(U3) Practicals

Question 1

Describe the details of the Benedict’s test for reducing sugars. (2)

What are the colour changes for positive results?

Answer 1

1. • add an equal vol of Benedict’s reagent to the test solution
   • heat to ~80ºC in a water bath
2. • Blue = -ve
   • green —> yellow —> orange = partially positive
   • brick-red ppt = +ve

Question 2

Describe the details of the Benedict’s test for non-reducing sugars. (4)

What are the colour changes for positive results?

Answer 2

1. • test part of solution for reducing sugar (benedicts test 1)
   • if none is present, hydrolyse by heating with HCl in a water bath
   • once cooled, neutralise with NaHCO₃
   • test with Benedict’s reagent
2. • blue = -ve
   • green —> yellow —> orange = partially positive
   • brick-red ppt = +ve

Question 3

Name a glucose specific test and state it’s colour changes

Answer 3

• clinistix strips
• pink to purple

Question 4

How do you test for starch?

What are the colour changes?

Answer 4

• add iodine solution
• yellow-brown (-ve) to blue-black (+ve)

Question 5

Outline the Biuret test

State the colour changes

Answer 5

1. • add equal volumes of KOH to the test sample
   • then a few drops of CuSO₄ solution
   • shake
2. Blue (-ve) to lilac/mauve (+ve)

Question 6

How do you calculate Rf values during chromatography?

What number should the value always be less than?

Answer 6

• (Distance moved by solute / distance moved by solvent front) x 100
• less than 1