9700 · 2.1
Testing for biological molecules flashcards
Revision flashcards for Cambridge 9700 Testing for biological molecules (syllabus 2.1). Flip, recall, then mark a real past-paper question.
Card
What reagent is used to test for reducing sugars?
Benedict's solution (containing copper(II) sulfate).
Card
What are the key steps and conditions for the Benedict's test?
Add an equal volume of Benedict's solution to the sample and heat the mixture in a water bath at 80-100°C for 5-10 minutes.
Card
Describe the sequence of positive colour changes for the Benedict's test, from low to high concentration.
Blue (negative) -> Green -> Yellow -> Orange -> Brick-red precipitate (high concentration).
Card
What two essential steps must be performed before testing for a non-reducing sugar like sucrose?
1. Hydrolysis: Boil the sample with dilute hydrochloric acid (HCl). 2. Neutralisation: Add sodium hydroxide (NaOH) to make the solution alkaline.
Card
What is the positive result for the iodine test for starch?
The solution changes colour from yellow-brown to blue-black.
Card
What is the principle behind the emulsion test for lipids?
Lipids are soluble in ethanol but insoluble in water. When the ethanol solution containing lipids is poured into water, the lipids form tiny droplets, creating a milky white emulsion.
Card
What is the name of the test for proteins?
The Biuret test.
Card
What chemical bond does the Biuret test detect, and what is the positive result?
It detects peptide bonds. A positive result is a colour change from blue to lilac or purple.