Heat will loosen a tight nut on a screw. How, scientifically, does this work?

Little brother's school project.Heat will loosen a tight nut on a screw. How, scientifically, does this work?
Heat expands the metal in the nut but the bolt stays cooler and is not expanded. The gap created causes it to be loose.





The metal expands because like everything, heat causes the molecules to get excited and vibrate more creating more space between them. More space over all the molecules expands the material.Heat will loosen a tight nut on a screw. How, scientifically, does this work?
Heat makes the nut expand.
Heat makes metal expand. The nut is bigger than the screw, so when both expand, the nut will expand more.
It chemically expands the ';molecules'; on the nut.


But not the one on the end of the wrench.
Since heat expands an object , it would seem that the nut would be harder to take off if it was heated but that is not the case- I would say it is a combination of two factors at play- it expands the nut outward taking some of the tightness off of the nut and softens the threads on the screw making it easier to unscrew.
It basically has two reasons for unscrewing.


1. the heat burns out all excess dirt the is between the threads and thus allows more freedom.


2. More importantly and if done right, The nut must be heated quickly on the outside while avoiding the heat from reaching the bolt. In such a case, since all mater expands, and since the expansion is directly proportional to the heat, the nut will expand more than the bolt and thus frees the nut from he bolt. This gets easier if the difference in temperatures between the bolt and nut is high!