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Q Why is Gibbs free energy called ‘free’? A Because it is that part of the energy which is in principle obtainable as mechanical work. N. K. Adam, Physical Chemistry, Clarendon Press, Oxford (1956) page 263.

Allotropic Transition in Pure Iron

o o A B A B 1-x Gibbs free energy per mole Concentration x of B   free energy of mechanical mixture G* x Mechanical Mixture

o o A B A B x Gibbs free energy per mole Composition   G{x} free energy of mechanical mixture ∆G M free energy of solution G*

o o A B A B x Gibbs free energy per mole Composition  1-x A  x B   G{x}