A student, Imran, wants to find out which conditions are needed for an iron nail to rust. He sets up four boiling tubes, each containing one iron nail:
Tube 1: water that has been boiled (to remove dissolved oxygen), with a layer of oil floating on top of the water.
Tube 2: tap water that has not been boiled, left open to the air.
Tube 3: anhydrous calcium chloride (a drying agent) surrounding the nail, in a stoppered tube containing dry air.
Tube 4: salt water, left open to the air.
He examines the nails after one week.
Figure (to be drawn): Four boiling tubes, each containing an iron nail, are set up as described in the prompt (boiled water with an oil layer; ordinary tap water open to the air; dry air with anhydrous calcium chloride, stoppered; salt water open to the air).
(a)State the independent variable in this investigation.(1)
(b)State one variable that Imran should control to make this a fair test.(1)
(c)State which TWO tubes would show little or no rusting after one week. For each tube you identify, explain why little or no rusting occurs, referring to the substances needed for rusting.(4)
(d)State which tube would show the most rusting, and explain why.(2)
(e)Imran only compared the nails by looking at them. Suggest how he could improve the investigation to collect quantitative (numerical) data, and explain why repeating each condition would improve the investigation.(3)
(f)Iron and steel garden railings are often coated in paint. Using the results of Imran's investigation, explain why painting is an effective way to reduce rusting, and explain what happens if the paint layer is scratched.(3)
(g)Give the general name for this type of rust-prevention method, which works by coating the metal to keep out oxygen and water.(1)
(Total for Question 6 is 15 marks)