def exp_chem_list(rdict):
    """extract the chemicals that were used.
    :param rdict: see DataStructures_README.md
    :return: a set of the chemicals used in all the experiments
    """
    chemicalslist = []
    for reagentnum, reagentobject in rdict.items():
        for chemical in reagentobject.chemicals:
            if chemical in chemicalslist:
                pass
            else:
                chemicalslist.append(chemical)
    return chemicalslist