Under what condition is phase rule applied

Under the equilibrium condition

Explanation:

according to the phase rule given by Willard Gibbs, under the condition where the equilibrium between the number of phases is not affected by the gravity, electricity magnetic force or by surface action and the system is under constant temperature, pressure and concentration then the degrees of freedom(F) is proportional to the concerntration (C) and the Phases(P) at equilibrium condition.  

the expression is given as : F= C-P+2

it would be d. henry moseley

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hurricanes form when warm, moist air from the ocean surface quickly rises into the atmosphere and forms storm clouds and rain. when the clouds are formed, heat is released which warms the cool air in the upper regions of the atmosphere so that the cycle can continue. which two phases of the water cycle are repeated in this process?