[Music] hi and welcome back to free science lessons. co.uk by the end of this video you should be able to describe how to carry out the chemical tests for carbohydrates proteins and lipids now this is a required practical and it contains a lot of detail the key things you need to learn are the chemicals used to test for each food group and the positive results try not to get too bogged down with specific volumes of chemicals I should point out that all of the chemicals on these tests are potentially hazardous so safety goggles must be worn okay so first we take our food sample and grind this with some distilled water using a mortar and pestel we want to make a paste we then transfer the paste to a beaker and we add more distilled water we now stir this so the chemicals and the food dissolve in the water next we filter the solution to remove suspended food particles at this stage we can now test our solution for the chemicals present we're going to start by testing for carbohydrates carbohydrates include starch and also sugars such as glucose to test for starch we place 2 cm cubed of our food solution into a test tube we then add a few drops of iodine solution which is an orange color if starch is present then the iodine solution will turn blue black however if there's no starch present then the iodine solution will stay Orange now testing for sugar such as glucose is a bit more tricky again we start with 2 cm cubed of our food solution we then add 10 drops of Benedict solution which is a blue color we place the test tube containing our Solutions into a beaker and we half fill the beaker with hot water from a kettle we now leave this for around 5 minutes if sugars are present then the Benedict solution will change color the color of the Benedict solution gives us an approximate idea of the amount of sugar present but it cannot tell us the exact amount a green color tells us that there's a small amount of sugar a yellow color tells us that there's more sugar present and a brick red color tells us there's a lot of sugar present now the benedic test only works for certain sugar such as glucose scientists call these reducing sugars the benedix test will not work for sugars which are non-reducing for example sucrose coming up we'll see how to test for proteins and for lipids okay let's look now at how to test for protein to do this test we take 2 cm cubed of a food solution and we add 2 cm cubed of bored solution which is a blue color if protein is present then the bied solution will change from Blue to a purple or lilac color okay the final test we're going to look at is for lipids or fats now in this test just like before we grind our food with distilled water using a mortum pestel however unlike the other tests we do not filter the solution when testing for lipids and that's because lipid molecules can stick to filter paper Okay so first we transfer 2 cm cubed of our food solution to a test tube we then add a few drops of distilled water and a few drops of ethanol we then gently shake this solution if lipids are present then a white cloudy Emulsion forms now you need to remember that ethanol is highly flammable so it's very important that no naked Flames are present remember you'll find plent you have questions on testing for the chemicals and Foods in my vision workbook and you can get that by clicking on the link above [Music]