Each student pair received three unknown samples dissolved in a solution of water. The below tests were performed in addition to any other physical observations we could make such as: colour, odour, clear or opaque, texture etc. Our results were recorded in a data table.
Benedict’s test – for simple (reducing sugar):
A small amount of each unknown liquids was taken in three separate boiling tubes. To each of them Benedict’s solution was added. All three tubes were heated gently for 2 minutes in the waterbath. A colour change from blue to orange was searched and the result was noted in the table.
Iodine test – for starch:
A small amount of each unknown liquids was taken in three separate boiling tubes. To each of them a couple of drops of Iodine solution were directly added. A colour change from orange to blue-black was searched and the result was noted in the table.
Biuret test – for proteins:
A small amount of each unknown liquids was taken in three separate boiling tubes. To each of them Biuret solution (sodium hydroxide and small amount of copper sulphate) was added. A colour change from blue to mauve was searched and the result was noted in the table.
Emulsion test – for Fats and Oils:
A very small amount of each unknown liquids was taken in three separate boiling tubes. To each of them ethanol was added. All three tubes were shaken and heated gently for two minutes in the water bath. Then to each of them a small amount of (tap) water was added. A white (milk-like) emulsion was searched and the result was noted in the table.
Results
All results (colour changes) were reported in the below data table. A colour change is marked as a positive result.
Discussion
In the test for sugar, we observed that not all the three samples show a positive result for simple sugar when Benedict’s solution is used. That’s because of the presence of simple (reducing) sugar in only one sample. It can be a simple sugar like glucose, fructose, etc.
We can conclude that the Sample C will give a positive result for non-reducing sugar as well. Although it will be better to perform the test for non-reducing sugar separately. That’s why we marked a positive for non-reducing sugar for sample C.
In the test for protein, we used water to make the emulsion. Fats are not soluble in water.
Conclusion
We conclude that sample C was a sugar (either a simple or a non-reducing sugar), sample B was a starch and sample A, and a protein. We conclude that none of these three samples is a lipid. That’s also obvious from the test for lipid. Because all the results were negative!
