From a macroscopic perspective, in classical thermodynamics the entropy is interpreted as a state function of a thermodynamic system : that is, a property depending only on the current state of the system, independent of how that state came to be achieved. In any process, where the system gives up Δ Δ E of energy to the surrounding at the temperature T , its entropy falls by Δ Δ S and at least T ⋅ ⋅ Δ Δ S of that energy must be given up to the system's surroundings as a heat. Otherwise, this process cannot go forward. In classical thermodynamics, the entropy of a system is defined if and only if it is in a thermodynamic equilibrium (though a chemical equilibrium is not required: for example, the entropy of a mixture of two moles of hydrogen and one mole of oxygen in standard conditions is well-defined).