CAPF AC 2020 Q10: Science & Technology – Chemistry

CAPF AC 2020, Paper I (General Ability and Intelligence) · Science & Technology

Which one of the following gases produces a red light when electricity flows through it?

Answer: (d) Neon

Explanation

Neon gives a red light

How to think: the red advertising sign is called a neon sign.

The sign is named after the gas inside it.

When electricity flows through a gas at low pressure, the gas glows.

Each gas gives its own colour.

Neon glows reddish-orange.

That is why red advertising signs are called neon signs.

Gas	Type	Main use
Helium	Noble gas	Balloons; cooling the magnets of MRI machines
Neon	Noble gas	Red-orange glow in discharge tubes and signs
Argon	Noble gas	Filling electric bulbs; shield gas in welding
Nitrogen	Not a noble gas	Making ammonia; food packaging

Why noble gases are used in lamps:

Reason	Detail
Unreactive	They do not attack the electrodes or filament
Coloured glow	Each gives a fixed colour under a discharge

Answer: (d) Neon.

(a) Incorrect. Helium is used in balloons and for cooling; it does not give the red glow.
(b) Incorrect. Argon fills electric bulbs and does not glow red.
(c) Incorrect. Nitrogen is not a noble gas and is not the gas of red signs.
(d) CORRECT. Neon produces a red-orange light when electricity flows through it.

Neon glows reddish-orange in a discharge tube (neon signs). Argon fills bulbs; helium fills balloons and cools MRI magnets.

Neon = red-orange glow (neon signs).

Assuming every coloured sign contains neon; other colours come from other gases or coated tubes.
Choosing argon because it is the gas inside ordinary bulbs.
Thinking noble gases burn to give light; they only glow under a discharge.

The sign is named after the gas: a neon sign is red because neon glows red.

NCERT Class XII Chemistry — The p-Block Elements (Group 18, uses of noble gases)
Encyclopaedia Britannica — neon