Question

Explain the following: You add \(100 \mathrm{mL}\) of water to a \(500 \mathrm{mL}\) round-bottomed 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 this phenomenon, the water appears to boil by cooling due to a decrease in pressure inside the closed flask. When the neck of the flask is cooled, the steam inside condenses, reducing the pressure. This decrease in pressure lowers the boiling point of the water, causing it to boil again even without an external heat source. Overall, the boiling occurs due to changing pressures rather than the cooling itself.

Step-by-step solution

Understanding Boiling

Boiling is a phase change where a liquid (in this case, water) turns into a vapor. This process occurs when the temperature of the liquid reaches its boiling point. At this point, the vapor pressure of the liquid equals the atmospheric pressure. In other words, as water molecules gain energy, the vapor pressure inside the flask increases until it is equal to the atmospheric pressure. When this happens, water starts to boil and form bubbles of vapor.

Heating the Flask

When water is added to the round-bottomed flask and heated, its temperature rises. When the temperature reaches the boiling point of water (100 degrees Celsius or 212 degrees Fahrenheit at sea level), it starts to boil. The liquid water turns into vapor (steam), and the vapor pressure increases inside the flask. As long as the heat source is present, the water continues to boil.

Removing Heat and Stoppering the Flask

With the heat removed, water stops boiling as it is no longer receiving the necessary energy to maintain the phase change. When the flask is stoppered, the steam cannot escape, and the system becomes closed. The pressure inside the flask remains constant because the vapor pressure cannot equalize with the atmospheric pressure.

Cooling the Neck of the Flask

Now, when cool water is run over the neck of the flask, the temperature of the vapor near the neck decreases as it loses heat. This causes the vapor to condense back into a liquid, reducing the pressure inside the closed system as the vapor pressure decreases. Since boiling is a process that depends on pressure (i.e., the boiling point of a substance depends on the pressure it is subjected to), the actual boiling point of the water inside the flask will drop due to the decrease in pressure.

Bringing Back the Boiling Process

Since the pressure inside the flask has decreased as a result of cooling the neck, the boiling point of the water inside the flask has also decreased. Therefore, with the existing temperature of the water now higher than the reduced boiling point, boiling resumes even without an external heat source, creating the illusion of boiling water by cooling it. In conclusion, the boiling observed during the cooling process is caused by a decrease in pressure inside the closed system rather than a decrease in the overall temperature of the water. By cooling the neck of the flask and condensing the steam, the pressure inside the flask decreases, which in turn lowers the boiling point of the water.

Key concepts

Vapor Pressure

Vapor pressure is a crucial concept in understanding boiling and phase change. It is the pressure exerted by a vapor in equilibrium with its liquid at a given temperature. When you heat up a liquid, its molecules gain energy and move faster. As these molecules escape from the liquid surface, they become vapor and exert a pressure.

• The vapor pressure increases with temperature.
• A liquid boils when its vapor pressure equals the surrounding atmospheric pressure.

In the problem, when the water in the flask is heated, its vapor pressure increases until it matches atmospheric pressure, leading to boiling. But, when the flask is stoppered, the vapor pressure cannot balance with the outside, creating a closed system.

Phase Change

During boiling, water undergoes a phase change from liquid to gas. A phase change occurs when a substance changes from one state of matter to another, like liquid to vapor. For water, this happens at its boiling point, where sufficient energy is supplied for water molecules to break free from the liquid state.

• This process requires energy input, usually provided by heat.
• This energy is used to overcome forces holding molecules in the liquid phase.

In our example, heating the flask provides the energy needed for the phase change, turning liquid water into steam. Once the heat is removed, the phase change stops, but it can restart if certain conditions change, like reduction of pressure within the closed system.

Atmospheric Pressure

Atmospheric pressure is the weight of the air pushing down on the earth. It plays a significant role in determining the boiling point of a liquid. A liquid boils when its vapor pressure equals the atmospheric pressure. When you heat water in an open flask, the vapor pressure builds until it matches the atmospheric pressure, leading to boiling. When the system is closed, as with our stoppered flask, the atmospheric pressure doesn’t have any effect inside the flask.

• If the surrounding atmospheric pressure is lower, water boils at a lower temperature.
• At high altitudes, atmospheric pressure is lower, and water boils below 100°C.

In the given scenario, cooling the neck reduces the internal pressure, simulating lower atmospheric pressure, thus allowing water to boil at a reduced temperature.

Condensation

Condensation is the reverse process of evaporation, where vapor turns back into liquid. This typically happens when vapor hits a cooler surface, loses energy, and the molecules slow down enough to transition back to the liquid state.

• Condensation releases heat, known as the latent heat of vaporization.
• This is why the surface often feels warmer where condensation is occurring.

In the flask scenario, cooling the neck causes the vapor in that area to condense. With condensation removing vapor, the pressure inside the closed flask decreases. This pressure drop allows the water's boiling point to lower and causes the water to start boiling again despite the cooling process.

Closed System

A closed system is one where no matter can enter or leave. This characteristic greatly affects pressure and phase changes occurring within it. In the example, once the flask is stoppered, it becomes a closed system.

• Pressure changes in a closed system can influence the boiling point.
• A decrease in vapor due to condensation leads to pressure reduction.

As we clarified, by cooling the neck, the vapor condenses and reduces internal pressure. Thus, the water's boiling point lowers, causing water to boil again without adding heat. The closed system allows control over pressure dynamics, enabling phenomena like boiling through cooling.