GATE 2022 AE – Question 42

Propulsion · Basics of thermodynamics · 2 marks · Multiple choice

Which of the following is NOT always true for a combustion process taking place in a closed system?

Show answer and explanation

Correct answer: (B) Total number of molecules is conserved

Explanation

A **closed system** has a fixed mass of material, but a chemical reaction rearranges it.

**A. Total number of atoms is conserved.** True. Atoms are neither created nor destroyed in chemical reactions.
**C. The number of atoms of each element is conserved.** True. This is what balancing a chemical equation means.
**D. Total mass is conserved.** True (conservation of mass).
**B. Total number of molecules is conserved.** **Not always true.** Molecules are broken and formed: for example $CH_4+2O_2\to CO_2+2H_2O$ has 3 molecules on the left and 3 on the right, but $2H_2+O_2\to2H_2O$ has 3 on the left and 2 on the right.

Answer **B**.