Fig. 1 shows an investigation into rusting of iron used for outdoor railings. Four identical iron nails were left for seven days in sealed tubes. Tube A con...
Question 1 Report
Fig. 1 shows an investigation into rusting of iron used for outdoor railings. Four identical iron nails were left for seven days in sealed tubes. Tube A contained dry air with calcium chloride. Tube B contained boiled water covered by a layer of oil. Tube C contained tap water and air. Tube D contained salt water and air. Each nail began with the same mass and had no protective oxide coating. The student recorded the amount of rust visible.
(a) State the two substances needed for iron to rust. [2]
(b) Which tube should show the greatest amount of rust? [1]
(c) Explain why the nail in tube B has little or no rust. [2]
(d) Give one method used to prevent rusting of an iron gate. [1]
(e) Explain why coating iron with zinc can still protect it when the coating is scratched. [2]
(f) Name the orange-brown substance formed when iron rusts. [1]
Answer Details
(a) Iron needs water and oxygen to rust. [2]
(b) Tube D should have the most rust because it contains both air and salt water; salt water speeds rusting. [1]
(c) Boiling removes most of the dissolved oxygen from the water. The oil layer then prevents oxygen from the air entering the water, so both conditions for rusting are not available. [2]
(d) One method is to paint the gate. Oil or grease, galvanising, and electroplating with tin are also acceptable. [1]
(e) Zinc is more reactive than iron. At a scratch, zinc oxidises in preference to iron, providing sacrificial protection even where the barrier is broken. [2]
(f) The orange-brown substance is rust, hydrated iron(III) oxide. [1]
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