Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
The test for starch uses Iodine solution (iodine in potassium iodide). A positive result changes the color from orange-brown to blue-black due to the formation of a polyiodide complex within the starch helix.
Reducing sugars (like glucose, ) are detected using Benedict's reagent. The mixture must be heated in a water bath at approximately to . A color change from blue to green, yellow, orange, or brick-red indicates the presence of reducing sugars.
Non-reducing sugars (e.g., sucrose, ) do not react directly with Benedict's reagent. They must first be hydrolyzed by boiling with dilute hydrochloric acid (), neutralized with sodium hydrogencarbonate (), and then tested with Benedict's reagent.
The Biuret test is used to detect proteins. It reacts with the peptide bonds () in the polypeptide chain. A positive result is a color change from light blue to purple or violet.
Lipids are tested using the Ethanol Emulsion test. Lipids are dissolved in ethanol () and then added to water. Because lipids are insoluble in water, they precipitate out to form a cloudy white emulsion.
Vitamin C (ascorbic acid, ) is tested using solution. The blue is decolorized (becomes colorless) as it is reduced by the ascorbic acid.
📐Formulae
💡Examples
Problem 1:
A student is given an unknown colorless solution. After adding Benedict's reagent and heating, the solution remains blue. However, after boiling a fresh sample with , neutralizing with , and re-testing with Benedict's, a brick-red precipitate forms. Identify the nutrient.
Solution:
The nutrient is a non-reducing sugar, most likely sucrose ().
Explanation:
The negative initial Benedict's test rules out reducing sugars. The positive result after acid hydrolysis indicates that a disaccharide was broken down into its constituent reducing monosaccharides ().
Problem 2:
Explain the observation when a piece of crushed peanut is shaken with ethanol, and the resulting liquid is poured into a test tube containing distilled .
Solution:
A cloudy white emulsion forms in the .
Explanation:
Peanuts contain lipids. Lipids are soluble in organic solvents like ethanol () but insoluble in water. When the lipid-ethanol mixture is added to water, the lipid molecules aggregate into tiny droplets that scatter light, creating the 'emulsion' effect.
Problem 3:
A juice sample requires to decolorize of , while a standard solution of Vitamin C requires . Compare the concentration of Vitamin C.
Solution:
The juice sample is twice as concentrated () as the standard Vitamin C solution.
Explanation:
The fewer the drops required to decolorize the , the higher the concentration of ascorbic acid (). Since it took half as many drops ( vs ), the juice has double the concentration.