# Question #76bc1

Apr 1, 2016

#### Explanation:

Starch

One way to distinguish starch is to use a cat toy — a simple laser pointer — to shine a beam of light through "solutions" of each.

Unlike the others, starch forms a colloidal dispersion, in which the starch particles are large enough to disperse the light.

Glucose

You could prepare a crystalline osazone.

Osazone crystals from different sugars have characteristic shapes.

Place 0.1 g sugar, 0.2 g phenylhydrazine hydrochloride, 0.3 g sodium acetate,

and 2 mL water in a small loosely-corked test tube and heat in boiling water.

Shake the test tube occasionally without removing it from the water.

The osazone should precipitate within about 5 min.

Observe the moist brush-shaped crystals under the microscope.

Sucrose

A simple colorimetric test for sucrose is the Seliwanoff test.

An indicator solution is prepared by dissolving 50 mg of resorcinol in 100 mL of 4 mol/L $\text{HCl}$.

Add about 0.5 mL of a solution of the unknown carbohydrate to a test tube containing 3 mL of the freshly prepared indicator and heat the mixture to boiling.

A cherry red color develops within about 5 min if sucrose is present.