Four identical iron nails were coated in different ways and each placed in a boiling tube of water, as shown in Fig. 6.1. Nail 1 was left uncoated, nail 2 w...
Question 1 Report
Four identical iron nails were coated in different ways and each placed in a boiling tube of water, as shown in Fig. 6.1. Nail 1 was left uncoated, nail 2 was painted, nail 3 was coated in oil and nail 4 was coated in candle wax.
Fig. 6.1
(a) Name the two substances in the tube that together cause the uncoated nail to rust. [2]
(b) Explain how painting the nail helps to prevent rusting. [2]
(c) Plan how the student could compare which coating gives the best protection against rusting. In your plan, state what you would keep the same and what you would observe. [4]
Answer Details
This tests barrier methods of rust prevention and the design of a fair test. All barrier coatings work the same way: they keep water and oxygen off the iron.
(a) Two substances that cause rusting [2]. Iron rusts only when both water [1] and oxygen [1] reach its surface.
(b) How painting prevents rusting [2]. The paint forms a continuous barrier / layer over the surface [1] that keeps water and oxygen (air) away from the iron [1], so the rusting reaction cannot start.
(c) Fair-test plan comparing coatings [4]. Any four of the following, clearly split into controls and observation:
- use identical iron nails, one for each coating [1];
- place each in the same volume of water in the same conditions [1];
- leave them all for the same length of time [1];
- keep the temperature the same (and repeat for reliability) [1];
- observe / compare the amount of rust formed on each nail [1] (max 4).
The coating that shows the least rust after the same time gives the best protection.
Examination tip: plans score on control variables (same nail, volume, temperature, time) plus a clear dependent variable to observe (amount of rust). Missing the "what to observe" step is the most common lost mark.
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