Why must the Seliwanoff test be time controlled?

The Seliwanoff test must be time-controlled because the reaction times for different sugars vary. Ketose sugars, like fructose, give a color change rapidly (within 1-2 minutes), while aldose sugars, like glucose, react much more slowly or not at all. If the test is not carefully timed, the color change for ketoses may not be noticeable, or a false negative could occur for aldoses if too little time is given for the reaction to occur. Therefore, precise timing ensures accurate results.