2020-2021 AS LEVEL BIOLOGY
PRACTICALS (1 to 9)
Edexcel International A level
,CORE PRACTICAL 1 – USE A SEMI-QUANTITATIVE METHOD WITH BENEDICT’S REAGENT TO
ESTIMATE THE CONCENTRATIONS OF REDUCING SUGARS AND WITH IODINE SOLUTION TO ESTIMATE THE
CONCENTRATIONS OF STARCH, USING COLOUR STANDARDS
Part 1:
1) 2% glucose solution to make five different concentrations
2) Label test tubes + fruit juice
3) Add same known volume of distilled water
4) Add Benedict’s reagent & glucose solution
5) Add Benedict’s reagent & fruit juice
6) Shake to ensure contents are mixed
7) Water bath to control temperature + 5 mins timer
8) Observe colour of precipitate formed & with colour chart
Independent variable: concentration of reducing sugar
Dependent variable: colour of precipitate formed & compared
i
Increasing concentration of reducing sugars
Part 2: Iodine solution method
1. 2% starch solution to make five different concentrations
2. Use waterproof pen to label the test tubes and beakers
3. Use syringes, distilled water & the 2% starch solution to create 5 cm3 of each solution
4. Add 0.5 cm3 of iodine & dilute with 10 cm3 distilled water
Independent variable: concentration of starch
Dependent variable: colour change of solution
BROWN TO BLUE-BLACK INDICATES THE PRESENCE OF STARCH
Semi-quantitative → non-numerical results that provide an estimate of how much of the measured
substance is present
1) How could these investigations have been adapted to produce quantitative results?
Using a colorimeter.
2) Explain why Benedict’s test is not a suitable test for measuring the total sugar content of fruit
juice.
Fruit juices don’t contain sucrose (non-reducing sugar), thus no reaction would be observed.
3) Suggest one way in which the results of the Benedict’s reagent investigation could have been
made more reliable.
Larger quantities of each sample concentration could have been made, so repeat readings could be
taken.
Calculate average from the results of the whole class.
Copyright © 2020–2021 – All Rights Reserved.
, 4) How could you evaluate the accuracy of your experimental value for the concentration of glucose
in the fruit juice?
Compare with the reference values on the nutrition level showing the glucose content.
Calculate the concentration of glucose in the juice & then compare with the estimate generated
experimentally.
5) Name a variable that you controlled in the iodine investigation, explain how you controlled it and
describe the possible effect on the result if it had not been controlled.
Concentration of iodine → controlled using same solution & diluting by same amount each time
If iodine had not been controlled → over or underestimation of the concentration of starch.
Copyright © 2020–2021 – All Rights Reserved.
PRACTICALS (1 to 9)
Edexcel International A level
,CORE PRACTICAL 1 – USE A SEMI-QUANTITATIVE METHOD WITH BENEDICT’S REAGENT TO
ESTIMATE THE CONCENTRATIONS OF REDUCING SUGARS AND WITH IODINE SOLUTION TO ESTIMATE THE
CONCENTRATIONS OF STARCH, USING COLOUR STANDARDS
Part 1:
1) 2% glucose solution to make five different concentrations
2) Label test tubes + fruit juice
3) Add same known volume of distilled water
4) Add Benedict’s reagent & glucose solution
5) Add Benedict’s reagent & fruit juice
6) Shake to ensure contents are mixed
7) Water bath to control temperature + 5 mins timer
8) Observe colour of precipitate formed & with colour chart
Independent variable: concentration of reducing sugar
Dependent variable: colour of precipitate formed & compared
i
Increasing concentration of reducing sugars
Part 2: Iodine solution method
1. 2% starch solution to make five different concentrations
2. Use waterproof pen to label the test tubes and beakers
3. Use syringes, distilled water & the 2% starch solution to create 5 cm3 of each solution
4. Add 0.5 cm3 of iodine & dilute with 10 cm3 distilled water
Independent variable: concentration of starch
Dependent variable: colour change of solution
BROWN TO BLUE-BLACK INDICATES THE PRESENCE OF STARCH
Semi-quantitative → non-numerical results that provide an estimate of how much of the measured
substance is present
1) How could these investigations have been adapted to produce quantitative results?
Using a colorimeter.
2) Explain why Benedict’s test is not a suitable test for measuring the total sugar content of fruit
juice.
Fruit juices don’t contain sucrose (non-reducing sugar), thus no reaction would be observed.
3) Suggest one way in which the results of the Benedict’s reagent investigation could have been
made more reliable.
Larger quantities of each sample concentration could have been made, so repeat readings could be
taken.
Calculate average from the results of the whole class.
Copyright © 2020–2021 – All Rights Reserved.
, 4) How could you evaluate the accuracy of your experimental value for the concentration of glucose
in the fruit juice?
Compare with the reference values on the nutrition level showing the glucose content.
Calculate the concentration of glucose in the juice & then compare with the estimate generated
experimentally.
5) Name a variable that you controlled in the iodine investigation, explain how you controlled it and
describe the possible effect on the result if it had not been controlled.
Concentration of iodine → controlled using same solution & diluting by same amount each time
If iodine had not been controlled → over or underestimation of the concentration of starch.
Copyright © 2020–2021 – All Rights Reserved.
