Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Test for Starch: Add a few drops of Iodine solution ( dissolved in ) to the sample. A color change from yellow-brown to blue-black indicates the presence of starch.
Test for Reducing Sugars (e.g., Glucose): Add Benedict's reagent to the sample and heat in a water bath at approximately . A color change from blue to green, yellow, orange, or a brick-red precipitate () indicates the presence and concentration of reducing sugars.
Test for Proteins: The Biuret test involves adding Biuret reagent (a mixture of and ). A color change from blue to violet or purple indicates the presence of peptide bonds.
Test for Lipids: The Ethanol Emulsion test involves dissolving the sample in ethanol (), then pouring the liquid into a test tube of water. A cloudy, milky-white emulsion indicates the presence of lipids.
Test for Vitamin C: Use DCPIP solution (blue dye). Add the sample drop by drop; if the blue color disappears (becomes colorless), Vitamin C () is present. The fewer drops required, the higher the concentration of Vitamin C.
📐Formulae
💡Examples
Problem 1:
A student is given two clear liquids, Solution A and Solution B. When Benedict's reagent is added and the mixture is heated to , Solution A remains blue while Solution B turns brick-red. Identify the substance in Solution B.
Solution:
Solution B contains a reducing sugar (such as glucose, ).
Explanation:
The Benedict's test is specific for reducing sugars. The heat facilitates the reduction of blue Copper(II) ions () to insoluble red Copper(I) oxide ().
Problem 2:
During a DCPIP titration, it takes drops of Orange Juice to decolorize of DCPIP, but drops of Lemon Juice to decolorize the same volume. Which juice has a higher concentration of Vitamin C?
Solution:
Orange Juice has a higher concentration of Vitamin C ().
Explanation:
Vitamin C acts as a reducing agent that decolorizes DCPIP. Since it took fewer drops ( vs ) of Orange Juice to neutralize the dye, the Orange Juice must contain a higher mass of Vitamin C per unit volume.