A mixture of gaseous reactants is put into a cylinder, where a chemical reaction turns them into gaseous products. The cylinder has a piston that moves in or out, as necessary, to keep a constant pressure on the mixture of . The cylinder is also submerged in a large insulated water bath. (See sketch at right.) The temperature of the water bath is monitored, and it is determined from this data that of heat flows out of the system during the reaction. The position of the piston is also monitored, and it is determined from this data that the piston does of work on the system during the reaction.