Question

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

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

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#

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#

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#

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.

Key concepts

Vapor Pressure

Vapor pressure is a crucial concept when understanding the boiling process. It is the pressure exerted by a vapor in equilibrium with its liquid phase at a given temperature. When you heat water, more molecules from the liquid phase gain enough energy to enter the vapor phase, increasing the vapor pressure.
When vapor pressure equals atmospheric pressure, boiling occurs. This is the point where water molecules have enough energy to escape into the air, overcoming the atmospheric pressure. In a sealed environment, like a stoppered flask, the vapor pressure plays a critical role. If the pressure inside the flask drops, as seen when cooling the neck of the flask, the vapor pressure can exceed the internal pressure, causing boiling to resume. Thus, vapor pressure is key in determining when and why a substance boils.

Gas Laws

Gas laws help explain the behavior of gases under various conditions. Particularly, Boyle's Law and Charles's Law are relevant when examining the boiling process in our experiment. Boyle's Law states that the pressure of a gas is inversely proportional to its volume at constant temperature, while Charles's Law states that the volume of a gas is directly proportional to its temperature at constant pressure.

• In the cooling of the flask's neck, the reduction in temperature decreases the pressure inside the flask.
• According to the gas laws, as the temperature decreases, the pressure decreases.

Because of this drop in pressure and Boyle’s Lie confirming pressure’s decrease with volume remains unchanged, the vapor pressure of the water inside surpasses the internal pressure, leading to boiling.

Hydrogen Bonds

Hydrogen bonds are strong intermolecular forces present in water, responsible for its relatively high boiling point. When heating water, the energy provided is used to break these hydrogen bonds. Only after enough bonds are broken does the water transition from liquid to vapor.
In the stoppered flask scenario, the hydrogen bonds are continuously broken and reformed. When reheating is stopped, fewer molecules can overcome these bonds to enter the vapor phase, but the sealed environment helps sustain equilibrium. This understanding of hydrogen bonding explains why water requires energy (in the form of heat) to boil, and how these bonds reform as water returns to the liquid phase when not heated.

Phase Change

Phase changes involve transforming a substance from one state of matter to another, such as liquid to gas in boiling. These changes are driven by temperature and pressure conditions. In our exercise, heating initiates the phase change by providing energy for water to transition from liquid to vapor.
Once you stop heating and cool the neck of the flask, a pressure drop inside the flask reverses this phase change. The lower pressure allows water to boil at a reduced temperature as it readjusts to match the vapor pressure with the prevailing pressure inside the flask.

• Boiling is essentially a phase change that allows equilibrium between liquid and vapor phases.
• The experiment aptly demonstrates how manipulating pressure enables phase transitions even when temperature conditions change.

This cycle of phase transitions illustrates the delicate balance of temperature, pressure, and molecular interactions in everyday physical phenomena like boiling.