Understanding the Triple Point in Thermodynamics

The question asks about the definition of the triple point of a substance.

In thermodynamics, the state of a substance can be represented on phase diagrams, which show the conditions (like pressure and temperature) under which different phases (solid, liquid, gas) exist in equilibrium.

Let's examine the options provided:

• Option 1:

  the saturated liquid state and saturated vapour state become identical

  This describes the critical point, not the triple point. At the critical point, the distinction between liquid and gas phases disappears.
• Option 2:

  the constant temperature line passes through a critical point

  This also refers to the critical point. A constant temperature line on a phase diagram that passes through the critical point indicates the behavior of the substance at that specific critical temperature.
• Option 3:

  all three phases coexist in equilibrium

  This is the precise definition of the triple point. It is a specific temperature and pressure at which the solid, liquid, and gaseous phases of a substance can coexist in thermal equilibrium. For pure water, the triple point is at 0.01 °C (273.16 K) and 611.657 Pascals (0.006037 atmospheres).
• Option 4:

  a mixture of dry steam and wet particles exist

  This describes a state within the two-phase region of the phase diagram (specifically, the liquid-vapor coexistence region), often referred to as wet steam when considering water. This state exists over a range of temperatures and pressures below the critical point, but it does not involve the solid phase and is not a single unique point like the triple point.

Based on the definitions of thermodynamic states and phase diagrams, the triple point is uniquely characterized by the coexistence of all three phases (solid, liquid, and gas) in equilibrium.

Therefore, the correct description of the triple point is where all three phases coexist in equilibrium.