The process of burning and increasing weight

Question

I saw a movie in which an object (a kind of steel wool in kitchens) was burning on the kitchen scale. When the burning process began, the weight of the object began to increase. Can anyone explain the cause of this phenomenon (increasing weight by burning) so that it is understandable to a high school student?

I didn't find any related question in StackExchange.

Edit ---> I didn't expect the following information to be necessary: Steel wools are generally made of low-grade carbon steel wire, aluminum, bronze or stainless steel. The Steel itself is an alloy of iron and many other elements may be present or added to it depending on the industrial purposes.

Answer 1

For steel wool the combustion reaction is roughly:

$$\require{mhchem} \ce{2 Fe (s) + 3/2 O2 (g) -> Fe2O3 (s)}$$ So the object 'absorbs' (and chemically binds) air oxygen and thus gains weight.

Answer 2

All oxidation reactions will result in a greater mass of oxide than of the original material, due to the added oxygen in the oxide. Oxygen has mass, after all.

The important part to note here though is that because of the mass of the original element(s) and the structure, the majority of the oxide falls to the surface, so its mass can be measured fairly easily on a simple set of scales. Many other oxides are gaseous and so simply dissipate into the air, most obviously carbon dioxide. The same experiment with wood would show the mass decreasing, of course.

This is the reason why early science came to the conclusion that burning was caused by a separate substance, which they called "phlogiston". They weighed a wooden sample, weighed the ashes, and found the ashes were lighter. Clearly something had left then - and their answer was that it had used up its phlogiston. It was not until proper calometric analysis was done which included gases before and afterwards that the concept of phlogiston was finally shown to be wrong.