Testing carbohydrates. Benedict’s and Iodine test.

1. Testing carbohydrates. Benedict’s and Iodine test.

2. Objectives • Recall the stages of the Benedict’s test and the outcome for reducing and non-reducing sugars. • Be able to describe how acid hydrolysis affects sucrose. • Recall the iodine test for starch.

3. To start... • On your whiteboard... Draw an α-glucose molecule.

4. Biological molecules Sugars can be placed into two groups...reducing and non-reducing sugars. Glucose Sucrose Lactose Maltose Fructose Galactose The only non-reducing sugar

5. Benedicts test Benedicts reagent is a mixture of copper sulphate and sodium hydroxide. Method -Add a few drops Benedicts reagent to the solution to be tested. -Heat for 5 minutes in a water bath. -Will turn from blue solution to an orange/Brick red precipitate if a reducing sugar is present. Why? The reagent contains blue copper (II) ions. In the presence of a reducing sugar these are reduced to red copper (I) ions. These form a precipitate which is insoluble in water.

6. What biological molecule does Benedicts test for? • How do you perform a Benedicts test? • What colour does it start? • What colour will it change for a positive result? • What observations would be made if glucose was tested with benedictssolution? • What observations would be made if sucrose was tested with benedicts solution? Benedicts Test

7. Test for starch • Iodine turns from orange to blue/black in the presence of starch. • A few drops of iodine need to be added to the foodstuff to be tested. • If the food stuff is solid, it may need to be crushed and water added to it before the iodine test can be carried out.

8. Iodine test • What biochemical molecule does iodine test for? • What colour will iodine turn if there is a positive result during this test?