food tests

Question 1

what is the test for proteins

Answer 1

biurets test

Question 2

what does biurets reagent contain

Answer 2

it is made up of an alkali and copper (II) sulfate

Question 3

outline how to carry out a biurets test + positive and negative results

Answer 3

• add biurets reagent to the sample in test tube
  POSITIVE: colour change from blue&raquo_space; lilac
  NEGATIVE: no colour change

Question 4

4 limitations of biurets test

Answer 4

• if amino acids or dipeptides are present the result will still be negative
• test is qualitative not quantitative
• exact amounts of substance cannot be found
• based on visual observations which leaves margin for error

Question 5

what is the test for lipids

Answer 5

emulsion test

Question 6

outline how to carry out an emulsion test for lipids + positive and negative results

Answer 6

• add a few drops of ethanol to ground sample
• add water and a stopper
• shake test tube
  POSITIVE: a milky white emulsion forms as a layer on top
  NEGATIVE: no observations

Question 7

3 limitations of emulsion test for lipids

Answer 7

• test is qualitative not quantitative
• exact amounts of substance cannot be found
• based on visual observations which leaves margin for error

Question 8

what is the test for starch

Answer 8

iodine test

Question 9

what reagent is used for iodine tests and why

Answer 9

potassium iodide is used as iodine is insoluble in water

Question 10

outline how to carry out the iodine test for starch + positive and negative result

Answer 10

• add a few drops of potassium iodide solution to sample
  POSITIVE: colour change from yellow/brown&raquo_space; purple/black
  NEGATIVE: no colour change

Question 11

3 limitations of iodine test for starch

Answer 11

• test is qualitative not quantitative
• exact amounts of substance cannot be found
• based on visual observations which leaves margin for error

Question 12

what is the test for reducing sugars

Answer 12

benedicts test

Question 13

what is benedicts solution made of

Answer 13

copper (II) sulfate (blue) which can be reduced to copper (II) oxide (red), which forms an insoluble precipitate

Question 14

outline how to carry out a reducing sugar test + positive and negative results

Answer 14

• place sample in boiling tube, as a liquid or ground up and blended in water
• add an equal volume of benedicts solution
• boil in a water bath for 5 mins
  POSITIVE: colour change from blue&raquo_space; red, extent of colour change depends on concentration
  NEGATIVE: no colour change

Question 15

what is the test for non-reducing sugars

Answer 15

benedicts test, this would be carried out after a negative reducing test

Question 16

outline how to carry out a test for non-reducing sugars

Answer 16

• boil with HCl, acid will hydrolyse glycosidic bonds and free up C=O groups, therefore sugar will be able to donate electrons
• add benedicts reagent
  POSITIVE: colour change from blue&raquo_space; red
  NEGATIVE: no colour change

Question 17

what is the difference between reducing and non reducing sugars

Answer 17

reducing sugars are able to donate electrons to reduce other molecules, and become oxidised themselves

Question 18

3 limitations of benedicts test for reducing and non-reducing sugars

Answer 18

• test is qualitative not quantitative
• exact amounts of substance cannot be found
• based on visual observations which leaves margin for error