What's the independent & dependent variable & control group?

Question

Some metals rust when exposed to year-round conditions. The metals that do not rust usually contain chromium, an element that does not rust but increases the cost of the product. Your friend has a chemical that he claims will prevent rust on any metal. He claims that his chemical will reduce the cost of metal products by eliminating the need of adding chromium. Before he can sell his chemical concoction, you convince him to do some scientific testing to provide data for his sales pitch. Write a procedure for testing the chemical. (Remember, the marketability of the chemical depends on reliable date that supports the claim of inhibiting rust.)

Answer 1

A somewhat lengthy experiment would involve simply letting pieces of steel rust outdoors. Take 1 piece of steel and treating it with his product, then put it in the same place as a piece of untreated steel and a piece of stainless (chromium-containing steel) and leave outside for either a predetermined amount of time or until significant corrosion is observed on the untreated steel. The untreated and stainless steels are the controls in the experiment, and all pieces should have the same surface area (use regularly-sized pieces such as sheets or blocks). The pieces can simply be visually inspected for signs of corrosion, but this may not be good enough to prove the effectiveness.

To do this more scientifically, the pieces should be weighed beforehand. After rusting, the pieces could be treated with sodium dithionite (often sold as "super iron out"), a rust remover that does not attack iron, until all corrosion has been removed. The pieces could then be reweighed to determine what percentage of the mass was lost to corrosion. You may want to do more than one sample of each material for improved accuracy.

A faster way to do the experiment would be to expose the steels to corrosive conditions within the laboratory, but this would be less indicative of real-world performance.

To sum up the answers to your questions, the independent variable is type/treatment of steel, the dependent variable is the amount of corrosion, in this case, a mass percentage, and the two control samples are the untreated and stainless steel. These help you see if the product actually improves corrosion resistance, and also how it measures up to chromium.