Biochemical is almost always a describing word, not the name of a subject. You put it in front of another noun: a biochemical reaction, a biochemical test, biochemical research. It means “about the chemistry that happens inside living things” — in a cell, in blood, in a leaf. When your body turns rice into energy, that is biochemical. When a hospital checks the chemicals in your blood, that is a biochemical test. Though actually, people stretch it. News writing sometimes says biochemical just to sound science-y, when chemical would already do. Scientists keep it tighter: factory chemistry is chemical; life-chemistry is biochemical. There is a quieter noun use too. Biochemicals are the actual stuff — enzymes, hormones, the small molecules a cell makes. You will meet that more in science class than in the street. Do not mix the adjective with biochemistry (the whole science) or with biological (about living things more broadly, not specifically their chemistry).