NDA (I) 2022 General Ability Test · Question 87 of 150

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.

• (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.

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.

• 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).

• 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.

No directly related past PYQ was found.

• 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.