Final answer:
The Decay stage of fire occurs as oxygen is depleted and fuel is consumed, leading to a reduction in energy and light as combustion reactions wind down. This stage reflects the end of the fire triangle's conditions - oxygen, fuel, and ignition - with the total mass of matter remaining constant due to the Law of Conservation of Mass.
Step-by-step explanation:
The fire stage that occurs as the fuel is consumed and the oxygen concentration falls is known as the Decay stage. This stage is marked by a significant reduction in the release of energy and light in the combustion process. During this Decay stage, the combustion reactions slow down as the available oxygen is depleted, which is essential for the burning of fuels, just as it is for cellular respiration.
The fire triangle, consisting of oxygen, fuel, and ignition, is no longer balanced, effectively causing the fire to diminish. Combustion reactions, including those in a campfire, are spontaneous and only continue until one of the reactants, either the fuel or oxygen, is exhausted. It is important to note that even though the fire consumes material, in accordance with the Law of Conservation of Mass, the total mass after the fire remains equal to the mass before, evidencing that burning does not destroy matter but rather it changes form – in this case into ashes, carbon dioxide, and water vapor.