The only definite evidence of a chemical reaction is

a) the production of new substances

b) changes in phase

c) formation of a gas

d) a color change

Which statement about hf is true? it is zero for any compound in its standard state. it is positive when the bonds of the product store more energy than those of the reactants. it is negative when a compound forms from elements in their standard states. it is zero for any element that is in the liquid state.