1. The problem statement, all variables and given/known data We want to heat up 200 liters of water in a bathtub from 15 ◦C to 30 ◦C. The temperature is increased by adding marble stones to the water. The initial temperature of the marble stones is 773, 15 K. Assume that the marble is rigid, with the heat capacity cm = 0.88 J ·K−1 · g −1 and the water is incompressible, with cp = 4.2 J · K−1 · g −1 and the density ρ = 1000 kg · m−1 . Neglect all heat losses to the environment or any change in kinetic or potential energy. After the process, the system is in balance. Calculate the minimal mass of marble needed for the process. Approach the question making the following assumptions: consider an instationary open system. Balance the system of marble stones and water as a whole. Hint: Begin with the first law in its general form, integrate over the time and consider the definition of enthalpy h and its connection to the inner energy u. 2. Relevant equations H= E+PV, deltaH=q, E=3/2*RT, E=q+w, w=-pdeltaV 3. The attempt at a solution Open systems are new to me. Qualitatively, I assume the heat flux and volume changes work the same way that they would for a closed system? Heat flux moves from the warm marbles to the colder water. Volume decreases because temperature increases, and they are inversely related. I assume since the system is open we are working with something where w=0. Therefore heat given off/absorbed comprises the energy of the system. So we then focus on H= E+PV. Am I on the right track here?