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?

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.

Did you not already ask this in the homework help section and get a perfectly adequate answer?

Yes. I'll close this and copy the relevant answers to the other.