Loading...
Iodine gives a blue-black response with starch under the usual test conditions. Benedict’s reagent, used with the approved heating method, reports reducing substances through a colour/precipitate change; it is not uniquely a glucose test. Biuret reagent detects peptide bonds, giving a lilac/violet response with suitable protein material rather than a general test for every nitrogen-containing molecule.
Video by Cognito. Foundation recap: the written lesson develops the A-level detail.
Viewing focus: Explain the chemical basis of common molecule tests
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Biuret reagent primarily tests for which feature?
Peptide bonds. Suitable proteins contain many peptide bonds; free amino acids do not give the same general inference.
After watching 2: What does a known positive control establish?
The method can detect the target under the test conditions. A functioning control makes an unknown’s result more interpretable.
What you'll be able to do
Iodine gives a blue-black response with starch under the usual test conditions. Benedict’s reagent, used with the approved heating method, reports reducing substances through a colour/precipitate change; it is not uniquely a glucose test. Biuret reagent detects peptide bonds, giving a lilac/violet response with suitable protein material rather than a general test for every nitrogen-containing molecule.
A non-reducing-sugar investigation compares suitable samples before and after the approved acid hydrolysis and neutralisation steps. A new reducing response after hydrolysis can support a non-reducing carbohydrate such as sucrose in the stated candidate set. The correct conditions and reagent controls are necessary; a failed first assay is not by itself evidence of a non-reducing sugar.
Test a known positive material under the same appropriate conditions to check that a reagent can respond. A negative control, such as the relevant solvent without the target, checks background or contamination. Use fresh portions and clean apparatus; previous additions can change the sample, add a target molecule or alter pH.
In the ethanol emulsion test, a suitable lipid extract gives a cloudy emulsion when mixed with water under the prescribed method. The cloudiness reflects dispersed lipid droplets, not a chemical count of individual triglycerides. Ethanol is flammable, so keep it separate from ignition sources and follow the approved heating and handling arrangements for the different tests.
Equal sample preparation, reagent quantities, temperature and reaction time are needed for meaningful comparisons. Food colour or turbidity can obscure a response; choose suitable blanks or another validated assay. A stronger colour can suggest a concentration difference only within the method’s demonstrated response range.
Qualitative results usually support the presence of a class or structural feature. They do not establish a complete molecular identity, nutritional value or exact concentration on their own. To quantify an unknown, use appropriate standards and a validated signal–concentration relationship, retaining dilution factors and uncertainty.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: A sample is Benedict’s-negative before treatment but positive after appropriate hydrolysis and neutralisation. What additional evidence makes the inference stronger?
A non-reducing carbohydrate is supported within that tested set; the controls make the before/after comparison interpretable.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.