Which one of the following is the colour of hydrogen gas?
- (a)Light yellow
- (b)Orange
- (c)Black
- (d)Colourless
Answer
Why
Correct — D, Colourless. Hydrogen (H₂) is a colourless, odourless and tasteless gas — it does not absorb visible light, so it has no colour of its own. It is also the lightest and most abundant element in the universe.
Why the others are wrong
- (a)Light yellow — A pale greenish-yellow colour belongs to a gas like chlorine, not to hydrogen, which is colourless.
- (b)Orange — No common form of hydrogen gas is orange; hydrogen has no colour.
- (c)Black — Hydrogen is a transparent, colourless gas — nothing about it is black.
Concept
Hydrogen is the simplest and lightest element (atomic number 1). As a diatomic gas (H₂) at room temperature it is colourless, odourless and tasteless, and it is highly flammable, burning with a pale, almost invisible blue flame.
The distractors tempt you into linking a reactive or 'energetic' gas with a strong colour, but colour comes from absorbing visible light — most common gases (hydrogen, oxygen, nitrogen) simply do not, so they are colourless.
Key facts
- Hydrogen gas (H₂) is colourless, odourless and tasteless.
- It is the lightest element and the most abundant in the universe.
- It burns in air with a pale-blue, nearly invisible flame.
- Only a few gases are visibly coloured — for example chlorine (greenish-yellow) and nitrogen dioxide (reddish-brown).
Most common gases, including hydrogen, are colourless; only a few (Cl₂, NO₂) are coloured.
Study next
Common traps
- Assuming a flammable or reactive gas must have a strong colour.
- Confusing the pale-blue flame of burning hydrogen with the colour of the gas itself.
A direct recall item on a physical property of hydrogen gas — it is colourless.
Related PYQs
Which one of the following statements about dihydrogen (H₂) is not correct?
- (a) H₂ is lighter than air and insoluble in water.
- (b) H₂ is inert at room temperature due to high H–H bond dissociation enthalpy.
- (c) H₂ reacts with alkali metals at high temperature to yield metal hydrides.
- (d) A mixture of NO₂ and H₂ is known as Syngas.
Answer(d) A mixture of NO₂ and H₂ is known as Syngas.
Another NDA item on the properties of hydrogen gas — that its being lighter than air and insoluble in water are among the physical properties (like being colourless) tested here.
Practice
- practice — not a real PYQ
Which one of the following gases is greenish-yellow in colour?
- (a)Hydrogen
- (b)Oxygen
- (c)Chlorine
- (d)Nitrogen
Answer(c) Chlorine — a greenish-yellow gas; the others are colourless. - practice — not a real PYQ
The lightest gas is
- (a)Helium
- (b)Hydrogen
- (c)Nitrogen
- (d)Oxygen
Answer(b) Hydrogen — the lightest element and lightest gas.