The change in free energy (ΔG) is the difference between the heat released during a process and the heat released for the same process occurring in a reversible manner.
The sign of ΔG gives an indication for the spontaneity of the reaction:
If ΔG is negative, the reaction is spontaneous.
If ΔG = zero, the reaction is at equilibrium.
If ΔG is positive, the reaction is non-spontaneous.
A compound is a pure substance because its molecule cannot be broken down into simpler particles by physical means. A compound is not a pure substance because it is not an element, and only elements are pure substances.
