Perform the qualitative tests for carbohydrates on the given sugar solutions (glucose, fructose, lactose, maltose and sucrose) and record your results.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Aim. To perform qualitative tests on glucose, fructose, lactose, maltose and sucrose and note what each sugar shows.
2 · Principle.
Molisch's: conc. H2SO4 forms furfural compounds that give a violet ring with α-naphthol (all carbohydrates).
Benedict's and Fehling's: reducing sugars reduce Cu2+ to red Cu2O in hot alkali.
Barfoed's: copper reduction in weak acid; only monosaccharides react within 2-3 minutes.
Seliwanoff's: ketoses give cherry red with resorcinol in HCl within about 1 minute.
3 · Requirements. 1-2% sugar solutions; Molisch reagent, conc. H2SO4, iodine, Benedict's, Fehling's A and B, Barfoed's and Seliwanoff's reagents; test tubes, boiling water bath, pipettes with a bulb, stopwatch.
4 · Procedure.
Molisch's: 2 mL sugar + 2 drops reagent; layer 2 mL conc. H2SO4 down the side.
Benedict's: 5 mL reagent + 8 drops sugar; boiling water bath 5 minutes.
Fehling's: 1 mL A + 1 mL B, boil; add 1 mL sugar; heat.
Barfoed's: 2 mL reagent + 1 mL sugar; boiling water bath 2-3 minutes only.
Seliwanoff's: 3 mL reagent + 1 mL sugar; boiling water bath, watch 1 minute.
5 · Observation. [Table: Test | Observation | Inference for each sugar, with exact colours and times]
6 · Result. Glucose and fructose: reducing monosaccharides (fructose a ketose). Lactose and maltose: reducing disaccharides. Sucrose: non-reducing disaccharide.
7 · Precautions. Gloves and goggles; acid down the side of a tilted tube; time Barfoed's and Seliwanoff's; never pipette by mouth.
Scoring tip: Examiners look for exact colours and timing. Write 'brick-red precipitate', 'violet ring', 'cherry red within 1 minute', not just 'positive'.
