Finding out what is present:
There are a number of simple chemical tests that can confirm the presence of various biological molecules within a sample. These tests indicate only the presence of the type of molecule, not how much present, so they are known as qualitative tests. These tests are often referred to as food tests, because they can be used to detect the presence of various biological molecules in food samples.
The tests all rely on the biological molecules going into solution. It may be necessary to grind or break up the sample in order to carry out the tests properly.
Tests for the presence of carbohydrates:
To show the presence of starch, you can add a solution of iodine (i potassium iodide) to the sample. If starch is present, the iodine solution changes colour from yellow-brown to blue-black.
All monosaccharide and many disaccharide sugars are known as reducing sugars. Put simply this means that a molecule can react with other molecules by giving electrons to them. (In chemistry, adding electrons to another molecule or particle is called reduction.)
When a reducing sugar is heated with Benedict's solution (alkaline copper sulfate), the solution changes colour from blue to orange-red. The orange-red is described as a precipitate because the orange-red substance comes out of solution and forms solid particles dispersed in the solution. This is Benedict's test.
Some sugars do not react with Benedict's solution at all, so a reducing sugar test would show up as negative (no colour change).
Sucrose is a non-reducing sugar. It is formed by. A condensation reaction forming…