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Chapter 10: Problem 3

Explain the following: You add 100 mL water to a 500-mL round-bottom flask and heat the water until it is boiling. You remove the heat and stopper the flask, and the boiling stops. You then run cool water over the neck of the flask, and the boiling begins again. It seems as though you are boiling water by cooling it.

Short Answer

Expert verified
In summary, the phenomenon of water boiling after cooling the neck of a round-bottom flask containing previously boiling water is due to the relationship between pressure and temperature in the boiling process. When the neck of the flask is cooled, the pressure inside the flask decreases, which allows boiling to occur again at a lower temperature, creating the appearance of boiling water by cooling it. In reality, it is the pressure change that is causing the water to boil again.

Step by step solution

01

Initial Boiling of water

The water is heated in a 500-mL round-bottom flask until it reaches its boiling point. At this point, the heat energy supplied is used to break the hydrogen bonds between water molecules, transforming it from a liquid to a vapor state. The boiling process occurs when the vapor pressure of the water becomes equal to the atmospheric pressure. #Step 2: Removal of heat and stoppering the flask#
02

Removal of heat and stoppering the flask

The heat source is removed, and the flask is immediately stoppered, sealing the flask and preventing the transfer of matter (water vapor) between the flask's interior and the surrounding atmosphere. Once the heat is removed, the boiling process stops as the water temperature starts to decrease, leading to a decrease in vapor pressure. #Step 3: Cooling the neck of the flask#
03

Cooling the neck of the flask

By running cool water over the neck of the flask, the temperature of the water vapor that is trapped in the flask decreases. According to the gas laws, when the temperature of a gas decreases, its pressure also decreases (given by the equation \(P\propto T\), where P is the pressure, and T is the temperature). #Step 4: Boiling resumes as a result of the pressure change#
04

Boiling resumes as a result of the pressure change

As the pressure inside the flask decreases due to the cooling, the vapor pressure of the liquid is now greater than the pressure inside the flask. This causes the water to boil again to establish an equilibrium between the vapor pressure of the water and the pressure inside the flask. In summary, cooling the neck of the flask creates a decrease in pressure, which allows boiling to occur again at a lower temperature than before. The phenomenon appears as if boiling the water by cooling it, but in reality, it is the pressure change that is causing the water to boil again.

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