CORE PRACTICAL 3: Food tests

Question 1

Iodine test for starch

Method: (3)

Answer 1

1- Place one spatula of the food sample on a dish or 1 cm3 if the sample is liquid.
2- Using a dropper, place a few drops of iodine solution onto the food.
3- Record any change in the colour of the solution.

Question 2

what is starch detected by? what does it do?

Answer 2

iodine solution

-turns blue-black in the presence of starch.

Question 3

Benedict’s test for reducing sugars

Method: (4)

Answer 3

1- Place two spatulas of the food sample into a test tube or 1 cm3 if the sample is liquid. Add about 1 cm3 depth of water to the tube and stir to mix.
2- Add an equal volume of Benedict’s solution and mix.
3- Place the tube in a water bath at about 95°C for a few minutes.
4- Record the colour of the solution.

Question 4

what are reducing sugars detected by? what does it do?

Answer 4

Benedict’s solution

-gradually turns from blue to cloudy orange or brick red when heated with a reducing sugar.

Question 5

why is sucrose not affected by Benedict’s solution?

Answer 5

sucrose is a non-reducing sugar and does not react with Benedict’s solution.

Question 6

Reducing sugars include: (2)

Answer 6

monosaccharides – such as glucose and fructose

disaccharides – such as maltose.

Question 7

Biuret test for proteins

method: (4)

Answer 7

1- Place one-two spatulas of the food sample into a test tube or 1 cm3 if the sample is liquid. Add about 1 cm3 depth of water to the tube and stir to mix.
2- Add an equal volume of potassium hydroxide solution to the tube and stir.
3- Add two drops of copper sulfate solution and stir for two minutes.
4- Record the colour of the solution.

Question 8

what is protein detected by? what does it do?

Answer 8

Biuret reagent.

-turns a mauve or purple colour when mixed with protein.

Question 9

Emulsion test for lipids

method: (5)

Answer 9

1- Place two spatulas of the food sample into a test tube or 1 cm3 if the sample is liquid.
2- Add 2 cm3 of ethanol to the tube. Cover the end of the tube and shake the tube vigorously.
3- Allow the contents to settle.
4- Pour the liquid from the top of the mixture into a test tube half-filled with water.
5- Record the level of the food and whether the water is cloudy or clear.

Question 10

what are lipids detected by? what does it do? (3)

Answer 10

the emulsion test.

1- The test substance is mixed with 2 cm3 of ethanol.
2- An equal volume of distilled water is added.
3- A milky-white emulsion forms if the test substance contains lipids.

Question 11

what is an emulsion?

Answer 11

cloudy mixture formed between a lipid and water

Question 12

what are the risks in this practical? (2)

Answer 12

1- Potassium hydroxide can be harmful to skin and eyes.

2- Hot water can scald.

Question 13

control measures in the practical: (4)

Answer 13

Wear eye protection.
Do not taste any of the foods.
Take care with hot water.
Wash off any splashes immediately.