The Hidden Word

Forward I am heavy, but backward I am not. What am I?

I am a word that becomes shorter when you add two letters to me. What am I?

What becomes more valuable the more it is broken?

I am a word. Remove my first letter, I remain the same. Remove my last letter, I remain the same. Remove my middle letter, and I remain the same. What word am I?

I am not alive, but I grow; I don't have lungs, but I need air; I don't have a mouth, but water kills me. What am I?