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Scheme to identify an unknown sugar sheet
Essay frame

Identify the given unknown carbohydrate by systematic analysis.

How to lay out the answer. Follow the frame, then write it in your own words.

  1. 1 · Aim. To identify the given unknown carbohydrate by systematic qualitative analysis.
  2. 2 · Principle. Sugars differ in reducing power (Benedict's), speed of reduction in acid (Barfoed's: monosaccharides), ketose nature (Seliwanoff's) and osazone shape. Tests in order rule sugars out until one fits.
  3. 3 · Requirements. Unknown solution; Molisch reagent, conc. H2SO4, iodine, Benedict's, Barfoed's, Seliwanoff's, phenylhydrazine mixture, conc. HCl, 10% NaOH, conc. HNO3; boiling water bath, microscope, stopwatch.
  4. 4 · Procedure. [Flow chart: Molisch's → iodine → Benedict's → Barfoed's → Seliwanoff's → osazone, with both branches at each step]
  5. 5 · Observation. [Table: Test | Observation | Inference for each test done on the unknown]
  6. 6 · Result. The given unknown carbohydrate is identified as ______, with the deciding test named.
  7. 7 · Precautions. Tests in order; time Barfoed's and Seliwanoff's; gloves, goggles, fume hood for acids; never taste or mouth-pipette.

Scoring tip: Write each inference as a narrowing list, e.g. 'reducing sugar: glucose, fructose, lactose or maltose'. Examiners reward the logic as much as the final answer.