For a two component system, the reduced phase rule equation is: 

CONCEPT:

Reduced Phase Rule Equation for Two-Component Systems

• The phase rule is a general principle used to determine the degrees of freedom (F) in a system, which refers to the number of variables (such as temperature, pressure, or composition) that can be independently changed without altering the number of phases in equilibrium.
• For a two-component system, the reduced form of the phase rule equation is:

  F = C - P + 2

  where:
  • F: Degrees of freedom
  • C: Number of components (for a two-component system, C = 2)
  • P: Number of phases in equilibrium
• For systems involving only temperature and pressure variables (such as condensed systems), the phase rule is further simplified by removing the "+2" term:

  F = C - P

  Substituting C = 2 for a two-component system:

  F = 2 - P

EXPLANATION:

• In the reduced phase rule equation for a two-component system, the degrees of freedom (F) depend on the number of phases (P) present in equilibrium.
• For example:
  • If there is one phase (P = 1), F = 2 - 1 = 1, meaning one variable can be independently changed.
  • If there are two phases (P = 2), F = 2 - 2 = 0, meaning no variables can be independently changed (the system is fully constrained).

• Option 1, "F = 3 - P," is incorrect because the "+2" term applies to general systems, not reduced systems.
• Option 2, "F = 2 - P," is correct for a two-component system in reduced form.

Therefore, the correct answer is F = 2 - P.