Consider the Gibbs-Helmholtz equation, G = H - TS. For each dissolving process, substitute the signs of G and H into the equation and predict the sign for the entropy (S). Does the sign for entropy change seem to make sense? Explain.

The answer is solid copper 

They cool it down and aluminum cools easily

In reversible reaction chemistry equilibrium is attained when the forward and reverse reactions are taking place at the same rate, thus, the concentrations of reactants and products remain the same. a reversible reaction is a chemical reaction where the reactants form products that, in turn, react together to give the reactants back. in irreversible reactions on the other hand, the reactants react to form the products, which cannot revert back into reactants.