3️⃣PAG 2: Testing for biological molecules

Question 1

Safety (7)

Answer 1

Do not eat any of the food.
Some people may have food allergies.
Wear chemical splash-proof eye protection.
Sodium hydroxide is an IRRITANT, Biuret reagent is an IRRITANT.
Ethanol is HIGHLY FLAMMABLE and HARMFUL, keep away from naked flames. Turn off Bunsen burner when using ethanol
Iodine solution is HARMFUL, avoid contact with skin (iodine solution will also stain skin and materials).
Water in the water bath will be HOT.

Tie hair back and wear safety goggles when performing the Benedict’s test using a Bunsen burner and hot water bath.
Handle Biuret solution with care as it contains copper sulphate (poisonous) and sodium hydroxide (corrosive). Wash immediately if it comes into contact with skin and wipe away any spills to surfaces.
Keep ethanol solution away from flames as ethanol is highly flammable.

Question 2

Equipment and materials

Answer 2

• a range of small pieces of different foods (e.g., cottage cheese, crisps, pasta, ham, bread, boiled sweets, nuts)
• test tubes
• test-tube rack
• spotting tile
• iodine solution- HARMFUL
• Benedict’s solution
• dilute sodium hydroxide solution and copper sulfate solution, or Biuret reagent- IRRITANTS
• disposable pipettes
• filter paper
• water bath or beakers and a supply of hot water
• sticky labels or waterproof pen
• ethanol- HIGHLY FLAMMABLE and HARMFUL

Question 3

Method - test for starch - 5 steps

Answer 3

Test for starch:
1 Place a small amount of food on the spotting tile or in a test tube.
2 Add a few drops of iodine solution to the food.
3 Orange iodine solution turns blue–black if starch is present.
4 Record your result in the results table.
5 Repeat steps 1–4 for other types of food.

Question 4

Method - test for sugar - 6 steps

Answer 4

1 Place a small amount of food in a test tube.
2 Add enough Benedict’s solution to cover the food.
3 Place the test tube in a warm water bath for 10 minutes.
4 Blue Benedict’s solution turns orange–red on heating if a sugar such as glucose is present (from blue).
5 Record your result in the results table.
6 Repeat steps 1–5 for other types of food.

Question 5

Method - test for lipids (fat) - 7 steps

Answer 5

1 Place a small amount of food into a test tube.
2 Add a few drops of ethanol to the test tube.
3 Shake the test tube and leave for 1 minute.
4 Pour the solution into a test tube of water.
5 Ethanol added to a solution gives a cloudy white layer if a lipid is present.
6 Record your result in the results table.
7 Repeat steps 1–6 for other types of food.

Question 6

Method - test for protein - 5 steps

Answer 6

1 Place a small amount of food in a test tube.
2 Add 1 cm3 of sodium hydroxide solution and then add a few drops of copper sulfate solution. Alternatively add 1 cm3 of Biuret reagent.
3 Pale blue colour turns purple if protein is present.
4 Record your result in the results table.
5 Repeat steps 1–4 for other types of food.

Question 7

Sources of error

Answer 7

Colour change of Benedict’s test and Biuret test may be subtle and difficult to judge if the concentration of the tested molecule is low.