Chapter 17: Change of Phase
energy is absorbed in the direction of: energy is released in the direction of:
- absorbed = solid>liquid>gas - released = gas>liquid>solid
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, 80 calories, 540 calories
What happens to the hexagonal open structure of ice when sufficient pressure is applied to it?
The crystals are crushed to form the liquid phase.
What cooks food faster in a pressure cooker?
The higher temperature of boiling water under pressure cooks the food faster.
boiling
change of phase (throughout the liquid) - cooling process - rapid evaporation beneath surface forms bubbles (if bubble is cooler than temp., it collapses at surface) - most energetic molecules escape (same as evaporation) *depends on pressure - by lowering pressure, water will boil at lower temp.
energy and phase change
during phase change, temp. is constant.
heat pump
evaporation & condensation (condensation coils)
4 phases of matter: (what do they depend on)
solid liquid gaseous plasma *depends of temperature and pressure
sublimation
solid > gas *requires low pressures & temperatures - cooling process ex: (dry ice) - fastest molecules at the surface of a solid have escape velocity and leave to become a gas. It leaves behind the slower, cooler molecules.
latent heat of fusion
solid > gas (sublimation) heat of fusion for water = 334 J/g Q = mLf
evaporation vs. boiling
Evaporation - surface of the liquid. boiling - bulk of the liquid.
Use these relationships to determine the number of calories to change 1.3 kg of 0∘C ice to 0∘C ice water.
Q = 100 kcal
Use these relationships to determine the number of calories to change 1.3 kg of 0∘C ice water to 1.3 kg of 100∘C boiling water.
Q = 130 kcal
Use these relationships to determine the number of calories to change 1.3 kg of 100∘C boiling water to 1.3 kg of 100∘C steam.
Q = 700 kcal
Use these relationships to determine the number of calories to change 1.3 kg of 0∘C ice to 1.3 kg of 100∘C steam.
Q = 940 kcal
Why does warm, moist air form clouds when it rises?
Rising air expands and cools. Water drops condense when moist air cools.
Changes of phase most always require _________
a transfer of energy
Which involves the greatest number of calories? a. condensing 1g of 100C steam to 100C water b. cooling 1g of 100C water to 1g of 0C ice c. cooling 1g of 0C ice to near absolute zero
a. condensing 1g of 100C steam to 100C water
What happens to water at room temperature if you decrease the atmospheric pressure around it?
boil then freeze
condensation
gas > liquid *gas molecules attracted to surface of liquid - warming process (KE absorbed by the liquid, increasing the temperature) - when temp. is low, water molecules in air move slowly, sticking together causing fog.
evaporation
liquid > gas - only at surface of liquid - cooling process (When the fastest molecules leave during evaporation, the slower, cooler ones remain, therefore average KE/temp. decreases) - helps maintain stable body temp. (sweating)
latent heat of vaporization
liquid > gas (evaporation) heat of vaporization for water = 2256 J/g Q = mLv
melting vs. freezing
melting: - solid > liquid - add energy freezing: - liquid > solid - remove energy
Ice is put in a picnic cooler. To speed up the cooling of cans of beverage, it is important that the ice:
melts (heat transfer - cans absorb ice)
What do you use a pressure cooker for?
to prevent boiling and cooling - It increases the amount of internal energy because it does not allow the heat to boil the water.
when snow forms in clouds, the surrounding air is:
warmed (condensation)
firewalking is best when your feet are
wet (evaporate water before heating foot)
