Understanding the Triple Point of Water

The question asks about a specific state where all three phases of water – solid, liquid, and gas – can exist together at the same time in equilibrium. This is a unique point on the phase diagram of a substance.

Exploring the Options

Let's look at the given options and understand what each one represents:

• critical state: The critical point is where the liquid and gas phases of a substance become indistinguishable. Above the critical temperature and pressure, the substance exists as a supercritical fluid, not separate liquid and gas phases. Therefore, it is not a state where all three phases coexist.
• saturated liquid line: This line on a phase diagram represents the boundary between the liquid phase and the liquid-vapor equilibrium region. Along this line, a substance is a saturated liquid, meaning it is at its boiling point for that pressure. It does not represent the coexistence of solid, liquid, and gas.
• saturated vapour line: This line on a phase diagram represents the boundary between the vapor (gas) phase and the liquid-vapor equilibrium region. Along this line, a substance is a saturated vapor, meaning it is at its condensation point for that pressure. It does not represent the coexistence of solid, liquid, and gas.
• triple point line: The triple point is a specific temperature and pressure at which the solid, liquid, and gaseous phases of a substance coexist in thermodynamic equilibrium. For water, this occurs at a temperature of 0.01 °C (273.16 K) and a pressure of 611.657 Pascals (approximately 0.006 atm). This is precisely the state described in the question.

The Triple Point Explained

The triple point is a single point on the phase diagram because it represents a specific, unique combination of temperature and pressure where the three phase boundaries meet. At this precise condition, solid water (ice), liquid water, and water vapor (gas) can all exist alongside each other in a stable state.

Consider the phase diagram of water:

Any slight change in temperature or pressure from the triple point will cause one or more of the phases to disappear as the system moves into a region on the phase diagram where only one or two phases are stable.

Conclusion

The state where all three phases of water can simultaneously coexist is known as the triple point. Therefore, the correct option is the one referring to the triple point.