Ateacher wants to perform a classroom demonstration that illustrates both chemical and physical changes. which would be the best demonstration that she could use?

The physical changes may be demonstrated by the teacher with the use of the phase changes in the water turning it to ice or steam. Moreover, the teacher may demonstrate the chemical changes by the reaction of different chemicals to form precipitate. 

Spontaneity tell us the direction of the reaction, but not how fast it goes. gibbs free energy brings together enthalpy, entropy and temperature in one easy formula. g = h - ts. and, finally the sign of delta g is always negative for a spontaneous reaction.