def remove_common_molecules(reactants, products):
    """
    Removes common species between two lists leaving only reacting species.

    Parameters
    ----------
    reactants, products : list of str
        List containing strings all molecular species.

    Returns
    -------
    tuple of str
        Reduced lists for both reactants and products such that only species
        that participate in the reaction remain.
    """
    reduced_react = reactants.copy()
    reduced_prod = products.copy()

    reduced_react.sort()
    reduced_prod.sort()

    if reduced_react == reduced_prod:
        raise Exception("Reactants and products are the same.")
    else:
        pass

    for mol in reactants:
        if mol in reduced_prod:
            reduced_prod.remove(mol)
            reduced_react.remove(mol)

    return (reduced_react, reduced_prod)