REQUIRED PRACTICAL 4: food tests

Question 1

Describe how to carry out chemical tests for carbs, proteins and lipids.

Answer 1

1. Take the food sample and grind this with distilled water using a mortar and pestle. We want to make a paste
2. we transfer the paste into the beaker and add more distilled water. stir so the chemicals in the food dissolve in water
3. We filter the solution to remove suspended food particles
4. at this stage we can now test our solution for the chemicals present

TESTING FOR CARBS:
5. to test for starch, place 2cm^3 of food solution into a test tube

6. add a few drops of iodine solution (orange colour)
7. if starch is present then the iodine solution will turn blue-black, if there is no starch present, the iodine solution will stay orange

TESTING FOR GLUCOSE: BENEDICT’S TEST
5.. place 2cm^3 of food solution into a test tube

6. Add 10 drops of benedicts solution which is a blue colour
7. place the test tube containing our solution into a beaker and half fill the beaker with hot water from a kettle. leave for 5 mins
8. after that, If sugars are present, the Benedict’s solution will change colour
   (green=small amount of sugar, yellow= average amount of sugar, brick red = a large amount of sugar present)

TESTING FOR PROTEINS: biuret solution

Proteins:
5.Take 2cm^3 of food solution and add 2cm^3 of biuret solution which is a blue colour

6.If protein is present then the biuret solution will change from blue to a purple/lilac colour

TESTING FOR LIPIDS : distilled Water & ethanol

1. take a food sample and grind it with distilled water using a mortar and pestle to make a paste
2. we transfer the paste into the beaker and add more distilled water and stir so the chemicals in the food dissolve in water
3. WHEN TESTING FOR LIPIDS WE DO NOT FILTER THE SOLUTION- BECAUSE LIPID MOLECULES CAN STICK TO THE FILTER PAPER
4. we transfer 2cm^3 of our food solution to a test tube and add a few drops of distilled water and a few drops of ETHANOL.
5. We then gently shake the solution
6. if lipids are present then a white cloudy emulsion forms

Question 2

IMPORTANT

what are the chemicals used to test for each food?

Answer 2

iodine - carbs
benedicts solution - reducing sugars
biuret solution- proteins
distilled water & ethanol - lipids

Question 3

when testing for proteins, what colour does the biuret solution go from and change to if proteins are present?

Answer 3

from blue to lilac/purple

Question 4

why must safety goggles be worn during this practical?

Answer 4

all chemicals used are hazardous

Question 5

what does the result of the colour of the benedicts solution in the test tube tell us?

Answer 5

gives us a. approximate idea of the amount of sugar present.
does NOT tell us the exact amount
Green colour = small amount of sugar present
yellow colour= more sugar present
brick red colour= a lot of sugar is present

Question 6

what does the benedicts test work for?

what does the benedicts test NOT work for?

Answer 6

works for reducing sugars like glucose

will NOT work for sugars which are non reducing for example sucrose

Question 7

when testing for lipids, what do we do differently and why?

Answer 7

we do not filter the solution because lipid molecules can stick to filter paper

Question 8

what is a safety precaution when testing for lipids?

Answer 8

ethanol is highly flammable. it’s very important that no naked flames are present.