International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

Which of the following prevents rust by forming a barrier between iron and moisture?

1. Oil

2. Hydrochloric acid

3. Sugar

4. Salt

The correct answer is: Oil

Oil prevents rust by forming a protective barrier between iron and moisture. This barrier effectively isolates the iron from exposure to water and oxygen, which are the two essential components that contribute to the rusting process. Rust, which is primarily composed of iron(III) oxide, forms when iron reacts with moisture and oxygen in the environment. By applying oil, a hydrophobic substance, it limits the access of water, thus significantly decreasing the likelihood of rust formation. Other options do not effectively prevent rust. Hydrochloric acid promotes corrosion rather than protection, as it reacts with iron and can remove any existing protective oxide layer. Sugar does not create a barrier; instead, it can attract moisture, which may contribute to rusting. Salt can lead to accelerated rusting due to its ability to enhance the conductivity of water, allowing for more efficient electrochemical reactions that cause rust.