Problem 67 - Chemistry: An Atoms-Focused Approach

Question

Preserving Food Freeze-drying is used to preserve food at low temperature with minimal loss of flavor. Freeze-drying works by freezing the food and then lowering the pressure with a vacuum pump to sublime the ice. Must the pressure be lower than the pressure at the triple point of \(\mathrm{H}_{2} \mathrm{O} ?\)

Step-by-Step Solution

Answer

Answer: Yes, the pressure during the freeze-drying process must be lower than the pressure at the triple point of water (\(611.657\) pascals) to ensure the direct transition of water from solid (ice) to vapor without passing through the liquid phase.

Step 1: Understand the freeze-drying process

Freeze-drying is a method used to preserve food by removing water from it. This process involves freezing the food and lowering the pressure around it using a vacuum pump so that the ice formed can sublime (transition directly from solid to gas without passing through the liquid phase). The goal is to maintain the food's flavor and texture while preserving it long-term.

Step 2: Know the triple point of water

The triple point of water is a unique condition where water can exist in three phases simultaneously - solid, liquid, and gas. For water, this occurs at a specific temperature and pressure: \(0.01^{\circ} \mathrm{C}\), and \(611.657\) pascals.

Step 3: Analyze phase diagram

A phase diagram is a graphical representation of the states of matter (solid, liquid, and gas) as a function of temperature and pressure. In a phase diagram of water, the sublimation curve represents the transition between solid (ice) and gas (water vapor) phases without passing through the liquid phase. During the freeze-drying process, the water in the food is first frozen and then sublimated, which eliminates it without creating a liquid phase.

Step 4: Determine the required pressure for the freeze-drying process

For sublimation to occur in the freeze-drying process, the pressure needs to be such that it enables the direct transition between the solid and gas phases. In the phase diagram, this occurs below the sublimation curve. Since the curve intersects the triple point, it means that for sublimation to take place, the pressure must be lower than the pressure at the triple point.

Step 5: Conclusion

The pressure required during the freeze-drying process must be lower than the pressure at the triple point of water (\({611.657}\) pascals), as this is needed to ensure the direct transition of water from solid (ice) to vapor without passing through the liquid phase.

Key Concepts

Triple Point of Water

The triple point of water is a fascinating concept in thermodynamics. It is the unique set of conditions at which water can exist in all three states: solid, liquid, and gas, simultaneously. This specific point occurs at a temperature of \(0.01^{\circ} \mathrm{C}\) and a pressure of \(611.657\) pascals.
Understanding the triple point is crucial because it acts as a reference point for the phase behavior of water.
In processes like freeze-drying, the pressure must be below this triple point to successfully convert ice directly to vapor without forming liquid water.

Sublimation

Sublimation is an important phase transition in freeze-drying. It refers to the process where a solid, like ice, changes directly into a gas, skipping the liquid phase. In freeze-drying, food is frozen first, and then the pressure is lowered around it. This allows the ice within the food to sublimate, thus removing moisture.

• This process preserves the original structure of the food, maintaining its texture and flavor.
• By bypassing the liquid phase, sublimation avoids adverse reactions that could affect the food's quality.

This is why pressure needs to be very carefully controlled, well below the triple point of water.

Phase Diagram of Water

The phase diagram of water is a powerful tool that shows how the states of water change with different temperatures and pressures. It features three distinct lines: the sublimation curve (solid to gas transition), the vaporization curve (liquid to gas transition), and the fusion curve (solid to liquid transition).

• The triple point is where these curves intersect, highlighting unique temperature and pressure conditions.
• The sublimation curve indicates conditions under which water transitions directly from solid to vapor, a critical aspect in freeze-drying.

Understanding the phase diagram helps determine the optimal conditions for freeze-drying, ensuring that pressure is below the triple point, facilitating effective sublimation.