Home · A-Level Biology 9700 · Biological molecules · Testing for biological molecules · Practice
AS · Practice questions · Testing for biological molecules

Name the reagent, state the colour.

Six original Cambridge-style questions on testing for reducing and non-reducing sugars, starch, lipids and proteins, and on turning Benedict's into a measurement.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Reagent, method, both colours.

01
Recall
[3 marks]

Describe how you would carry out the emulsion test for lipids and state the result for a positive test.

  • Shake the sample with ethanol so that any lipid dissolves. ✓
  • Pour the ethanol into a test tube of water (or add water). ✓
  • A milky-white emulsion (cloudiness) shows lipid is present; a clear solution means no lipid. ✓
02
Explain
[4 marks]

A solution gives a negative result with Benedict's solution. Describe how you would show that it contains a non-reducing sugar.

  • Boil a fresh sample with dilute hydrochloric acid to hydrolyse the glycosidic bonds. ✓
  • Cool, then neutralise with sodium hydrogencarbonate (test with pH paper) because Benedict's needs alkaline conditions. ✓
  • Add Benedict's solution and heat in a water bath. ✓
  • A colour change from blue to green, yellow, orange or brick red shows a non-reducing sugar was present. ✓
03
Analysis
[4 marks]

Describe how you would use Benedict's solution and a set of colour standards to estimate the concentration of glucose in a fruit juice.

  • Make a dilution series of glucose solutions of known concentration. ✓
  • Add the same volume of each standard and of the juice to the same (excess) volume of Benedict's solution. ✓
  • Heat all tubes in the same water bath at the same temperature for the same time. ✓
  • Compare the colour of the juice with the standards; the concentration is that of the closest standard, or between two. ✓
04
Data
[3 marks]

A student times how long glucose solutions take to show the first colour change with Benedict's: 0.1% takes 180 s, 0.5% takes 60 s, 1.0% takes 25 s. An unknown takes 90 s. Estimate its concentration and explain one weakness of the method.

  • The time lies between those for 0.1% and 0.5%, so the concentration is between 0.1% and 0.5% (about 0.3 to 0.4% read from a graph of time against concentration). ✓
  • Weakness: deciding the moment of the first colour change by eye is subjective, so the timings are imprecise. ✓
  • (Also accept: only three standards, so the graph is poorly defined in that range.) ✓
05
Explain
[2 marks]

Explain why sucrose gives a negative result with Benedict's solution but glucose gives a positive result.

  • Glucose has a free reducing group (on carbon 1) that reduces Cu2+ to Cu+, forming a brick-red precipitate. ✓
  • In sucrose the reducing groups of glucose and fructose are both used in the glycosidic bond, so sucrose cannot reduce Cu2+. ✓
06
Data
[3 marks]

A food sample gives these results: Benedict's stays blue; iodine turns blue-black; biuret turns lilac; emulsion test stays clear. State which biological molecules are present and which are absent.

  • Present: starch (blue-black with iodine) and protein (lilac with biuret). ✓
  • Absent: reducing sugars (Benedict's stayed blue) and lipids (no emulsion). ✓
  • Non-reducing sugars cannot be ruled out without the acid hydrolysis test. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.