Question

why do apples get a reddish-brown substance when kept in open air?

Answer 1

Apple is highly rich in Iron. And, we know that when Iron is exposed to moist air, it forms rust. Similarly, apple also gets covered with a layer of rust since the air around us is usually moist.

Fe + H2O + O2 ----> Fe2O3.xH2O


Hope it'll help.. :-D