Question

why does iron surface get covered with a powdery brown material when kept in moist air

Answer 1

This is so because iron reacts with moist air this is also due to the action of light and water on it. This is called rusting. This can be represented as- Fe+O2 (from air) + H2o (water)= Fe203 (rust). This is a chemical change because two substances are reacting together to make a new substance.

Answer 2

because of rusting iron surface get covered with a brown material when kept in moist air