Why is there a colour change in a reagent when a nutrient is present in food tests ?
I'm trying to decide if it has anything to do with hard foods having to be crushed and water added when being tested with a reagent.
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Any time you are doing a chemistry lab that involves a colour change, the colour change is being used as an indicator. If one is conducting food tests, the colour change is probably indicating a presence of one of the three main constituents of foods, those being a carbohydrate, a protein, or a fat. Carbohydrates may be broken down into two subgroups, sugars and starches. Colour changes have long been used in the chemistry lab to indicate presence of a certain substance. For example potassium cyanate (KSCN) will turn a fiery blood red in the presence of iron (Fe+2) ions. Bromine solution, which is a deep orange in color, will turn a dark, bluish black color when it detects the presence of starch, such as that present in a potato. So the colour change is part of an interaction with that specific substance, and the colour change is a physical observation used to detect the presence of that substance.
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