If I wanted to open the jar of jam but the lid was too tightly capped I would pour very hot water on the metal lid, and I could open it easily, why does it work?

If the lid is made of plastic, would pouring very hot water on this plastic lid work just as well, ? why is it so?

What are the thermal expansion coefficients? Which ones are larger?

Remember, heat makes things expand -- if the lid expands, it doesn't fit as tightly on the jar.

more specifically, the lid expands more than the jar which makes it looser.

Yeh it is basically depend upon thermal(heat) expansion. As rightly stated by the previous poster I think you need to have a look at thermal expansion coefficient. That will tell you whether you are going to have similar effect on plastic lid jar as you had with metal lid one from pouring hot water over it.

Note: Some posts copied from duplicate thread.

and another factor to consider also is that the Jam is put in the jars Hot and then capped, the cooling creates a vacuum, heating it lowers this vacuum anti-pressure making the lid a little easier to turn by lowering the friction.

This is true(all said above). I have a funny little story to add. When I first learned that heat makes things expand (I must have been in 3rd grade or so), my Grandma was trying to open a jar of jam. She was having major problems doing so. I had just gotten home from school after learning that and I thought "Gee! If heat makes things expand, then I bet running hot water over the lid would make it open" and she tried it and it worked. I was shouting "It worked! It worked!" - guess I was a science freak all my life lol!