When a flask of gas is cooled the pressure of the gas will change what would happen to the gas if it were cooled to an extremely low temperature?

Introduction

A small amount of water is added to an aluminum soda can and brought to boiling on a hot plate or with a Bunsen burner.  The water gas molecules will occupy all the space inside the can since the air molecules have been pushed out. The hot gas molecules are the same pressure as the air outside the can. When the can is placed in cold water upside down, the hot gas water molecules are cooled very rapidly. Some of the gas molecules are condensed back into liquid water so there are less molecules of water in the gas phase inside the can. The cold water will also cool any remaining gas molecules, decreasing their kinetic energy and therefore decreases the number of collisions with the walls of the can. This decreases the pressure inside the can.  Since the air pressure outside the can is stronger than that inside the can, it causes the can to collapse.

H2O(g)  à  H2O(l)

To Conduct Demonstration

1. Place the can containing water on a hot plate (turned to high) or a ring stand with a Bunsen burner underneath.
2. Allow several minutes for the water to come to a full boil.
3. Steam must displace the air inside the can; wait until you see a steady flow of steam exiting the spout, then immediately remove                   the can from the heat and place in the ice water bath.
4. As the hot steam cools and condenses to water, a vacuum is created inside the can and atmospheric pressure will crush it.

NOTES:

1. 250 ml water to a 5 gallon can
2. 20 min to boil, 1 or 2 min to collapse.  Collapsing will take longer if the can is left     to heat longer and   it itself gets hot.
3. Requires a large hotplate.

Safety 

If using a large can do not continue heating the can after inserting the rubber stopper as pressure will increase. 

A flask contains water and water vapor at 100oC. Each time ice water is poured over the outside of the flask, the water inside the flask boils. Although the temperature of the water in the flask has decreased, in each instance boiling is observed.

Keywords

vapor pressure, equilibrium, phase changes, physical properties, gases and liquids, solids and liquids, gases, gas laws

Multimedia

_Play movie (45 seconds, 2.7 MB)

 

 

 

 

 
Water boils in a flask, and later the flask is stoppered and removed from the hot plate. When ice water is poured over the flask, the water begins to boil. When the flask is cooled, the pressure of the water vapor decreases, which decreases the boiling point of the water. Note also that the temperature of the water in the flask decreases in the process. Ice water may be poured over the flask several times as the water cools. Boiling occurs each time ice water is poured over the flask.

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_Play movie (24 seconds, 1.1 MB)

 

 
When ice is brought to the bottom of the flask, the liquid in the flask is cooled. The liquid does not boil.

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Discussion

Initially, the liquid and vapor phases of the water are in equilibrium. When ice water is poured over the top of the flask, the vapor cools more rapidly than the liquid. After cooling, the pressure of the vapor phase is less than the vapor pressure of the liquid, so the liquid will boil until the pressure of the vapor increases to the vapor pressure of the liquid. At a location such as Denver, where atmospheric pressure is lower than a sea level, water boils below 100oC for the same reason as in this demonstration.

Additional still images for the first part of this chapter

Additional still images for the second part of this chapter

Demonstration Notes: Warnings, Safety Information, etc.

Exam and Quiz Questions

1. Explain why pouring ice water over the flask makes water boil.

2. Does pouring ice water over the flask increase, decrease or leave unaffected the pressure of the water vapor in the flask?

3. What might be the effect if 100 oC water were poured over the flask? Would the water inside boil?

4. Why doesn't the boiling process build up sufficient pressure to pop the stopper off the top of the flask?

Page 2

A pellet of 1-2-3 yttrium barium copper oxide (YBa2Cu3O7-x) is cooled with liquid nitrogen. A magnet placed above the pellet is levitated. At a higher temperature the magnet rests on the pellet, but cooling once again causes the magnet to rise. The levitated magnet is spun with plastic tweezers

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Keywords

electrical conductivity, practical applications, phase changes, physical properties, solid state, solids and liquids, real life/environment, f block

Multimedia

_Play movie (30 seconds, 1.8 MB)

 

 

 

A 1-2-3 yttrium barium copper oxide superconducting pellet is cooled in liquid nitrogen and placed on an inverted foam cup. When a magnet is placed above the pellet, it is levitated by the superconductor.

