We have a piston with ideal gas in it and a weight. The weight is placed on the piston. The gas is heated externally and the gas expands. Will the expansion be isobaric or isothermal? One argument would be: the expansion will be isobaric because the weight is providing constant pressure. The other is: "all" heat goes into mechanical work done by the gas (i.e. lifting the weight, W = mgΔh), and the temperature of the gas will not rise meaning the expansion is isothermal. So the question is: which one is it? What are the circumstances under which this can be determined?