In a chemical reaction, if a color change is detected, if an odor change is detected, or if a precipitate is formed, then:

a chemical reaction has definitely occurred

only physical reactions have occurred

a chemical reaction may have occurred

both physical and chemical reactions have definitely occurred

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.