Answer #1:
Physical.
Explanation:
The only thing mentioned to change is the color. Nothing about the gas in general is changing
Answer #2:
Chemical.
Explanation:
The fact this interacted with another element to completely turn something else rather than just being that color makes this a chemical change.
Answer #3:
Physical
Explanation:
Well, how does an element's density have to do anything with a chemical change?
Answer #4:
Chemical.
Explanation:
Fire/heat is involved, and it makes it change states.
