Controlled factor

A controlled factor in chemistry is a part of a chemical reaction that is kept the same throughout all tests.

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An example of this would be to see whether ice melts more or less with salt. To have a controlled reation would be to have both ice cubes in the same temperature, same light, and all other factors the same between them, except the fact that one has salt added. Of course this would be close to impossible to do precisely, but it is usually considered a fair test if they are kept as similar as possible.