[Lewis Thomson (OP)]

Mark was asked this question by his high school chemistry teacher years ago, who didn't know the answer, and wonders if The Naked Scientists could help.

"What happens to the potential energy of a compressed coil spring that is dissolved in acid?"

Let's all spring to action and leave the answers in the comments below...

[chiralSPO]

Good question!

The compressed spring will probably decompress as it corrodes. So, for the most part, the answer is, "wherever energy goes when a compressed spring is released without any load on it." The mechanical energy is dissipated, with some of it turning into sound, some into heat, and possibly some other minor components.

However, this doesn't answer the core question here (at least not directly). Some of the spring must have dissolved from the "compressed state". What happened to that energy?

So, the act of compressing spring increases the potential energy of the spring. Because energy is conserved, there must be a way to account for it. So there are a few possibilities:

• Either dissolving the compressed spring results in a solution with the excess energy stored in it somehow (probably as heat, so it would be warmer than a solution otherwise identically prepared from the uncompressed spring.)

• Or, the solutions resulting from dissolving the spring in either state are identical, and the energy was released (again, probably as heat).

• Or, there is a range of intermediate cases, in which some of the energy goes into the solution, and some is released.

So that's the pure thermodynamic approach. But how can we explain it in a more concrete way?

On an atomic level, the energy stored in the spring can be thought of as strain on many different bonds between atoms—the bonds are either too short (compressed) or too long (extended) compared to the optimal bond length. Either way, this means that the bonds between atoms will be easier to break. Because dissolving the spring in acid requires breaking the bonds, it will take less energy to do this part for the "energized" spring, leaving more energy for motion of the resulting ions (heat).

A similar approach can be taken for thinking about springs that are in the same state of compression, but different temperatures (or one that is molten, and one that is solid).

[alancalverd]

My father was asked the same question in the 1940s - it's a real classic.

Fortunately James Joule answered it on his honeymoon when he measured the mechanical equivalent of heat, 4.2 joules per calorie. If you dissolve two identical springs, one compressed, and measure the heat of reaction, one will release a bit more energy. 

[paul cotter]

It will largely go in heat. You would however need very sensitive equipment to measure this as the spring will not possess much energy compared to the energy of dissolution of the metal and assuming an aqueous system water has a high specific heat.

[Zlatagor]

The potential energy of the compressed coil spring is converted into thermal energy as the spring is dissolved in the acid. The thermal energy is released into the environment as heat.