Question

explain how painting of an iron gate prevents it from rusting​

Answer 1

Rusting is a process of attack by water,moisture, chemicals, etc.

When iron comes in contact with moisture or oxygen present in the air it starts oxidizing and forms iron oxidize rust. Painting prevents the molecules of oxygen and the moisture from attacking on iron surface.

Answer 2

By painting ,a coating on a surface of iron is applied which prevents direct contact from moisture and air and water  hence prevents rusting