Acids
If a cut piece of lime is left on a knife for about a day without washing it, the part the lime touched is seen to rust. Activity 12.3.4 looks at how acidity affects rusting.
Activity 12.3.4
You will need: three boiling tubes, water, lime juice, dilute hydrochloric acid (HCl)
- Put a cleaned iron nail into each of the three boiling tubes.
- Add ordinary cold water to the first tube, water mixed with lime juice to the second, and water mixed with dilute hydrochloric acid to the third.
- Leave for about a day, observe and record.
Figure to be added
The observation
The nails in tubes (B) and (C) have rusted more than the nail in tube (A). So acids are a factor that increases the speed of rusting.
Why acid helps rust along
Remember from 12.1 what happens to a metal in acid: the hydrogen ions take electrons from it, and the metal dissolves. Iron is above hydrogen in the activity series, so in an acid there is a second way for iron to lose electrons, on top of the oxygen-and-water reaction. The acid also puts many more ions into the water, so the water conducts much better and the tiny cells on the iron surface work faster. Lime juice contains citric acid, which is why a knife left with lime on it rusts where the juice touched it.
Acid rain and polluted air
Iron structures near factories that release acidic gases rust faster, for the same reason. Wash lime, vinegar or tamarind off steel knives promptly.
Acids speed up rusting
Nails in lime juice or dilute hydrochloric acid rust more than a nail in plain water.