Heterogeneous materials are always mixtures. are homogeneous materials always pure substances? explain.

A homogeneous material is not always a pure substance.
For example, a solution is a homogeneous mixture, but it is not a pure substance. Salty water is homogeneous, but it is not a pure substance. There are two ingredients: salt, and water. 

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.

Density and pressure