Question

What happens to the weight of an iron bar when it rusts? a. There is no change because mass is always conserved. b. The weight increases. c. The weight increases, but if the rust is scraped off, the bar has the original weight. d. The weight decreases. Justify your choice and, for choices you did not pick, explain what is wrong with them. Explain what it means for something to rust.

Short answer

The correct answer is choice b, "The weight increases." The weight of an iron bar increases when it rusts because the iron reacts with oxygen in the environment to form iron oxide, which adds to the overall mass of the metal.

Step-by-step solution

Definition of Rusting

Rusting is a chemical reaction that occurs when iron or iron alloy, like steel, is exposed to oxygen and moisture for an extended period, forming iron oxide or rust. Rust is a reddish-brown substance that weakens the metal and causes it to decay over time.

Conservation of Mass

The law of conservation of mass states that mass cannot be created nor destroyed during a chemical reaction. The total mass of reactants equals the total mass of products in any chemical reaction. In the context of rusting, the mass of the iron bar plus the mass of oxygen used during rusting should equal the mass of the rusted bar.

Evaluate the Choices

a. There is no change because mass is always conserved: This statement is partially true. Mass is always conserved, but the weight of an object may change if new substances (like oxygen from rusting) are added to it. Hence, this is an incorrect choice. b. The weight increases: When an object rusts, the iron reacts with the oxygen present in the environment to form iron oxide (rust). This addition of oxygen increases the overall mass of the metal and increases its weight. Therefore, this choice is correct. c. The weight increases, but if the rust is scraped off, the bar has the original weight: When rusting happens, oxygen is chemically bonded to the iron, making a separate compound (iron oxide). If rust is removed, typically through mechanical means like scrubbing or chemical processes, the weight of the remaining iron bar will be lesser than the original weight as some iron atoms have been removed as part of the rusting process. This choice is incorrect. d. The weight decreases: As explained earlier, the weight of an iron bar increases due to the addition of oxygen during the rusting process. Therefore, this choice is incorrect.

Conclusion

The correct answer is choice b, "The weight increases." The weight of an iron bar increases when it rusts because of the addition of oxygen to the iron to form iron oxide. The other choices are incorrect as they don't accurately describe the changes in weight during the rusting process or violate the law of conservation of mass.

Key concepts

Chemical Reactions

A chemical reaction is a process that transforms one or more substances into different substances. In the context of rusting iron, the chemical reaction involves iron (Fe) combining with oxygen (O₂) from the environment, particularly when water or moisture is present.

This reaction is not just a surface phenomenon but involves the migration of atoms to form new chemical bonds. As a result, it leads to the formation of iron oxide, commonly referred to as rust. An important detail to note is that a chemical reaction entails changes at the atomic level, which can result in different physical properties and changes in mass of the substances involved.

Chemical equations are symbolic representations of chemical reactions. They show the reactants on the left side of the equation and the products on the right side, abiding by the Law of Conservation of Mass, which states that the total number of atoms for each element in the reactants must be equal to that of the products.

Conservation of Mass

The principle of conservation of mass is a fundamental concept in chemistry stating that mass cannot be created or destroyed in a closed system through ordinary chemical reactions or physical transformations. In the rusting of iron, we can apply this principle to understand why the mass of the iron bar increases.

When iron rusts, it reacts with oxygen to form iron oxide. The mass of the reactants (iron and oxygen) before the reaction equals the mass of the product (iron oxide) after the reaction. However, the mass we observe changes because we add the mass of oxygen from the air—a reactant that was not part of the iron bar initially.

The misunderstanding of the conservation of mass often leads to confusion about the weight of the rusted iron; however, when we account for the added mass of oxygen during the rusting process, it becomes clear why the weight increases. If we were to completely isolate the system and account for all atoms present before and after the reaction, we would find that mass is indeed conserved—as the Law of Conservation of Mass dictates.

Iron Oxide Formation

Iron oxide formation is the specific chemical change that takes place during the rusting of iron. This process turns solid iron and gaseous oxygen into a new substance, iron oxide (rust), with a different molecular structure and appearance.

Iron oxides can take on a range of chemical compositions and names—magnetite, hematite, and goethite are all forms of iron oxide—but the most common rust is a hydrated form of iron(III) oxide. The chemical reaction for forming rust can be represented as follows: 4Fe + 3O₂ → 2Fe₂O₃.

During rusting, the color, texture, and even the strength of the iron object changes due to the iron molecules bonding with oxygen to form the iron oxide compound. As students encounter concepts like rusting, recognizing that new compounds have distinct properties and compositions from the original substances is key to understanding chemical reactions.