2.2 Food Tests

Question 1

Iodine Test for Starch

Answer 1

• Place sample in test tube
• Add iodine

Positive- blue-black
Negative- Orange-yellow

Question 2

Biuret Test for Proteins

Answer 2

• Place sample in test tube
• Add equal volume of biuret solution (made of sodium hydroxide and copper sulphate solution)
• Mix by swirling

Positive- Lilac (purple)
Negative- no change

Question 3

Emulsion Test for Lipids

Answer 3

• Crush substance
• Add to test tube with ethanol
• Filter ethanol into test tube with cold water
• Discard the solid residue

Positive- Milky emulsion
Negative- no change

Question 4

Benedict’s Test for Reducing Sugars

e.g Glucose, fructose, most sugars

Answer 4

• Add substance to a test-tube
• Add same volume of Benedict’s solution
• Heat to 80°C in a water bath

Positive- Blue to green to yellow to orange to red
Negative- no change

Question 5

Benedict’s Test for Non-Reducing Sugars

Answer 5

• Negative reducing sugar test
• Add hydrochloric acid
• Heat in a water bath
• Cool the test tube and add sodium hydrogencarbonate
• Repeat Benedict’s test

Positive- Positive to Benedict’s
Negative- no change

Question 6

Colorimeter

Answer 6

• A colorimeter shines light through a sample
• Photoelectric cell picks up light
• Place solution of sample in a chamber between the light and the cell
• More light = less reducing sugar

Question 7

Colorimeter using Benedict’s reagent

Answer 7

-Do a Benedict’s test on known concentration of sugar

• filter precipitate
• do colorimeter test
• calibration curve
• test on unknown sample
• read off graph to find concentration

Question 8

Lipid

Answer 8

Positive = white emulsion

Question 9

Protein

Answer 9

biurets reagent

Question 10

Starch

Answer 10

positive = blue-black

Question 11

Reducing sugar

Answer 11

• use heat

- Benedict’s reagent

Question 12

Non-reducing sugar

Answer 12

• use heat
• Benedict’s reagent
• Boil with acid

Question 13

Chromatography

Answer 13

used to separate mixtures into their biological molecules

Question 14

stationary phase

Answer 14

Chromatography paper or TLC plate

Question 15

what is chromatography paper made out of?

Answer 15

cellulose

Question 16

what is TLC made out of?

Answer 16

• plastic coated with silica gel or aluminium hydroxide

Question 17

mobile phase

Answer 17

solvent for the biological molecules

Question 18

examples of mobile phase

Answer 18

water ethanol

polar non-polar

• glucose fats
• some amino acids oils

Question 19

cations

Answer 19

positively charged ions

Question 20

anion

Answer 20

negatively charged ions