Why do naphthalene balls become smaller in size on prolonged exposure to air?
Naphthalene, while changing its state, does not go from solid to liquid to gaseous state. It sublimes - goes directly from the solid state to the gaseous state. The bonds between the naphthalene molecules are sufficiently weak - even heat from ordinary room temperature is sufficient to break the intramolecular forces and move apart. The freed molecules, especially from the surface, can go into the gas phase immediately. This is the reason why you see the naphthalene balls become smaller in size when they exposed to air.
Why does table salt absorb moisture? (what is a deliquescent compound ?)
Table salt (NaCl) in its pure form does not absorb moisture. A substance is said to be deliquescent if it absorbs moisture from the air and dissolves in the water. NaCl is not a deliquescent compound. Commercial variety of table salt has a little bit of magnesium chloride (MgCl2) - which is deliquescent. It absorbs moisture from air and then dissolves in it. This makes the table salt moist, especially in the rainy season.
Why calcium oxide (CaO) is used in desiccators - to keep substance in the desiccator dry and free of moisture?
Substances that absorb moisture from the air, without dissolving, are called hygroscopic substances. Compounds such as CaO, silica gel, CuO are examples of hygroscopic compounds. They are used in dessicators as drying agents - to keep moisture of air away from the substances placed in the desiccators.