The Test for Starches. No one test can be used for all the carbohydrates, but we can test for any starch by dissolving the substance supposed to contain it in hot water and then adding a drop of iodine. The solution will turn blue if starch be present. No substance other than starch will act this way under these conditions.
The Test for Grape Sugar. There is no one test for all sugars, but grape sugar (glucose) is very common and can be easily distinguished from our household (beet or cane) sugar by what is known as the Fehling Test so named from the man who devised it.
Two solutions are used in the Fehling test, one colorless, and one blue. When these are added in equal amounts to a similar amount of the substance to be tested, and the mixture heated, a yellow-brown solid will form if grape sugar be present. Cane or beet sugar will not act this way.
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