In option 1, common salt is exposed to air. This situation does not result in a chemical reaction. Common salt, also known as sodium chloride (NaCl), is a stable compound and does not react with oxygen or moisture in the air. Therefore, when common salt is exposed to air, there is no chemical reaction taking place.
Option 2 states that coal is burnt in air. This situation does result in a chemical reaction known as combustion. When coal is burnt in the presence of oxygen, it undergoes a chemical reaction called combustion, producing carbon dioxide, water, and releasing energy.
Option 3 mentions that sodium is placed in water. This situation does lead to a chemical reaction. When sodium reacts with water, it undergoes a vigorous reaction, producing hydrogen gas and sodium hydroxide.
Option 4 states that iron is kept in moist air. This situation does result in a chemical reaction. When iron is exposed to moisture in the air, it undergoes a chemical reaction called oxidation, forming iron oxide or rust.
Therefore, the correct answer is option 1, as in that situation, a chemical reaction does not occur.