9.3 Qualitative testing for biological molecules - glucose

Independent variable

Type of sample (A, B, C, D)

Dependent variable (what you measure)

Colour change after Benedict’s test
(blue → green/yellow/orange/brick-red depending on sugar presence

Control variables (what you keep the same)

• Volume of sample (4 cm³)

• Volume of Benedict’s reagent (~2 cm³)

• Temperature of water bath (100°C)

• Heating time (2 min + 2 min)

Limitations

• Subjective colour judgement → different people may interpret colours differently

• Qualitative only → does not give concentration of sugar

• Heat loss variability → tubes may not all be at exactly 100°C

Improvements

• Use a colorimeter instead of visual judgement → more objective results

• Use a thermostatically controlled water bath to maintain constant temperature

• Use accurate measuring equipment (pipettes instead of syringes)

• Repeat experiment and calculate a mean result

Benedict’s reagent

• Contains copper(II) ions → reduced to copper(I) oxide if reducing sugar present

• Causes colour change, indicating glucose

Water bath (100°C)

• Provides consistent heating needed for the reaction

• Ensures reaction occurs at the correct rate

Hydrochloric acid (2 mol dm⁻³)

• Hydrolyses sucrose into glucose + fructose

• Converts non-reducing sugar → reducing sugars

Sodium hydrogen carbonate

• Neutralises acid after hydrolysis

• Ensures pH is suitable for Benedict’s reaction

AIM

• To identify the presence of glucose (reducing sugar) using Benedict’s reagent

• To distinguish between reducing sugars (glucose) and non-reducing sugars (sucrose)

• To confirm sucrose after acid hydrolysis + neutralisation