A test for carbohydrate – Iodine is the test for starch. Mixing iodine and starch causes the formation of a starch – iodine complex that is blue – black in color. If you place iodine on a sample of food and it remains an amber or light brown color there is no starch present, but it there is starch present, then the color will change to a dark blue – black color.
Benedict’s reagent is used for single sugars (called monosaccharide) presence of monosaccharide causes the benedict’s solution to change to various colors when heated. The color change depends on the concentration of the sugar present.
Tests for lipids – There are two tests that can be used to test for the presence of lipids.
1) IV – Sudan IV is a special biological stain that is soluble in non polar solvents such as lipids and turn from a pink to red color. Polar compounds will not take up the pink or red color. For example, if you take a test tube of water and vegetable oil (a lipid) and mix them together and then add a small amount of Sudan IV and shake the mixture, only the oil will turn red or pink not the water.
2) The second test is often called the translucence test for lipid. This is the test that uses unglazed brown paper (same type of brown paper you find in a brown paper bag like the ones form the co-op used to rap bottles). If lipid is present the lipid is absorbed into the paper and causes it to appear translucent which means that light is able to pass through and it looks like a greasy spot.