Problem 99 - Chemistry

Question

A \(\mathrm{CO}_{2}\) fire extinguisher is located on the outside of a building in Massachusetts. During the winter months, one can hear a sloshing sound when the extinguisher is gently shaken. In the summertime there is often no sound when it is shaken. Explain. Assume that the extinguisher has no leaks and that it has not been used.

Step-by-Step Solution

Answer

The sloshing sound heard in winter is due to the CO2 being in a liquid state because of the low temperature and high pressure inside the extinguisher. During summer, the increase in temperature causes the liquid CO2 to convert back to a gaseous state, which doesn't produce a sloshing sound when shaken.

Step 1: Understand the properties of CO2

Carbon dioxide (CO2) is a gas at room temperature. However, under low temperature or high pressure conditions, the CO2 can change state to become a solid or liquid respectively. In the given case of a fire extinguisher, CO2 is stored under high pressure which can force it to become liquid.

Step 2: Explain the condition in winter

During the winter months in Massachusetts, the temperature drops significantly. This decrease in temperature combined with the high pressure inside the extinguisher makes the CO2 remain in liquid state. Hence, when the extinguisher is shaken, the liquid CO2 moves around producing a sloshing sound.

Step 3: Explain the condition in summer

In the summer months, the temperature increases. Even though the CO2 is under high pressure inside the extinguisher, the increased temperature causes some or all of the liquid CO2 to change back to a gaseous state. In this gaseous state, the CO2 would not produce a sloshing sound when the extinguisher is shaken.

Key Concepts

CO2 Properties

Carbon dioxide (CO₂) is a unique substance with properties that make it highly useful in various applications, including fire extinguishers. At standard atmospheric conditions, CO₂ is a colorless and odorless gas. It is denser than air, which allows it to smother fires effectively by displacing oxygen.

One of the interesting characteristics of CO₂ is its phase behavior. Unlike most substances, it does not transition from a solid to a liquid under normal atmospheric pressure. It sublimates, meaning it transitions directly from a solid to a gas at temperatures below its triple point. However, when placed under high pressure, such as inside a fire extinguisher, CO₂ can be compressed into a liquid form. This liquid CO₂, when released from the extinguisher, rapidly expands and cools, helping to extinguish flames.

Phase Changes

Phase changes refer to the transitions between different states of matter: solid, liquid, and gas. Each substance, including CO₂, has specific temperatures and pressures at which it will change from one phase to another.

For instance, water transitions from solid (ice) to liquid at 0°C under standard atmospheric pressure. With CO₂, the transitions are more complex due to its ability to sublimate. In the application of a CO₂ fire extinguisher, understanding the phase change is crucial. The fire suppression effect relies on the rapid phase change from liquid to gas which occurs when the CO₂ is released, absorbing heat and reducing the oxygen available to the fire. The 'sloshing' sound heard when shaking the extinguisher in the colder months indicates the presence of liquid CO₂, implying phase changes within the canister.

Temperature and Pressure

Temperature and pressure are fundamental concepts in understanding how substances behave and change phase. For every substance, there are specific combinations of temperature and pressure that define its state of matter. The behavior of CO₂ in a fire extinguisher is a prime example of this relationship.

In the cooler winter months, the lower temperature and high pressure inside the extinguisher ensure that CO₂ remains in a liquid state. This is because the ambient temperature is low enough to keep the CO₂ from reaching its sublimation point even under high pressure. Conversely, during the summer, the higher ambient temperatures raise the internal temperature of the extinguisher, which may be enough to push the CO₂ past its sublimation point, causing it to change from liquid to gas thereby eliminating the sloshing sound. As temperature and pressure constantly influence each other, they are pivotal to the phase changes and general behavior of CO₂ in various conditions.