paper 3

Question 1

starch test

Answer 1

method:
-add 2cm3 of starch suspension
-add few drops of iodine or potassium iodide solution

positieve results: yellow to black
conclusion: starch reacts with iodine forming a poly iodide complex

Question 2

protein test

Answer 2

method:
add 2cm3 of protein solution to clean test tube
add equal volume of KOH or Naoh solution
add 10 drops of coppersulphate solution

positieve results: blue to violet
conclusion: the peptide chain of protein forms purple complex with dilute coppersulphate alkaline medium

Question 3

vitamin C test

Answer 3

method:
add 2cm3 of vitamin C solution to clean test tube
add 10 drops of DCPIP

positieve results: blue black to colorless
conclusion: vitamin C reduces the blue black DCPIP into colorless

Question 4

lipid test

Answer 4

method:
add 2cm3 of fats
add few drops of ethanol to form lipid droplets
add 5 cm3 of water and shake vigorously

positieve results: clear to cloudy
conclusion: lipids are insoluble in water

Question 5

qualitative

Answer 5

to test the presence or absent of the food mol

Question 6

semi-qualitative

Answer 6

to test the conc of food mol in unknown sample

Question 7

reducing sugars test

Answer 7

all sugars but sucrose
method:
add 2cm3 of reducing sugar solution to clean test tube
add equal volume of benedict solution
put in wb above 80°c
positieve results: blue to brick red
conclusion: the unknown sol contains red sugars that can reduces benedict

Question 8

non reducing sugars test

Answer 8

method:
add 2cm3 of sucrose solution to clean test tube
add 1cm volume of acid
boil for at least 2 min using boiling wb
cool
neutralize using NAHCO3 powder or liquid
add at least 3 cm3 of benedict sol
boil again using wb
positieve results: blue to brick red
conclusion: sucrose is hydrolyzed by dilHCL into fructose and glucose reducing the benedict reagent

Question 9

explain how you can identify unknown concentration of reducing sugars sample

Answer 9

prepare: 5 diff conc of reducing sugars by simple dilution
carry out: add 2cm3 benedict sol on unknown concentration as well as 5 known concentrations and boil in water bath
compare: color intensity unknown with the diff known conc

Question 10

explain how you can identify unknown concentration of starch sample (3 marks)

Answer 10

prepare: 5 diff conc of starch by simple dilution
carry out: starch test using 10 drops of iodine on unknown conc
compare: color intensity unknown with the diff known conc

Question 11

How to modify the experiment to improve the accuracy of the estimate for unknown concentration? Or How to increase the confidence in the estimate of the unknown concentration? (Usually 3marks)

Answer 11

1-Replicate several times exclude anomalous results and calculate mean
2- Use more concentrations (smaller intervals) between (……&….. .%) using Simple dilution
3- Use colorimeter or white background to identify the color intensity of the unknown concentration and compare it to the known concentrations
4- Plot calibration curve and read off the unknown concentration.
5- Carry out the test on known and unknown concentration at the same time.

Question 12

how to modify the independent to get an accurate estimate of unknown conc? if the independent is different conc (2 marks)

Answer 12

use more concentration (smaller intervals)

Question 13

How to modify the dependent to get an accurate estimate of unknown concentration? “ If the dependent is color change”(1mark)

Answer 13

1-Use colorimeter or white background to identify the color intensity of the unknown concentration and compare it to the known concentrations,
2- Replicate several times exclude anomalous results and calculate mean