Which of the following is NOT one of the fixed rules for balancing combustion equations?

The assertion that reactants must always form products is indeed not one of the fixed rules specifically governing the balancing of combustion equations. In the context of combustion reactions, the primary focus is on ensuring that mass and atoms are conserved throughout the chemical process.

The rules that are adhered to when balancing combustion equations include maintaining the total mass constant, which aligns with the law of conservation of mass. Consequently, each element should have the same total number of atoms on both sides of the equation, further reinforcing the idea that mass and atoms are conserved during the reaction.

Energy conservation is also a significant principle in overall chemical reactions but does not specifically pertain to the mechanics of balancing combustion equations themselves. In summary, while reactions typically move from reactants to products, the notion that reactants must always yield products does not fit within the rigid framework of rules established for balancing such equations.