For which of the following activities might you want to hire a chemist? a. examining the structure of plant cells b. building a fence c. building a wood storage shed d. testing a rock sample for gold content which of the following procedures involves a physical change in one of the substances? a. creating salt by reacting sodium metal and chlorine gas b. allowing a nail rust c. separating a salt solution by evaporating the water d. building a model rocket propelled by mixing baking soda and vinegar