Re: Conservation of energy
Since I do not want to embark on a complex study of springs to understand exactly how they store energy I have to admit my answer is somewhat speculative. Taking this experiment from "thought" mode into a real physical experiment would be challenging because the ability of the solvent to dissolve the spring implies some significant energy exchanges as the solvent will react with the spring metal to release energy independently of the metal being under tension. Taking the explanation down to the molecular/atomic level I would imagine that the tension of the spring comes about due to some deformation of the crystal lattice or perhaps some minuscule deformation of orbiting electrons. I wonder if quantum mechanics could be brought into the discussion here? But I won't go there. So as the solvent attacks the crystal lattice and pulls off the metal atom by atom the tiny amount of energy held by each particular atom being under some kind of deformation will likely impart some that energy into movement (kinetic energy) and pass that energy into the solvent around it thereby increasing the temperature. It might be possible to do two experiments placing a large number of springs into a large vat of acid once under tension and once not under tension and see if there if an observable difference in temperature behavior.