Q.S.L. Miller in 1953, to support the theory of chemical evolution, created conditions in the closed flask that included : (A) CH3, O2, NH3, H2O vapour at 1800°C (B) CH4, H2, NH3, H2O vapour at 800°C (C) CH4, CO2, H2, H2O vapour at 1800°C (D) CH4, NH4, SO2, H2O vapour at 800°C

Concept: Miller-Urey experiment — simulating early Earth conditions to test abiogenesis.

The key was to recreate a reducing atmosphere (no free oxygen) with energy sources like electric sparks (simulating lightning) to form organic monomers.

Reasoning:

1. Miller used a mixture of methane (CH₄), hydrogen (H₂), ammonia (NH₃), and water vapour — these were believed to be the gases of the primitive reducing atmosphere. …

The question tests your recall of one of the most famous experiments in the history of biology — the Miller-Urey experiment (1953). This experiment was designed to test the Oparin-Haldane theory of chemical evolution, which proposed that organic molecules could form from inorganic precursors under the reducing atmosphere of early Earth.

The key idea is that the early atmosphere was reducing (rich in hydrogen and lacking free oxygen), and energy sources like lightning or UV radiation could drive the formation of amino acids and other organic compounds. Miller and Urey built a closed apparatus that circulated gases through a spark discharge chamber, then condensed the products into a liquid trap.

Let’s examine each option carefully.

1. Option (A): CH₃, O₂, NH₃, H_2O at 1800^circC

   • CH₃ is not a stable gas — it’s a methyl radical, not a compound used in the experiment.
   • More critically, oxygen (O₂) was absent in the Miller-Urey setup. Oxygen would have oxidised any organic products and prevented the reducing conditions needed for synthesis.
   • The temperature 1800^circC is far too high — that would break down most molecules, not form them.
   • ✗ Incorrect.

2. Option (B): CH₄, H₂, NH₃, H_2O vapour at 800^circC

   • This matches the actual gas mixture used by Miller and Urey: methane (CH₄), hydrogen (H₂), ammonia (NH₃), and water vapour (H_2O).
   • The temperature of 800^circC is consistent with the hot region near the spark discharge (the flask was heated to produce steam, and the spark itself created local high temperatures).
   • The spark discharge simulated lightning, providing energy for chemical reactions.
   • ✓ Correct.

3. Option (C): CH₄, CO₂, H₂, H_2O vapour at 1800^circC

   • Carbon dioxide (CO₂) was not part of the original Miller-Urey mixture. CO₂ is an oxidised form of carbon; the experiment required a reducing atmosphere.
   • Again, 1800^circC is too extreme.
   • ✗ Incorrect. …