Question

Consider an iron bar on a balance as shown. a. The balance will read less than \(75.0 \mathrm{~g}\). b. The balance will read \(75.0 \mathrm{~g}\). c. The balance will read greater than \(75.0 \mathrm{~g}\). d. The balance will read greater than \(75.0 \mathrm{~g}\), but if the bar is removed, the rust is scraped off, and the bar replaced, the balance will read \(75.0 \mathrm{~g}\).

Short answer

The correct answer is option d: "The balance will read greater than \(75.0 \mathrm{~g}\), but if the bar is removed, the rust is scraped off, and the bar replaced, the balance will read \(75.0 \mathrm{~g}\)."

Step-by-step solution

Determine How Rust Will Affect the Iron Bar's Mass

Rust is an oxide formed due to the corrosion of iron in the presence of oxygen and moisture. The rust is a chemical reaction that combines iron and oxygen to form iron oxide and it will add weight to the iron bar. It means that the mass of the iron bar will increase due to the presence of rust on it.

Evaluate the Given Options

Now that we know rust adds weight to the iron bar, we can eliminate options a and b as the balance will not read less than or equal to \(75.0 \mathrm{~g}\) when rust is present on the iron bar. Now let's examine option d: "The balance will read greater than \(75.0 \mathrm{~g}\), but if the bar is removed, the rust is scraped off, and the bar replaced, the balance will read \(75.0 \mathrm{~g}\)." This option suggests that when rust is present, the balance will read more than \(75.0 \mathrm{~g}\) which is true. Also, if the rust is scraped off, the balance should read the original weight of \(75.0 \mathrm{~g}\) because the iron bar is back to its original state without rust. Hence, option d is the right choice.

Conclusion

In conclusion, the correct answer is option d: "The balance will read greater than \(75.0 \mathrm{~g}\), but if the bar is removed, the rust is scraped off, and the bar replaced, the balance will read \(75.0 \mathrm{~g}\)."

Key concepts

Rust Formation

Rust formation is a common phenomenon that occurs when iron is exposed to moisture and oxygen over time. This process is a type of chemical change called oxidation. During oxidation, iron combines with oxygen from the air and water to form rust, which is primarily made up of iron oxide. Rust is a reddish-brown flaky substance that often appears on iron tools and structures.

Here are some important points to understand about rust formation:

• Rust occurs because of an electrochemical reaction, which involves both chemical and electrical aspects.
• The presence of salt, acidic conditions, or pollutants can accelerate rust formation, making rust a significant problem in coastal and industrial areas.
• Rust not only changes the appearance of iron objects but also weakens them, affecting their structural integrity.

Understanding rust formation helps in taking preventive measures, such as painting or galvanizing iron to protect it from contact with moisture and oxygen.

Iron Oxide

Iron oxide is a chemical compound formed when iron reacts with oxygen. It is known in various forms including hematite (Fe2O3) and magnetite (Fe3O4), but in the context of rust, we are usually concerned with iron (III) oxide (Fe2O3). Iron oxide features key characteristics:

• It is usually reddish-brown or yellowish in color.
• This compound is what gives rust its distinct color.
• Iron oxides are very stable, which means once rust forms, it doesn't easily revert back to pure iron.

The formation of iron oxide increases the mass of an iron object. This increase is due to the addition of oxygen from the air combining with the iron to create the heavier compound. As a result, if you weigh an iron object before and after rusting, you'll notice the increase in weight.

Chemical Reaction

A chemical reaction involves the transformation of substances through chemical change, leading to the formation of new products. In rust formation, the chemical reaction involves iron and oxygen reacting to form iron oxide.

During this process:

• The reactants are iron and oxygen, each contributing elements to the product, iron oxide.
• Energy is involved, often released as heat when iron rusts, depicting that it is an exothermic reaction.
• The reaction can be represented by the equation: \(4Fe + 3O_2 \rightarrow 2Fe_2O_3\).

Understanding chemical reactions is crucial in many fields, from industrial processes to everyday life. In the case of rust, comprehending the chemical nature of the reaction helps in planning and implementing strategies to prevent or slow down the process.