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 this exercise, the water initially boils due to heating, increasing the vapor pressure to match the atmospheric pressure. When the heat is removed and the flask is stoppered, the system cools down and the pressure inside decreases as the vaporization rate decreases. Running cool water over the neck further reduces the pressure inside the flask. As pressure decreases, the boiling point of water also decreases, allowing the water to reach its new boiling point at a lower temperature. This is why it appears as if the water is boiling again when cooled.

Step-by-step solution

Understand the phenomenon

First, let's understand what is happening in the exercise. We start with 100 mL of water in a 500-mL round-bottom flask. We heat the water until it boils, which means it reaches its boiling point. We then remove the heat source, stopper the flask, and watch the boiling stops as the system cools down. We cool the flask further by running cool water over its neck, and the boiling restarts.

Explain the boiling process

The boiling process depends on both temperature and pressure. When a liquid reaches its boiling point, the vapor pressure equals the external pressure. Initially, we heat the water until it starts boiling. This means that the temperature increases, the particles in the liquid have more energy, and the vapor pressure of the liquid increases. Once the vapor pressure equals the atmospheric pressure, the liquid starts boiling.

Understand the role of pressure in cooling

When we remove the heat source and stopper the flask, the system starts to cool down and the vapor pressure inside the flask decreases. However, the pressure inside the flask also decreases as the cooling process continues. The decrease in pressure is due to the difference in vaporization rate between the liquid and the vapor under the stopper. As the flask's neck is cooled, the vaporization rate decreases, leading to a lower pressure inside the flask.

The re-boiling process

When we run cool water over the neck of the flask, the pressure inside the flask decreases further. This means that the liquid inside experiences a lower pressure environment, which can make it easier to reach its boiling point. The boiling point of a liquid decreases when the pressure decreases. As the pressure drops, the water's boiling point also drops, and it reaches its new boiling point at a lower temperature. This is why it seems as if the liquid is boiling again when it is cooled.

Key concepts

Vapor Pressure

To fully comprehend the boiling phenomenon, one must first understand vapor pressure. It is the pressure exerted by the vapor released by a liquid or solid as they evaporate into the air above them. At any given temperature, a liquid's molecules are in constant motion, and some possess enough energy to escape the liquid surface into the atmosphere.

As more molecules escape, they collide and exert pressure, known as vapor pressure. This is measured in a closed system where equilibrium is reached—meaning, the rate of evaporation equals the rate of condensation of the vapor.

Impact of Temperature on Vapor Pressure

When you increase the temperature, the average kinetic energy of the liquid's molecules also increases. Consequently, more molecules can overcome the intermolecular forces binding them together, boosting the rate at which they evaporate. This leads to a higher vapor pressure.

On many occasions, understanding an exercises solution requires awareness of these underlying phenomena. For instance, when we cool the neck of the flask, it indirectly affects the vapor pressure within it.

Boiling Process

The boiling process is a fascinating interplay between temperature, pressure, and the energy of particles within a liquid. Boiling happens when a liquid's vapor pressure equals the surrounding pressure, causing the liquid to form bubbles and transform into a vapor.

How the Boiling Process Unfolds

Heating a liquid increases the average kinetic energy of its particles. Once their vapor pressure matches atmospheric pressure, bubbles form not only at the surface but throughout the liquid. This is because these bubbles can now exist without being crushed by external pressure.

Our exercise showcases such an occurrence. Initially, the flask is heated to reach boiling, and when cooled—the boiling stops because the vapor pressure drops below the atmospheric pressure. By then running cold water over the flask, we surprise the system, and boiling resumes—an effect attributed to changes in pressure, as explained later.

Effect of Temperature on Boiling

The relationship between temperature and boiling is direct and significant. If you elevate the temperature of a liquid, its particles gain kinetic energy and move more vigorously. This increases the chances of particles passing the energy barrier to evaporate, elevating the vapor pressure.

Lower Temperature, Lower Vapor Pressure

As one might infer, when the temperature decreases, the reverse happens. The vapor pressure falls because fewer particles have the energy to escape the liquid's surface. The cooling in our exercise initially caused the water to cease boiling by lowering the vapor pressure below atmospheric pressure.

Effect of Pressure on Boiling

The role of pressure in the boiling process is just as crucial as temperature. Atmospheric pressure is the invisible hand that determines the boiling point of a liquid.

Lower Pressure Equals Lower Boiling Point

As we decrease the surrounding pressure, the boiling point of the liquid drops. This phenomenon is dynamic; for every location and situation where the external pressure varies, so too will the boiling point.

Drawing from the earlier exercise, when the heat is removed and the top of the flask cooled, the water vapor in the flask condenses. This leaves less vapor to exert pressure on the liquid, thereby decreasing the internal pressure. Consequently, the boiling point of the water inside the flask plunges, and the water starts boiling again, albeit at a lower temperature. Such vivid examples help students visualize how pressure modulation can lead to intriguing outcomes such as boiling through cooling.