2. Water

Water can be used as a heat transfer fluid in two phases: liquid and vapor. In its liquid form, water's properties make it an excellent heat transfer medium, serving as a reference fluid for other fluids. Its use, in its pure state, is limited to temperatures above 0.01°C (triple point) and below 300°C, given the high saturation pressures and installation costs these high temperatures entail. As it can dissolve a great many mineral species in the form of $\left({\text{Ca}}^{++},{\text{Mg}}^{++},{\text{Cl}}^{-},{\text{HCO}}_3^{-},{\text{SO}}_4^{--}\right)$...