Discussion
Which of the following is NOT always true for a combustion process taking place in a closed system?
Answer: B
The statement that is not always true is Total number of molecules is conserved.
Combustion rearranges atoms into new molecules, and the number of molecules usually changes. For example, \(2H_{2} + O_{2} \rightarrow 2H_{2}O\) turns three molecules into two, while \(CH_{4} + 2O_{2} \rightarrow CO_{2} + 2H_{2}O\) happens to keep three.
The other three are always true in a closed system. Chemical reactions neither create nor destroy atoms, so the total number of atoms and the number of atoms of each element are conserved, and mass is conserved because no matter crosses the boundary (the mass equivalent of the heat released is negligible).
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