A student left a clean iron nail on a damp windowsill for two weeks. Fig. 2.1 shows the corroded nail (on the right) next to a fresh iron nail (on the left)...
Question 1 Report
A student left a clean iron nail on a damp windowsill for two weeks. Fig. 2.1 shows the corroded nail (on the right) next to a fresh iron nail (on the left) for comparison.
(a) Name the substance formed on the surface of the iron nail after two weeks. [1]
(b) State two changes in the appearance of the nail caused by the corrosion. [2]
(c) Explain why the nail is described as corroded and not simply wet. [1]
Answer Details
Labelled answer diagram:
This tests recognising rusting as a chemical change (corrosion) rather than a physical one. On a damp windowsill the iron is exposed to both water and oxygen, the two things needed for rust.
(a) Substance formed [1]. The orange-brown layer is rust, chemically hydrated iron(III) oxide [1].
(b) Two changes in appearance [2]. Any two of:
- the shiny grey surface turns orange-brown / red-brown [1];
- the surface becomes rough, flaky or pitted and loses its shine [1].
(c) Corroded, not simply wet [1]. "Wet" is only a physical, temporary state - dry the nail and it is unchanged. Corrosion means the metal has chemically reacted with oxygen and water to form a new substance (rust) and has been eaten / worn away, a change that does not reverse on drying [1].
Examination tip: the word "corroded" must be linked to a chemical reaction forming a new substance, not just moisture on the surface - that distinction is the whole mark here.
Download The App On Google Playstore
Everything you need to excel in your exams
