Question

A chemistry instructor performed the following mystery demonstration. Just before the students arrived in class, she heated some water to boiling in an Erlenmeyer flask. She then removed the flask from the flame and closed the flask with a rubber stopper. After the class commenced, she held the flask in front of the students and announced that she could make the water boil simply by rubbing an ice cube on the outside walls of the flask. To the amazement of everyone, it worked. Give an explanation for this phenomenon.

Short answer

The ice lowers the internal pressure, allowing the water to boil at a lower temperature.

Step-by-step solution

Understanding the Situation

The instructor has a flask filled with boiling water, and she seals it with a rubber stopper after removing it from the flame. The critical point here is that the vapor inside the flask was at power temperature as the flask was closed at boiling point.

Known Physical Principles

Water boils when its vapor pressure equals the atmospheric pressure. This means that under normal conditions, this pressure is at a high temperature, such as the boiling point of water (100°C or 212°F at sea level).

Effect of Ice on Vapor Pressure

By rubbing ice on the flask, the external temperature around the flask decreases, causing the vapor inside to cool down slightly. This causes the pressure inside the flask to drop.

Boiling Without Direct Heat

As the pressure decreases (due to cooling by ice), the vapor pressure of water matches the reduced internal pressure at a temperature lower than normal boiling point. Thus, with reduced pressure inside, the water boils again even at a lower temperature.

Conclusion

The cooling action by the ice reduces the pressure inside the flask. Since the water can boil at a lower temperature due to the decrease in pressure, the water seems to boil again without additional heat.

Key concepts

Boiling Point

Understanding the boiling point of water is crucial to explain the demonstration. The boiling point is the temperature at which a liquid turns into vapor. For water, this is typically 100°C (212°F) at sea level, where the air pressure is standard. At the boiling point, the vapor pressure inside the liquid equals the surrounding atmospheric pressure. However, this balance can be altered by changing the pressure. In the experiment, when the flask was closed at the boiling point, this crucial balance was preserved even as the external temperature conditions were changed, showcasing the interplay between pressure and temperature.

Pressure Reduction

Pressure plays a key role in the behavior of liquids and gases. In the mystery demonstration, when the flask was closed after boiling, the vapor pressure of the water inside matched the atmospheric pressure at that temperature. When an ice cube is applied to the flask, it cools the outside, causing a decrease in internal pressure.
This happens because lower temperatures usually lead to reduced kinetic energy in molecules, which results in decreased pressure. With less pressure inside the flask, the boiling point of the water drops. This means water inside the flask can start boiling again at temperatures lower than its standard boiling point. This surprising result illustrates how pressure reduction can lead to phase changes without additional heat.

Phase Changes

Phase changes, like the transition from liquid to vapor, depend on both temperature and pressure. The demonstration effectively shows a phase change occurring under unusual conditions. Normally, water needs enough thermal energy to change from liquid to gas, which happens at the boiling point under standard pressure.
By lowering the pressure inside the flask through cooling, the energy required for the phase change is reduced. This allows water to boil even at a cooler temperature, without adding more heat. It's a clear example of how manipulating environmental conditions impacts phase changes.

Thermal Principles

The thermal principles underlying the demonstration involve the relationship between heat, temperature, and pressure. Temperature is a measure of the average kinetic energy of particles in a substance. When you rub ice on the flask, it absorbs heat from the flask, lowering the temperature.
This cooling effect reduces pressure inside, illustrating that pressure can affect the boiling point of a liquid. The thermal principle at work here is that a liquid's boiling point decreases as the ambient pressure decreases, allowing it to boil at lower temperatures. Understanding these principles helps explain thermal behaviors vital in many scientific and practical applications.