Preventing rusting by galvanising
4 marks · NormalA bicycle frame made from iron is left outside in the rain.
State the two conditions required for rusting of iron to occur.
Describe how painting the bicycle frame prevents rusting.
Explain why coating the frame with zinc protects the iron even if the paint or zinc layer is scratched, using the idea of the reactivity series.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Oxygen and water are required for rusting of iron.
(b) Painting acts as a barrier to stop water and oxygen reaching the iron.
(c) Zinc is more reactive than iron, so even if the coating is scratched, zinc will react (corrode) in preference to iron (sacrificial protection). This prevents the iron from rusting.
Also accept: Oxygen and moisture for conditions; References to air and water; Accept mention of other barrier methods
Common mistakes: Saying only water or only oxygen is needed for rusting; Only describing barrier effect for zinc
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States both oxygen and water are required for rusting of iron
B2 (b) · 1 mark · Explains that painting acts as a barrier to oxygen and water
M1 (c) · 1 mark · Identifies that zinc is more reactive than iron (in the reactivity series)
A1 (c) · 1 mark · Explains zinc corrodes in place of iron, protecting the iron even if coating is scratched (sacrificial protection).
Worked solution
a) Rusting of iron requires both oxygen and water to be present.
b) Painting the surface forms a barrier, preventing oxygen and water from reaching the iron, so rusting cannot occur.
c) Zinc is more reactive than iron in the reactivity series. When the zinc layer is scratched, zinc will corrode or react in preference to iron, so the iron does not rust. This is called sacrificial protection.