The filament of an electric bulb is made of Tungsten. The bulbs are usually filled with which of the following chemically inactive gases to prolong the life of the filament?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Nitrogen and Argon
A tungsten filament glows at over 2500°C. In air it would react with oxygen and burn out at once, so the bulb is filled with gases that do not react with hot tungsten. Nitrogen and argon are both chemically inactive here and are cheap, since both are taken from air; the heavy argon atoms also slow down the evaporation of tungsten from the filament.
Why the other options are incorrect
Option (1) Nitrogen and Helium — Incorrect. Helium is inert, but its light atoms carry heat away from the filament quickly and do little to stop tungsten evaporating.
Option (3) Hydrogen and Argon — Incorrect. Hydrogen is not chemically inactive; it is a flammable, reactive gas.
Option (4) Hydrogen and Helium — Incorrect. Neither is used: hydrogen is reactive and helium cools the filament.