_Play movie (28 seconds, 1.7 MB)

 

 

 

A magnet placed on the pellet that is above the superconductivity transition temperature does not float. Cooling the pellet through the phase change again levitates the magnet.

_Play movie (25 seconds, 1.5 MB)

 

 

The magnet can be spun, showing the dynamic response of the magnetic field expulsion. Here plastic tweezers are used to spin the magnet.

Discussion

When cooled below a "critical" temperature, some substances lose all resistance to the flow of electrical current, becoming superconductors. A current induced in a superconducting material can flow forever, providing that the temperature remains below the critical temperature. Another property of a superconductor is its interaction with a magnetic field. A magnet induces a current in the superconductor, creating a magnetic field that repels the field of the magnet. This repulsive interaction is responsible for the levitation observed in the demonstration.

It has been determined that substances with the formula YBa2Cu3O7-x behave as superconductors at relatively high temperatures (above the boiling point of liquid nitrogen). The discovery of these materials was an important breakthrough, since liquid nitrogen is cheaper and easier to handle than the liquid helium required to bring low temperature superconductors below their critical temperatures. The designation 1-2-3 refers to the relative amounts of yttrium, barium and copper in the superconductor. Note that the compound is nonstoichiometric; that is, the subscript for oxygen is not an integer (in our compound x is a little less than 0.1). Note also that the material contains a mixture of Cu2+ and Cu3+ ions.

Superconductors are already employed in instruments requiring large magnetic fields, such as nuclear magnetic resonance spectrometers. Other potential applications include resistanceless power transmission cables and levitated vehicles.

Additional still images for this topic

Demonstration Notes: Warnings, Safety Information, etc.

Exam and Quiz Questions

1. Why is liquid nitrogen necessary in this demonstration?

2. Why is the small magnet levitated?

Page 3

A novelty store "Drinking Bird" repeatedly dips is beak into a beaker of water and then bobs back to an upright position. Visible movement of a liquid in the bird's body accompanies the bobbing; the movement is caused by changing vapor pressure of the liquid due to different temperatures at the bird's head and the bird's body. Using a heat lamp on the head does not cause bobbing, but using a heat lamp on the body does cause bobbing.

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Keywords

vapor pressure and temperature, liquids, evaporation, vapor pressure, evaporative cooling, solids and liquids, gases

Multimedia

Introduction

_Play movie (36 seconds, 2.1 MB)

 

 

 

 
This drinking bird is available in some novelty stores. The vapor pressure differential between the head and the base causes the liquid to rise in the neck. When the liquid enters the head, it causes the bird to tip over into the water for another drink. The bird's head is covered with a material that absorbes water. The base is closed at the top and so the only way that liquid and vapor can escape is through the tube which extends almost to the bottom of the base.

Effect of heat

_Play movie (47 seconds, 2.6 MB)

 

 

 

 

 

Heating the head of the bird forces the liquid into the base. This does not help the bird move.

Heating the liquid in the base causes an increase of the vapor pressure in the base. This causes a change in the differential of the vapor pressure between the head and the base, which causes the liquid to rise in the tube, ultimately causing the bird to tip over and drink.

Effect of water (condensation/cooling)

_Play movie (1 minute 15 seconds, 4.3 MB)

 

 

 

 

 

Evaporating water cools the liquid in the base. This lowers the vapor pressure and pulls the liquid back into the base.

Evaporating water cools the head of the bird. This decreases the vapor pressure in the head. The vapor pressure difference between the head and the base pulls the liquid into the head, ultimately causing the bird to tip over and drink.

Discussion

Additional still images for this topic

Demonstration Notes: Warnings, Safety Information, etc.