When a comet nears the sun what happens to its nucleus
it becomes solid
it becomes visible
it disappears
it becomes gas

It becomes gas. 
Comets are objects composed mostly of ice and dust that grow tails when they approach the sun. All comets have a nucleus, which is the hard rock/ice object. When a comet nucleus nears the sun, solar energy begins to heat the ice and vaporize it.

Alright, i'm gonna try to explain this as good as possible. the law of conservation is like a collection overtime. it never changes though. no matter how you change the constituent parts.

Where's the picture of the model?