Lowest Pressure for Liquid Water Existence

Water, like many substances, can exist in different physical states or phases depending on the surrounding temperature and pressure. These phases are solid (ice), liquid (water), and gas (steam or water vapor).

Understanding Water's Phase Diagram

A phase diagram visually represents the conditions (temperature and pressure) under which each phase of a substance is stable. For water, this diagram shows distinct regions for solid, liquid, and gas phases. Lines on the diagram represent conditions where two phases can coexist in equilibrium.

The Significance of the Triple Point

At a specific combination of temperature and pressure known as the triple point, all three phases of water (solid, liquid, and gas) can exist together in stable equilibrium. For water, this unique point occurs at a temperature of approximately 0.01 degrees Celsius ($0.01 \text{ }^{\circ}\text{C}$) and a pressure of about 611.657 Pascals (Pa).

To convert Pascals to kilopascals, we divide by 1000:

$$ \text{Pressure (kPa)} = \frac{\text{Pressure (Pa)}}{1000} $$

Therefore, the triple point pressure is approximately:

$$ \frac{611.657 \text{ Pa}}{1000} = 0.611657 \text{ kPa} \approx 0.611 \text{ kPa} $$

Determining the Lowest Pressure for Liquid Water

The question asks for the lowest pressure at which water can exist in the liquid phase in stable equilibrium. This corresponds to the pressure value on the phase diagram where the solid-liquid equilibrium line (melting/freezing curve) meets the vapor-liquid equilibrium line (boiling/condensation curve). This intersection point is precisely the triple point.

Below this specific pressure, ice does not melt into liquid water when heated; instead, it transitions directly into water vapor through a process called sublimation. Conversely, above this pressure, liquid water can exist.

Thus, the lowest pressure required for the stable existence of liquid water is the triple point pressure.

Identifying the Correct Option

• 101.325 kPa: This is the standard atmospheric pressure at sea level, where water boils at 100 degrees Celsius ($100 \text{ }^{\circ}\text{C}$). Liquid water is stable well below this pressure.
• 0.311 kPa: This pressure is below the triple point pressure.
• 22.06 kPa: This pressure corresponds to the boiling point of water at approximately 70 degrees Celsius ($70 \text{ }^{\circ}\text{C}$). Liquid water is stable below this.
• 0.611 kPa: This value closely matches the triple point pressure of water, which is the minimum pressure required for liquid water to exist stably.

Based on the properties of water's phase diagram, the lowest pressure at which water can exist in the liquid phase in stable equilibrium is approximately 0.611 kPa.