A substance in the air which can change the surface of limestone would be oxygen, O2.
<span>A process or reaction which takes in heat from the surroundings is said to be </span><u>Endothermic.</u>
A iron combines with oxygen to produce rust, which is the compound named iron
After one half-life, one half is remaining. After two half-lives one fourth is remaining. So that would be 25%
