Question

You decide to cool a can of soda pop quickly in the freezer compartment of a refrigerator. When you take out the can, the soda pop is still liquid; but when you open the can, the soda pop immediately freezes. Explain why this happens.

Short answer

The soda pop freezes when opened because it has been supercooled in the freezer - chilled beyond its freezing point but still in liquid form due to lack of nucleation points for ice crystals to form. When the can is opened, the release of carbon dioxide creates bubbles which serve as nucleation sites, leading to immediate freezing.

Step-by-step solution

Definition of Supercooling

Supercooling, also known as undercooling, is the process of reducing the temperature of a liquid or a gas below its freezing point, without it becoming a solid.

Understanding the Supercooling in the Given Context

When the soda pop is placed in the freezer, the temperature is lower than its normal freezing point. However, the freezing process doesn't occur because the formation of ice requires a nucleation point (an impurity or a surface to start forming). As the soda is in a sealed and smooth container, this lacks the required conditions for nucleation.

The Trigger of the Freezing Process

Upon opening the can, bubbles form due to the release of carbon dioxide (the gas that makes the soda fizzy). These bubbles act as nucleation sites, triggering the rapid freezing of the supercooled soda.

Key concepts

Nucleation

Nucleation is a critical process needed for the transformation of a liquid into a solid. In simple terms, it's the start of crystal formation, where tiny particles serve as the center for further growth. This is like starting a snowball that grows as it rolls downhill.
Markers for nucleation sites can be impurities or surfaces. These provide places where molecules can gather and start forming a solid structure. Without these sites, even if a liquid is cooled below its freezing point, it may not solidify. This is because molecules need an initial gathering place to form a solid structure.
In your soda experiment, nucleation didn't happen in the sealed can until a trigger was introduced - carbon dioxide bubbles when the can was opened. The release of these bubbles created ideal sites for nucleation, leading to the soda's freezing.

Freezing Point

The freezing point is the temperature at which a liquid turns into a solid, as molecules slow down enough to take on a fixed position. For plain water, the freezing point typically occurs at 0°C (32°F). However, whether in pure substances or mixtures like soda, specific conditions can affect this.
In your exercise, supercooling lowers the effective freezing point below typical conditions. Even when soda is cooled below this point, it remains liquid until triggered to freeze. This means that, despite its low temperature, the absence of nucleation sites prevents it from becoming a solid. The freezing point essentially marks the boundary where liquid becomes solid under normal conditions, yet it can be altered by factors like supercooling or nucleation presence.

Carbon Dioxide

Carbon dioxide, a colorless and odorless gas, plays a vital role in the effervescence of soda. It is dissolved under pressure to give soda its fizz. Carbon dioxide also becomes a key player in the phenomenon discussed in this exercise.
When the can of soda is opened, the pressure inside changes, allowing carbon dioxide to escape as bubbles. These bubbles fulfill an unexpected, crucial role: providing the nucleation sites necessary for freezing in the supercooled soda. Therefore, instead of being just the gas that makes the drink fizzy, carbon dioxide instigates the rapid phase change from liquid to solid by providing sites where ice crystals can begin to form.
Through this simple act of opening the can, carbon dioxide shifts from a component of the soda to a catalyst in the freezing process, demonstrating the unique interplay between gas and liquid dynamics.