National League for Nursing (NLN PAX) Practice Exam

When steel wool (mostly iron) burns, its mass increases. What best explains this observation?

1. Hot iron has more mass than cold iron

2. The iron oxide formed has more mass than the iron alone

3. The iron is magnetized by the burning process

4. The iron changes to another allotropic form

The correct answer is: The iron oxide formed has more mass than the iron alone

The observation that the mass of steel wool increases when it burns can be best explained by the formation of iron oxide during the combustion process. When steel wool is heated, it reacts with oxygen in the air. This reaction produces iron oxide, which is a compound formed by the combination of iron and oxygen. The key point here is that the mass of the resulting iron oxide is greater than the mass of the original iron alone because it includes not only the mass of the iron but also the mass of the oxygen that was absorbed from the environment during the reaction. Therefore, the increase in mass is due to this addition of oxygen, which contributes to the overall mass when the iron reacts to form the oxide. This phenomenon illustrates the principle of conservation of mass, where the total mass of reactants (iron and oxygen) equals the total mass of products (iron oxide). Thus, the burning of steel wool results in an increase in mass because the products formed (iron oxide) incorporate the mass of the reactants involved in the reaction.