Does rusting need water?

Now find out whether rusting needs water.

Activity 12.3.3

You will need: four cleaned iron nails, two boiling tubes with corks, anhydrous calcium chloride (CaCl₂)

Fix two cleaned iron nails into each rubber cork, as shown in the figure.
Fit one cork into an empty boiling tube, and the other into a boiling tube containing anhydrous calcium chloride or silica gel.
Observe after a few days and record.

Figure to be added

Figure 12.3.3 — tube A: two nails held in a cork in an empty tube; tube B: the same with anhydrous calcium chloride at the bottom.

Anhydrous calcium chloride can absorb the water vapour in the air.

The observation

The nails in tube A have rusted on the parts both inside and outside the tube. The nails in tube B have rusted only on the parts outside the cork, open to the atmosphere. Inside tube B there is no water vapour; all other factors are common to both tubes. This confirms that rusting needs water.

Why the outside of the nail is useful too

Each nail in tube B is its own comparison. The part inside the dry tube and the part outside in the room are the same piece of iron, at the same temperature, for the same time. Only the part that meets damp air rusts. Put this beside the last two activities: without dissolved air the nail did not rust, without oxygen the steel wool stopped, and without water vapour the nail stayed bright. Rusting needs both oxygen and water together, and taking away either one stops it — the idea behind every way of preventing rust later in the chapter.

Rusting needs oxygen and water

Nails kept in dry air over anhydrous calcium chloride do not rust. Rusting needs both oxygen and water.