A contract entered into by one municipal corporation for the supply of water to another, as provided by section 743.18 of the Revised Code, shall be terminated by an annexation under section 743.20 of the Revised Code, and so much of the debt incurred by either, in the construction of a water works, as remains unpaid, shall thereafter be a charge upon the united municipal corporation, to the same extent that the separate debt of either was, before the union, a charge upon the municipal corporation which constructed such water works.