In an electric bulb, why is argon gas commonly used along with a tungsten filament instead of leaving the bulb as a vacuum or filling it with ordinary air?

Introduction / Context:
This question is about the practical application of chemistry and physics in everyday technology, specifically incandescent electric bulbs. It asks why argon gas is used inside a bulb along with a tungsten filament. Understanding the role of inert gases in light bulbs helps explain how manufacturers increase filament life and bulb efficiency. It also reinforces general knowledge about reactive versus inert gases and high temperature behaviour of metals.

Given Data / Assumptions:
The filament in a traditional incandescent bulb is made of tungsten, which operates at very high temperatures to emit visible light.

Argon is a noble gas that is chemically inert under ordinary conditions.

Ordinary air contains oxygen, nitrogen and other gases that can react with hot metals.

The goal in bulb design is to prevent rapid evaporation and oxidation of the tungsten filament while allowing light to be emitted.

Concept / Approach:
At the high operating temperatures of a tungsten filament, exposure to oxygen would lead to rapid oxidation and burning of the filament, drastically shortening the life of the bulb. A vacuum would reduce oxidation but can encourage faster evaporation and thinning of the tungsten filament. Filling the bulb with an inert gas like argon reduces the evaporation rate of tungsten by providing a gas environment that does not react chemically with the filament. This slows down filament degradation and significantly increases the bulb lifetime compared with an evacuated or air filled bulb.

Step-by-Step Solution:
Step 1: Consider a filament in air. At high temperature, tungsten would react with oxygen in the air to form tungsten oxides and would burn out quickly, so air is unsuitable. Step 2: Consider a pure vacuum. Although this prevents oxidation, it allows more rapid evaporation of tungsten atoms from the hot filament surface, causing it to thin and break prematurely. Step 3: Consider filling the bulb with argon. Argon is inert and does not react with tungsten, so it prevents oxidation. It also helps reduce the evaporation rate of tungsten by providing a gas that slows the escape of tungsten atoms from the filament. Step 4: As a result, the filament lasts longer before breaking, which directly increases the useful life of the bulb. Step 5: Therefore, the primary reason for using argon gas in an electric bulb is to increase the life of the bulb by protecting the filament.

Verification / Alternative check:
Manufacturing descriptions of incandescent bulbs typically state that the bulb is filled with an inert gas such as argon or a mixture of argon and nitrogen. This is done specifically to slow down the evaporation of the tungsten filament and to prevent chemical reactions that would damage it. Experimental and practical experience show that bulbs filled with inert gas last longer than those with a vacuum or air. These facts confirm that the main purpose is to prolong the bulb lifetime, rather than to significantly reduce electricity consumption or to colour the light.

Why Other Options Are Wrong:
Reducing the consumption of electricity is not the main effect of filling a bulb with argon. The power consumption depends primarily on the electrical characteristics of the filament and supply voltage, not on the filler gas, so option B is incorrect.

Making the emitted light coloured is wrong because the gas inside the bulb is essentially transparent; the colour of the light is determined mainly by the filament temperature and not by argon.

Reducing the cost of the bulb is not the main reason either, since adding argon gas and sealing the bulb may actually increase manufacturing complexity. The main benefit is longer life, which justifies any added cost.

Common Pitfalls:
A common misunderstanding is to think that the filler gas directly changes the energy efficiency of the bulb or the colour of light emitted. Another pitfall is assuming that any gas could be used instead of argon. Reactive gases such as oxygen would quickly destroy the filament. Remembering that argon is inert and that the goal is to protect the filament at high temperature helps to see that the main purpose is to increase the bulb life.

Final Answer:
Argon gas is used in an electric bulb with a tungsten filament primarily to increase the life of the bulb by reducing oxidation and evaporation of the filament.