Subject: Some questions about fresh/salt water?

Hey just before I ask my question I did search the site for answers but I didn't
get as much as I would of liked.

Anyways in school my class has started a new unit about oceans and bodies of
water.  It would help me understand more if you could give me some detailed
answers to the questions that I'm confused about.

1)  What is Salinity?
2)  What is the composition of salt water?
3)  Why does the ocean contain salt?
4)  How does the salt water affect density?
5)How have volcanoes contributed to salt levels of the ocean?

I know that questions are supposed to be limited to 250 characters but it would
be a big help if you can explain these to me.  Thanks.

Re: Some questions about fresh/salt water?