Question

why do apple peices becames reddish brown if kept in air for long time

Answer 1

When you cut an apple, you are exposing it's insides to the oxygen in the air. This causes the PPO (polyphenol oxidase) enzyme in the apple's flesh to oxidize the polyphenols, turning them into some other brown colored products. Hence, the color change.

Answer 2

because apple react with air people oxygen so that's why apple becomes reddish