Problem 50 - Chemistry and Chemical Reactivity

Question

During thunderstorms in the Midwest, very large hailstones can fall from the sky. (Some are the size of golf balls!) To preserve some of these stones, we put them in the freezer compartment of a frost-free refrigerator. Our friend, who is a chemistry student, tells us to use an older model that is not frost- free. Why?

Step-by-Step Solution

Answer

Use a non-frost-free model to prevent hailstones from melting during defrost cycles.

Step 1: Understanding Frost-Free Refrigerators

Frost-free refrigerators work by periodically defrosting themselves to prevent ice buildup. This is done by briefly warming the freezer coils, which melts any frost, and then this melted frost evaporates.

Step 2: Effect on Stored Items

Due to the periodic warming, items stored in a frost-free freezer might experience slight temperature fluctuations. This could cause any stored ice, like hailstones, to partially melt and refreeze, altering their natural structure.

Step 3: Why Use a Non-Frost-Free Model?

Older non-frost-free models maintain a constant low temperature without periodic warming. This ensures that hailstones remain fully frozen and retain their original structure and properties as they do not go through thaw and refreeze cycles.

Key Concepts

Frost-Free Refrigerators

Frost-free refrigerators are a modern marvel of convenience, designed to prevent the inconvenient and messy buildup of ice in your freezer. They operate by occasionally raising the temperature of their freezer coils to melt any frost that might have formed. This melted frost is subsequently evaporated, maintaining a clear, ice-free environment.
However, this heating and cooling cycle can affect temperature-sensitive items. Although handy, this feature may be a drawback when freezing certain items, such as hailstones, where maintaining a constant temperature is important. Understanding how frost-free technology works can help you make informed decisions about what to store in your freezer.

Phase Changes

Phase changes refer to the transformation of a substance from one state of matter to another. In the context of hailstones in frost-free refrigerators, this primarily concerns the change between solid (ice) and liquid (water) states.
Temperature fluctuations in frost-free freezers could induce partial melting of hailstones: this is a shift from solid to liquid. Upon returning to cooler temperatures, the water refreezes, transitioning back to a solid. However, these cycles can affect the integrity and texture of the hailstones. They may not preserve their original structure or appearance due to repeated melting and refreezing.

Temperature Fluctuations

Temperature fluctuations within a frost-free freezer occur during the defrost cycles. In these cycles, the temperature inside rises slightly to melt away any frost buildup.
This intentional warming could impact items stored inside by causing partial melting. Slight changes may not be noticeable for most frozen foods but can significantly affect objects sensitive to exact temperatures, like hailstones or specialized specimens.

• Temperature consistency is critical for maintaining the structural integrity of stored items.
• Fluctuations may result in altered textures or properties.

Understanding these fluctuations can guide your choice of storage methods and equipment.

Cryopreservation

Cryopreservation is a process used to preserve cells, tissues, or other items at very low temperatures to maintain their biological integrity. It hinges on keeping temperatures consistently low to prevent damaging phase changes.
In the context of hailstones or similar samples, using a constant-temperature freezer, like an older non-frost-free model, helps prevent thawing and refreezing, key factors that could disrupt the preserved state.
Utilizing such techniques ensures that the preserved object retains its original characteristics, making an older model freezer a better option for tasks requiring gentle temperature maintenance.