Question

Explain the following: You add \(100 \mathrm{~mL}\) water to a \(500-\mathrm{mL}\) 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, running cool water over the neck of the stoppered flask reduces the pressure inside, causing the boiling point of the water to decrease. The water inside the flask is still hot enough to boil at this new lower boiling point, hence the boiling resumes. This phenomenon is due to the change in pressure and subsequent lowering of the boiling point, not the act of cooling the water itself.

Step-by-step solution

Understand the initial situation

Initially, we have a round-bottom flask containing 100 mL of water. The flask is heated until the water reaches boiling point (approximately \(100^\circ \mathrm{C}\) at sea level).

Observe the change after removing heat

After the heat source is removed and the flask is stoppered, the boiling stops because the energy source (heat) is removed, and the flask is sealed to not allow any more heat or gas exchange from the surroundings.

Analyze the effect of running cool water on the neck

When cool water is run over the neck of the stoppered flask, the pressure inside the flask starts decreasing. Cooling the neck of the flask causes the vapor inside to condense back into a liquid as the temperature drops, reducing the gas pressure inside the flask.

Understand the relation between pressure and boiling point

The boiling point of water depends on the atmospheric pressure. At a lower pressure, the boiling point of the water decreases, meaning that water boils at a lower temperature than it would at sea level.

Relate the reduced pressure in the flask with boiling point

As the pressure inside the flask decreases due to the cooling of the neck, the boiling point of the water inside the flask also decreases. Since the flask still has heat energy stored from the initial boiling, the cool water running outside the neck doesn't significantly affect the temperature of the water inside the flask. In simpler terms, the water inside is still hot enough to boil at the reduced pressure.

Explain the observed phenomenon

Now that the pressure inside the flask has been reduced, the boiling point of the water has also decreased, allowing the still-hot water to start boiling at the new lower boiling point. Thus, the phenomenon observed, where boiling seems to begin again while cooling, is actually due to the change in pressure and the resulting decrease in the water's boiling point within the flask.

Key concepts

Pressure and Temperature Relationship

In thermodynamics, the relationship between pressure and temperature is crucial in understanding many natural phenomena. When you heat water to its boiling point, you're adding energy to it. This energy makes the water molecules move faster, increasing their kinetic energy. At boiling point, these molecules have enough energy to escape into the air as vapor.
When the flask is stoppered after boiling, and heat is removed, the pressure inside stabilizes, and boiling ceases. This is because the vapor pressure produced by the fast-moving molecules is no longer sufficient to overcome external pressure without the added energy from heat. The key takeaway is that a liquid only boils when its vapor pressure equals the surrounding pressure, which is why changes in pressure and temperature affect boiling behavior.

Phase Changes

Phase changes occur when a substance transitions from one state of matter to another, for example, from liquid to gas. Boiling is a type of phase change where liquid water becomes gaseous steam. This process requires energy because the strong bonds holding water molecules together in the liquid state must be broken for the molecules to escape as gas.
Removing heat or changing pressure, as in the flask experiment, can reverse this phase change. When the temperature drops due to cooling, pressure decreases, altering the conditions for phase change. The water remains hot enough to boil because the reduced pressure lowers the energy needed for the molecules to remain in the gas phase. Thus, the cooling of the flask’s neck indirectly promotes continued vaporization by reducing external pressure.

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid. This occurs when molecules escape liquid and enter the gaseous phase. At any given temperature and in a closed system, such as your stoppered flask, vapor pressure reaches a balance with atmospheric pressure.
When you introduce cold water to the flask's neck, the system's equilibrium is disrupted. The vapor inside condenses as the temperature and pressure drop, but the heat stored in the water maintains sufficient vapor pressure. This enables boiling at the new, lowered boiling point. Vapor pressure, therefore, directly influences when and at what temperature water will change phases.

Condensation

Condensation is the process by which vapor transforms back into liquid. This occurs when vapor molecules lose energy, usually due to a drop in temperature. In the flask experiment, running cool water over the flask's neck promotes condensation by reducing the temperature and pressure in the vapor space above the water.
The decreased pressure inside the flask allows the remaining water to boil again even though external cooling occurs. This is because while condensation lowers the vapor pressure, it also enables a new boiling equilibrium due to the reduced atmospheric pressure, demonstrating the interconnected nature of condensation and boiling.

Heat Transfer

Heat transfer is the movement of thermal energy from one body or substance to another. This process occurs through conduction, convection, or radiation. In the flask scenario, heat transfer occurs initially when you heat the water until boiling. The water absorbs energy until the molecules have enough energy to transition into steam.
When removing the heat source and cooling the flask's neck, heat transfer still occurs. Although the neck cools the vapor, the heat energy in the water persists. This retained heat supports continuous boiling at the newly achieved lower boiling point once pressure decreases. Thus, understanding heat transfer helps explain why energy movement allows boiling to restart in a seemingly counterintuitive way.