Practicals

Question 1

Write down the 4 steps to prepare a food sample for a food test.

KEYWORDS: break, ground up food, distilled water, dissolve, solid food,

Answer 1

1) Break the food into smaller pieces using a pestle and mortar.
2) Transfer the ground up food to a beaker and add some distilled water.
3) Stir the mixture well with a glass rod to dissolve some food
4) Filter the solution using a funnel lined filter paper to get rid of the solid food.

Question 2

What is used to test for reducing sugars?
Write down the 5 steps to test for sugars.

Answer 2

Benedict is used to test for reducing sugars.

1) Prepare the food sample and transfer 5cm³ to a test tube.
2) Prepare and heat a water bath to 75°C
3) Add 10 droplets of benedicts solution using a pipette
4) Put the test tube in the water bath using a tube holder and leave it in for 5 minutes, ensuring the tube is pointing away from you.
5) IF the sample contains reducing sugar, the solution with start to change colour, from normal blue to green > orange > brick red.

The colour change depends on how much sugar there is.

Question 3

What is used to test for starch?
Write the 4 steps to test for starch.

Answer 3

Iodine solution is used to test for starch.

1) Prepare a food sample and tranfer 5cm³ of the sample to a test tube.
2) Add 4 droplets of the iodine solution to the test tube.
3) Gently shake the tube to mix the solution
4) IF the sample contains starch, it will change from browny-orange > black OR blue-black.

Question 4

What is used to test for proteins?
Write down the 4 steps to test for proteins.

Answer 4

Biuret is used to test for proteins.

1) Prepare the food sample and transfer 2cm³ of the sample into a test tube.
2) Add 2cm³ of biuret solution to the sample
3) Gently shake the sample to mix the solution.
4) IF the food contains proteins, the colour change from blue > pink > purple

IF there’s no protein, the solution stays blue.

Question 5

What is used to test for fats (lipids)? (there are 2)

Write down the 4 steps to test for fats for each of the test.

Answer 5

• Sudan III stain solution is used to test for fats (lipids)

1) Prepare the sample and transfer 5cm³ into a test tube. IT DOESN’T NEED TO BE FILTERED.
2) Add 3 drops of the stain solution using a pipetteto the sample.
3) Gently shake to mix it.
4) The sudan III stain solution with stain the fats (lipids) and IF it contains fats, the mixture will seperate into 2 layers. The top layer will be bright red. If it doesnt contain fats, the separate red layer will not form at the top.

• Ethanol is also used to test for fats.

1) Prepare the food sample and tranfers 5cm³ to a test tube.
2) Add 2cm³ of ethanol to the sample
3) Shake it vigorously
4) Add 2cm³ of distilled water
5) if it contains lipids, it will turn into a cloudy white colour and if it’s no lipids, nothing will change.

Remember to say no filtering as AQA doesn’t require this.

Question 6

Write the practical for osmosis.

Answer 6

1) Using cork borer, take 3 samples of the food. This allows to give the same shape with same diameter to all the samples
2) Cut the ends off to take of the skin, as it’s impermeable.
3) Cut each sample into 3cm length
4) Measure the mass for each and record both mass and length
5) Put each sample into a boiling tube, in which one contains 10mm distilled water, 0.25 moles sucrose, and 0.5 moles sucrose
6) Make sure to label the samples to avoid mixing.
7) Add a bun to the boiling tube to prevent any escape of gases.
8) Leave the samples for 24 hours
9) Pour the solution out and take the food sample out
10) Gently remove the excess liquid
11) Measure the length and mass of each sample again and record it.
12) Measure differences
13) It make calculation easy, calculate the percentage change
14) Plot a line graph of results; where the line of best fit meets the x-axis tells you how much sugar was in the potatoes.