Why are bulbs filled with chemically inactive nitrogen or argon gas?
A) To prevent oxidation of tungsten filament
B) To prevent dehydration of Tungsten filament
C) To prevent oxidation of steel filament
D) None

Hint: Incandescent bulbs produce light when the electricity in their filament heats it up and consequently, it glows. For the long life of the bulb, it is important that no chemical changes occur with the filament and it remains pure. For a filament kept in the open, the atmosphere can chemically react with it.

Complete step by step answer:
In an incandescent bulb, usually a tungsten filament is used which glows and produces light when it gets heated up due to the passage of electricity through it. Tungsten is used as the material of choice for the filament because filament has a very high melting point and thus, can withstand the extreme heat produced when electricity passes through it.
However, the life of a bulb is reduced if the tungsten filament is damaged or it is changed due to chemical reactions with the surroundings. Usually, if air is filled inside the bulb, the oxygen in the air reacts with it and the tungsten gets oxidized.
So, chemically inactive gases such as nitrogen or argon are filled in the glass bulb, so the tungsten filament does not chemically react with its surroundings.
Therefore, the correct option is A) To prevent oxidation of tungsten filament.

Note:
Students must remember that tungsten is used as the filament material in an incandescent bulb and not steel as the other options suggest. This is because steel does not have as high a melting point as tungsten. Also, steel does not get oxidized or rusted. In the past carbon filaments were also used in incandescent bulbs but they were quickly replaced due to their low power efficiency and low life.