boiling is also a change of phase below the water surface.

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

Decreased temperature means decreased motion, which means less chance of molecule separation and more chance of sticking together.

Why is evaporation a cooling process?

Evaporation is a cooling process because energy that is taken from the remaining liquid by those molecules undergoing a phase change causes the temperature of the remaining liquid to decrease.