REQUIRED PRACTICALS

Question 1

Describe the Benedict’s test for reducing sugars

Answer 1

reducing sugar - any sugar that acts as a reducing agent.

1) add an equal volume of Benedict’s reagent to a sample.

2) Heat the mixture in an electric water bath at 100•c for 5 mins

3) positive result - colour change from blue to orange and brick red precipitate forms.

Question 2

Describe the Benedict’s test for non/ reducing sugars.

Answer 2

1) if result of reducing sugars negative - could be non -reducing present . (like sucrose - have to break down into monosaccharides)

2) get new sample test solution : add 1cm^3 dilute hydrochloric acid + carefully heat in water bath (5mins)

3) neutralise it with sodium hydrocarbonate + carry out benedict’s test normally

4) if test positive = coloured precipitate but if negative = remain blue .

Question 3

describe the test for starch.

Answer 3

starch is the storage carbohydrate in plant cells.

1) add iodine dissolved in potassium iodide solution

2) positive result: colour change from orange/brown to blue-black.

Question 4

outline how colorimetry could be used to give qualitative results for the presence of sugars and starch.

Answer 4

1) make standard solutions with known conc. record absorbance or % transmission values .

2) plot calibration curve- absorbance or % transmission (y axis), conc (x axis)

3) record absorbance or % transmission values of unknown samples - use calibration curve to read off concentration

Question 5

Describe how to test for lipids in a sample.

Answer 5

1) add 2cm^3 of food sample to a dry and grease-free test tube.

2) add 5cm^3 of alcohol to food sample and shake test tube

3) add 5cm^3 water to food sample and shake test tube

4) positive result - milky white emulsion forms but if solution remains colourless - no lipid was present in sample.

(mixture of microscopic lipid droplets + water - emulsion + prevents light passing through.)

Question 6

describe how to test for proteins in a sample .

Answer 6

biuret test confirms presence of peptide bond

1) add equal volume of sodium hydroxide to sample at room temp
2) add drops of dilute copper (II) sulfate solution (0.05%) . swirl to mix ( steps 1/2 to make reagent.)

3) positive result : colour changes from blue to purple
- negative result : solution remains blue .

4) can control test with pure water - can mistake blue colour for positive result .