Question

Sted wool wetted with vinegar is stuffed into a narrow-mouth round glass bottle. A rubber balloon is then sealed over the mouth of the bottlc. After several hours, the balloon is inflated into the bottle in an inverted manner. Explain.

Short answer

The balloon inflates inward due to the creation of a partial vacuum inside the bottle from vinegar vapor dynamics.

Step-by-step solution

Understanding the Scenario

The problem involves a wool wetted with vinegar placed inside a glass bottle with a narrow mouth. A rubber balloon is sealed over the bottle's mouth, and after some time, the balloon inflates inside the bottle in an inverted manner. Our task is to explain this phenomenon.

Chemical Reaction

Vinegar, which is an acetic acid, mixed with the air forms acetic acid vapors. These vapors can initiate a reaction like the sublimation of solid particles in the air or interact with the moisture inside the bottle and cause a change in gas volume and pressure with time.

Pressure and Temperature Dynamics

The interaction of vinegar with the enclosed air changes the pressure and temperature dynamics inside the bottle. As the acetic acid vapors fill the space inside the bottle, they increase the pressure slightly. However, since vinegar evaporates slowly, there is a significant time delay before noticeable pressure changes occur.

Formation of a Partial Vacuum

As the acetic acid vapors gradually disperse and potentially undergo condensation or reactions inside the bottle, the overall gas volume could reduce, creating a partial vacuum (lower pressure) inside the bottle compared to the external environment.

Balloon Inflation Mechanism

Due to the lower pressure inside the bottle than the outside atmosphere, the atmospheric pressure can push the balloon inward. This causes the balloon to inflate inside the bottle in an inverted manner, as the external air pressure is greater than the internal pressure without any additional inflation from external sources.

Key concepts

Chemical Reactions

In this interesting experiment, we witness the interaction between acetic acid vapors and the air inside the bottle. These vapors come from vinegar, which is essentially a diluted form of acetic acid. When the wool soaked in vinegar is placed in the bottle, the acetic acid tends to evaporate slowly. As it does so, it mixes with the air already present inside.
These chemical interactions are key to what happens next. They can involve various processes, such as the vaporization of vinegar molecules and possible interactions with any moisture in the air. These processes can change the composition of the gases inside the bottle. This subtle transformation is the precursor to the pressure changes that will drive the inflation of the balloon.

Pressure Dynamics

Pressure dynamics play a crucial role in this experiment. As the acetic acid evaporates, this increases the number of gas molecules inside the bottle. Although the increase in the number of vapor molecules initially increases the internal pressure, it stabilizes over time.
This is because the vinegar evaporates very slowly, leading to an imbalance. During this phase, the vapors spread out and interact within the bottle, eventually leading to a decrease in pressure as compared to the outside air.
The dynamics of pressure shift gradually, making the internal pressure less than the external atmospheric pressure. It's these dynamics that allow external air pressure to play such a big role in the balloon's behavior.

Acetic Acid

Acetic acid, the main component of vinegar, is a weak acid known for its sour taste and pungent smell. It’s this component that plays a critical role in the experiment. When vinegar is used, it introduces acetic acid into the bottle. Over time, the acetic acid molecules begin to evaporate.
This evaporation leads to the creation of acetic acid vapors. These vapors will then fill the bottle, modifying the air composition and setting the stage for pressure changes. This slow transformation due to the properties of acetic acid is often the key to unlocking processes in scientific experiments involving pressure and gases.
Understanding acetic acid's role in this setup helps us see how such ordinary substances can lead to interesting scientific phenomena.

Vapor Pressure

Vapor pressure is a fundamental concept in understanding how the balloon inflates in this setup. Every liquid, including acetic acid, exerts a vapor pressure in an enclosed space. This corresponds to the pressure exerted by the vapors in equilibrium with the liquid at a given temperature.
In the bottle setup, this vapor pressure shifts over time. As vinegar evaporates into the space of the bottle, the vapor's pressure starts building up. However, the process isn't instantaneous.
Eventually, as the vapors stabilize and interact, the pressure inside the bottle decreases, leading to the balloon reversing inward. The difference between external and internal pressure leads to this reverse inflation phenomenon, illustrating how vapor pressure plays a significant role over a longer duration.