Red brown precipitate result indicates reducing sugars present
Disaccharide hydrolysis test and Benedicts test
Carbohydrates – non-reducing sugars such as sucrose
- Add 2cm3 of solution to a test tube .
- Add 1cm3 dilute hydrochloric acid.
- Boil for one minute.
- Carefully neutralise with sodium hydrogen carbonate
- Carry out Benedicts test
Brick red precipitate indicates reducing sugars present, if above test is negative
Heavier precipitate indicates reducing sugars present. if above test is positive
Starch Test
Starch
- Add 2cm3 1% solution to a test tube
-
Add a few drops of I2/KI solution.
A blue-black colouration indicates starch present
To Identify Bio chemicals in an unknown substance
Method
Lipids Test
Lipids such as oils, fats and waxes.
- Add 2cm3 of absolute ethanol to a test tube
- Add 2cm3 of solution to the test tube
- Dissolve the solution by shaking vigorously
- Add an equal volume of cold water
A cloudy white suspension indicates lipids are present
Biuret Test
Add 2cm3 of solution to a test tube
Add an equal volume of 5% potassium hydroxide solution and mix
Add 2 drops of 1% copper sulphate solution and mix
A mauve or purple colour indicates peptide bonds are present
Results
Benedicts test
An unknown substance was tested and the test results were as follows:
Benedicts test – reducing sugars
As the solution was heated the substance changed from a brown to a light orange solution indicating a positive test.
Disaccharide hydrolysis test and Benedicts test
After boiling the litmus paper was dipped into the solution to check the Ph. The Ph turned red indicating a Ph of 1. Sufficient sodium hydroxide was added until the litmus paper turned olive green indicating a neutralised solution.
The Benedict’s test was carried out on the neutralised solution. The solution turned orangey red after heating, indicating a positive result.
Starch test
After the I2/KI solution was added there was no change to the substance it did not have a blue-black colouration indicating a negative result.
To Identify Bio chemicals in an unknown substance
Results
Lipid test
After dissolving the solution in ethanol a cloudy white suspension did not form, indicating a negative result.
Biuret test
After adding potassium hydroxide and copper sulphate no colour developed indicating a negative result.
Conclusion
The unknown substance is a protein containing both reducing and non-reducing sugars.
