A student investigated which conditions cause an iron nail to rust (corrode). Five clean iron nails of the same size were each sealed in a separate test tube under different conditions and left undisturbed for 7 days.
Rusting is the corrosion of iron, in which the metal reacts to form a flaky reddish-brown solid. A student set out to find which conditions are needed for an iron nail to rust. Five identical iron nails were first cleaned with sandpaper to bare metal and weighed. Each nail was then sealed in its own test tube under a different set of conditions and left at room temperature for 7 days.
The five conditions are listed in Table 1. Setup 1 held the nail in dry air together with a drying agent that absorbs water vapor, so the nail met air but no water. Setup 2 half-covered the nail in ordinary tap water open to the air. Setup 3 was the same as Setup 2 but used salt water in place of tap water. In Setup 4 the water was boiled first (to drive out dissolved air) and then covered with a thin layer of oil, so the nail met water but almost no air. Setup 5 was the same as Setup 2 but with a small piece of zinc metal clamped in contact with the iron nail.
After 7 days each nail was examined and given a rust rating from 0 (no visible rust) to 5 (heavily coated). The loose rust was then scraped off, dried, and weighed to give the mass of rust in milligrams (mg). The results are shown in Table 1.
Table 1: Conditions and rust formed on each iron nail after 7 days
| Setup | Conditions | Rust rating (0-5) | Rust mass (mg) |
|---|
| 1 | dry air + drying agent (no water) | 0 | 0 |
| 2 | air + tap water | 3 | 22 |
| 3 | air + salt water | 5 | 48 |
| 4 | boiled water + oil layer (no air) | 0 | 0 |
| 5 | air + tap water + zinc contact | 1 | 3 |
